TY - JOUR T1 - Coupling of a regional atmospheric model (RegCM3) and a regional oceanic model (FVCOM) over the maritime continent JF - Climate Dynamics Y1 - 2014 A1 - Jun Wei A1 - Paola Malanotte-Rizzoli A1 - Eltahir, Elfatih A B A1 - Pengfei Xue A1 - Danya Xu KW - Air–sea interactions KW - Climate variability KW - Regional atmosphere–ocean coupled model KW - Southeast Asian maritime continent AB -

Climatological high resolution coupled climate model simulations for the maritime continent have been carried out using the regional climate model (RegCM) version 3 and the finite volume coastal ocean model (FVCOM) specifically designed to resolve regions characterized by complex geometry and bathymetry. The RegCM3 boundary forcing is provided by the EMCWF-ERA40 re-analysis. FVCOM is embedded in the Global MITgcm which provides boundary forcing. The domain of the coupled regional model covers the entire South China Sea with its through-flow, the entire Indonesian archipelago with the Indonesian through-flow (ITF) and includes a large region in the western Pacific and eastern Indian oceans. The coupled model is able to provide stable and realistic climatological simulations for a specific decade of atmospheric–oceanic variables without flux correction. The major focus of this work is on oceanic properties. First, the coupled simulation is assessed against ocean-only simulations carried out under two different sets of air–sea heat fluxes. The first set, provided by the MITgcm, is proved to be grossly deficient as the heat fluxes are evaluated by a two-dimensional, zonally averaged atmosphere and the simulated SST have anomalous cold biases. Hence the MITgcm fluxes are discarded. The second set, the NCEP re-analysis heat fluxes, produces a climatological evolution of the SST with an average cold bias of ~−0.8 °C. The coupling eliminates the cold bias and the coupled SST evolution is in excellent agreement with the analogous evolution in the SODA re-analysis data. The detailed comparison of oceanic circulation properties with the International Nusantara Stratification and Transport observations shows that the coupled simulation produces the best estimate of the total ITF transport through the Makassar strait while the transports of three ocean-only simulations are all underestimated. The annual cycle of the transport is also very well reproduced. The coupling also considerably improves the vertical thermal structure of the Makassar cross section in the upper layer affected by the heat fluxes. On the other hand, the coupling is relatively ineffective in improving the precipitation fields even though the coupled simulation captures reasonably well the precipitation annual cycle at three land stations in different latitudes.

UR - http://link.springer.com/article/10.1007/s00382-013-1986-3 ER - TY - JOUR T1 - Droughts and floods over the upper catchment of the Blue Nile and their connections to the timing of El Niño and La Niña events JF - Hydrology and Earth System Sciences Y1 - 2014 A1 - M. A. H. Zaroug A1 - Eltahir, Elfatih A B A1 - Giorgi, F ER - TY - JOUR T1 - Enhancement of rainfall and runoff upstream from irrigation location in a climate model of West Africa JF - Water Resources Research Y1 - 2014 A1 - Im, Eun-Soon A1 - Elfatih A. B. Eltahir AB - This study investigates the impact of potential medium-scale irrigation (about 60,000 km2) on the climate of West Africa using the MIT Regional Climate Model. We find that irrigation at this scale induces an atmospheric response similar to that of large-scale irrigation (about 400,000km2) which was considered in our previous theoretical study. While the volume of water needed for large-scale irrigation is about 230–270 km3, the medium-scale irrigation requires about 50 km3, and the annual flow of the Niger river in the relevant section is about 70 km3. The remote response of rainfall distribution to local irrigation exhibits a significant sensitivity to the latitudinal location of irrigation. The nature of this response is such that irrigation from the Niger River around latitude 18°N induces significant increase in rainfall of order 100% in the upstream sources of the Niger River and results in significant increase in runoff of order 50%. This additional runoff can potentially be collected by the river network and delivered back toward the irrigation area. By selecting the location of irrigation carefully, the positive impacts of irrigation on rainfall distribution can be maximized. The approach of using a regional climate model to investigate the impact of location and size of irrigation schemes, explored in this study, may be the first step in incorporating land-atmosphere interactions in the design of location and size of irrigation projects. However, this theoretical approach is still in early stages of development and further research is needed before any practical application in water resources planning VL - 50 UR - http://onlinelibrary.wiley.com/doi/10.1002/2014WR015592/abstract ER - TY - JOUR T1 - Impact of potential large-scale irrigation on the West African Monsoon and its dependence on location of irrigated area JF - Journal of Climate Y1 - 2014 A1 - Im, Eun-Soon A1 - Marcella, Marc P. A1 - Eltahir, Elfatih A B KW - Atmosphere-land interaction KW - Climate sensitivity KW - Regional models AB -

This study investigates the impact of potential large-scale irrigation on the West African monsoon using the Massachusetts Institute of Technology regional climate model (MRCM). A new irrigation module is implemented to assess the impact of location and scheduling of irrigation on rainfall distribution over West Africa. A control simulation (without irrigation) and eight sensitivity experiments (with irrigation) are performed and compared to discern the effects of irrigation location and scheduling. It is found that the irrigation effect on soil moisture could force significant changes in spatial distribution and magnitude of rainfall, depending on the latitudinal location of irrigation. In general, the large irrigation-induced surface cooling owing to anomalously wet soil tends to suppress moist convection and rainfall, which in turn induces local subsidence and low-level anticyclonic circulation. These local effects are dominated by a consistent reduction of local rainfall over the irrigated land, irrespective of its location. However, the remote response of rainfall distribution to irrigation exhibits a significant sensitivity to the latitudinal position of irrigation and the intraseasonal variation of supplied irrigation water. The low-level northeasterly airflow associated with an anticyclonic circulation centered over the irrigation area, induced at optimal location and timing, would enhance the extent of low-level convergence areas through interaction with the prevailing monsoon flow, leading to a significant increase in rainfall. As the location of the irrigation area is moved from the coast northward, the regional rainfall change exhibits a significant decrease first, then increases gradually to a maximum corresponding to irrigation centered around 20°N, before it declines again.

VL - 27 IS - 3 ER - TY - JOUR T1 - Improving the simulation of the West African Monsoon using the MIT Regional Climate Model JF - Journal of Climate Y1 - 2014 A1 - Im, Eun-Soon A1 - Gianotti, Rebecca L. A1 - Eltahir, Elfatih A B AB -

This paper presents an evaluation of the performance of the MIT Regional Climate Model (MRCM) in simulating the West African Monsoon. MRCM is built on Regional Climate Model Version 3 (RegCM3) but with several improvements; including coupling of Integrated Biosphere Simulator (IBIS) land surface scheme, a new surface albedo assignment method, new convective cloud and convective rainfall autoconversion schemes, and modified scheme for simulating boundary layer height and boundary layer clouds. To investigate the impact of these more physically realistic representations when incorporated into MRCM, a series of experiments were carried out implementing two land surface schemes (IBIS with a new albedo assignment, and BATS: Biosphere-Atmosphere Transfer Scheme) and two convection schemes (Grell with the Fritsch-Chappell closure, and Emanuel in both the default form and modified with the new convective cloud cover and rainfall autoconversion scheme). The analysis primarily focuses on comparing the rainfall characteristics, surface energy balance and large scale circulations against various observations. This work documents significant sensitivity in simulation of the West African Monsoon to the choices of the land surface and convection schemes. Despite several deficiencies, the simulation with the combination of IBIS, modified Emanuel scheme with the new convective cloud cover and rainfall autoconversion scheme shows the best performance with respect to the spatial distribution of rainfall and the dynamics of the monsoon. The coupling of IBIS leads to representations of the surface energy balance and partitioning that show better agreement with observations compared to BATS. The IBIS simulations also reasonably reproduce the dynamical structures of the West African monsoon circulation.

VL - 27 IS - 6 JO - Improving the simulation of the West African Monsoon using the MIT Regional Climate Model ER - TY - JOUR T1 - Introducing an Irrigation Scheme to a Regional Climate Model: A Case Study over West Africa JF - Journal of Climate Y1 - 2014 A1 - Marcella, Marc P. A1 - Eltahir, Elfatih A B AB -

This article presents a new irrigation scheme and biome to the dynamic vegetation model, Integrated Biosphere Simulator (IBIS), coupled to Regional ClimateModel version 3 (RegCM3-IBIS). The new land cover allows for only the plant functional type, crop, to exist in an irrigated grid-cell. Irrigation water (i.e. negative runoff) is applied until the soil root zone reaches relative field capacity. The new scheme allows for irrigation scheduling (i.e. when to apply water) and for the user to determine the crop to be grown. Initial simulations show a large sensitivity of the scheme to soil texture types, how the water is applied, and the climatic conditions over the region.

Application of the new scheme is tested over West Africa, specifically Mali and Niger, to simulate the potential irrigation of the Niger River. A realistic representation of irrigation of the Niger River is performed by constraining the land irrigated by the annual flow of the Niger River and the amount of arable land in the region as reported by FAO. A thirty-year simulation including irrigated cropland is compared to a thirty year simulation that is identical but with no irrigation of the Niger. Results indicate a significant greening of the irrigated land as evapotranspiration over the crop fields largely increases—mostly via increases in transpiration from plant growth. The increase in the evapotranspiration, or latent heat flux (by 65-150 W/m2), causes a significant decrease in the sensible heat flux while surface temperatures cool on average by nearly 5°C. This cooling is felt downwind where average daily temperatures outside the irrigation are reduced by 0.5-1.0°C. Likewise, large increases in two meter specific humidity (which will be referred to as QA) are experienced across the irrigated cropland (on the order of 5 g/kg) but also extend further north and east, reecting the prevailing surface southwesterlies. Changes (decreases) in rainfall are found only over the irrigated lands of west Mali. The decrease in rainfall can be explained by the large surface cooling and collapse of the boundary layer (by approximately 500m). Both lead to a reduction in the triggering of convection as the convective inhibition, or negative buoyant energy is never breached. Nevertheless, the new scheme and land cover allows for a novel line of research that can accurately reflect the effects of irrigation on climate and the surrounding environment using a dynamic vegetation model coupled to a regional climate model.

UR - http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-13-00116.1 ER - TY - JOUR T1 - Local feedback mechanisms of the shallow water region around the Maritime Continent JF - Journal of Geophysical Research: Oceans Y1 - 2014 A1 - Pengfei Xue A1 - Eltahir, Elfatih A B A1 - Paola Malanotte-Rizzoli A1 - Jun Wei KW - air-sea interaction KW - cloud feedbacks KW - evaporative cooling AB -

The focus of this study is the local-scale air-sea feedback mechanisms over the shallow shelf water region (water depth <200 m) of the Maritime Continent (MC). MC was selected as a pilot study site for its extensive shallow water coverage, geographic complexity, and importance in the global climate system. To identify the local-scale air-sea feedback processes, we ran numerical experiments with perturbed surface layer water temperature using a coupled ocean-atmosphere model and an uncoupled ocean model. By examining the responses of the coupled and uncoupled models to the water temperature perturbation, we identify that, at a local-scale, a negative feedback process through the coupled dynamics that tends to restore the SST from its perturbation could dominate the shallow water region of the MC at a short time scale of several days. The energy budget shows that 38% of initial perturbation-induced heat energy was adjusted through the air-sea feedback mechanisms within 2 weeks, of which 58% is directly transferred into the atmosphere by the adjustment of latent heat flux due to the evaporative cooling mechanism. The increased inputs of heat and moisture into the lower atmosphere then modifies its thermal structure and increases the formation of low-level clouds, which act as a shield preventing incoming solar radiation from reaching the sea surface, accounts for 38% of the total adjustment of surface heat fluxes, serving as the sec- ond mechanism for the negative feedback process. The adjustment of sensible heat flux and net longwave radiation play a secondary role. The response of the coupled system to the SST perturbation suggests a response time scale of the coupled feedback process of about 3–5 days. The two-way air-sea feedback tightly links the surface heat fluxes, clouds and SST, and can play an important role in regulating the short- term variability of the SST over the shallow shelf water regions. 

VL - 119 UR - http://onlinelibrary.wiley.com/doi/10.1002/2013JC009700/abstract ER - TY - JOUR T1 - Regional Climate Modeling over the Maritime Continent. Part I: New Parameterization for Convective Cloud Fraction JF - Journal of Climate Y1 - 2014 A1 - Gianotti, Rebecca L. A1 - Eltahir, Elfatih A B KW - climate models KW - Cloud parameterizations KW - Convective clouds KW - Feedback KW - Radiation budgets AB -

This paper describes a new method for parameterizing convective cloud fraction that can be used within large-scale climate models, and evaluates the new method using the Regional Climate Model, version 3 (RegCM3), coupled to the land surface scheme Integrated Biosphere Simulator (IBIS). The horizontal extent of convective cloud cover is calculated by utilizing a relationship between the simulated amount of convective cloud water and typical observations of convective cloud water density. This formulation not only provides a physically meaningful basis for the simulation of convective cloud cover, but it is also spatially and temporally variable and independent of model resolution, rendering it generally applicable for large-scale climate models. Simulations over the Maritime Continent show that the new method allows for simulation of an essential convective–radiative feedback, which was absent in the existing version of RegCM3–IBIS, such that moist convection not only responds to diurnal variability at the earth’s surface but also impacts the solar radiation received at the surface via cumulus cloud production. The impact on model performance was mixed, but it is considered that appropriate representation of the convective–radiative feedback and improved physical realism resulting from the new cloud fraction parameterization will likely have positive benefits elsewhere. The role of convective rainfall production in the convective–radiative feedback and a new parameterization for convective autoconversion are addressed in Part II of this paper series.

VL - 27 UR - http://journals.ametsoc.org/doi/full/10.1175/JCLI-D-13-00127.1 IS - 4 ER - TY - JOUR T1 - Regional Climate Modeling over the Maritime Continent. Part II: New Parameterization for Autoconversion of Convective Rainfall JF - Journal of Climate Y1 - 2014 A1 - Gianotti, Rebecca L. A1 - Eltahir, Elfatih A B KW - climate models KW - Convective-scale processes KW - Maritime Continent KW - Rainfall KW - Subgrid-scale processes AB -

This paper describes a new method for parameterizing the conversion of convective cloud liquid water to rainfall (“autoconversion”) that can be used within large-scale climate models, and evaluates the new method using the Regional Climate Model, version 3 (RegCM3), coupled to the land surface scheme Integrated Biosphere Simulator (IBIS). The new method is derived from observed distributions of cloud water content and is constrained by observations of cloud droplet characteristics and climatological rainfall intensity. This new method explicitly accounts for subgrid variability with respect to cloud water density and is independent of model resolution, making it generally applicable for large-scale climate models. This work builds on the development of a new parameterization method for convective cloud fraction, which was described in Part I.

Simulations over the Maritime Continent using the Emanuel convection scheme show significant improvement in model performance, not only with respect to convective rainfall but also in shortwave radiation, net radiation, and turbulent surface fluxes of latent and sensible heat, without any additional modifications made to the simulation of those variables. Model improvements are demonstrated over a 19-yr validation period as well as a shorter 4-yr evaluation. Model performance with the Grell convection scheme is not similarly improved and reasons for this outcome are discussed. This work illustrates the importance of representing observed subgrid-scale variability in diurnally varying convective processes for simulations of the Maritime Continent region.

VL - 27 UR - http://journals.ametsoc.org/doi/full/10.1175/JCLI-D-13-00171.1 IS - 4 ER - TY - JOUR T1 - The role of mineral aerosols in shaping the regional climate of West Africa JF - Journal of Geophysical Research: Atmoshperes Y1 - 2014 A1 - Marcella, Marc P. A1 - Elfatih A. B. Eltahir KW - West Africa;regional climate modeling;mineral aerosols;rainfall;AOD;climate AB -

This article examines the role of mineral aerosols in the regional climate of West Africa. Analysis is completed by comparing two 30 year simulations using a regional climate model (RegCM3-IBIS). The two simulations are identical in structure except one includes the representation of mineral aerosols via a fully coupled radiatively interactive dust emissions and aerosol tracer model; the other simulation does not. To discern the impact of dust on West Africa’s climate, comparisons are made between the two simulations’ surface climatology as well as atmospheric dynamics. It is found that RegCM3-IBIS and its dust model perform well in simulating the temporal and spatial distributions of mineral aerosols over the Sahel and Sahara. Consistent with previous studies over the region, RegCM3-IBIS simulates high-dust loading over the region (aerosol optical depth of 0.5–1.1), which results in significant incident shortwave radiation attenuation (25–50 W/m2) and temperature cooling (0.5ıC–1.25ıC). Depending on the underlying surface brightness, the top of atmosphere net radiative forcing may be positive (bright desert surfaces) or negative (dark, vegetated surface) with important implications on surface temperature cooling. Here it is proposed that the effects of dust on West African rainfall are distinctly different across the ocean-land border and the desert border region of the Sahel/Sahara. Nevertheless, in both regions, the change in rainfall is less than 10% of the total annual values. Therefore, this work concludes that the current, observed, dust loading over West Africa does not significantly affect rainfall via changes in the radiation budget. However, it is important to note that this work does not include mineral aerosol effects on sea surface temperatures, which may be significant in influencing the results 

VL - 119 UR - http://onlinelibrary.wiley.com/doi/10.1002/2012JD019394/abstract ER - TY - JOUR T1 - Role of the Indian Ocean sea surface temperature in shaping the natural variability in the flow of Nile River JF - Climate Dynamics Y1 - 2014 A1 - Mohamed S. Siam A1 - Guiling Wang A1 - Marie-Estelle Demory A1 - Eltahir, Elfatih A B ER - TY - JOUR T1 - Simulating the connections of ENSO and the rainfall regime of East Africa and the upper Blue Nile region using a climate model of the Tropics JF - Hydrology and Earth System Sciences Y1 - 2014 A1 - M. A. H. Zaroug A1 - Giorgi, F A1 - E. Coppola A1 - G. M. Abdo A1 - Eltahir, Elfatih A B KW - CLIMATE KW - ENSO KW - MODEL KW - Rainfall KW - RegCM4 KW - Upper Blue Nile AB -

We simulate the observed statistical relationship between ENSO and the rainfall regime of the upper Blue Nile using the tropical-band version of the regional climate model RegCM4 (or Reg-TB). An ensemble of nine simulations for the 28-year period 1982–2009 is completed to investigate the role of ENSO in modulating rainfall over the upper Blue Nile catchment. Reg-TB shows a good skill in simulating the cli- matology of temperature, outgoing long-wave radiation pat- terns as well as related atmospheric circulation features dur- ing the summer season (i.e. the rainy season over the Blue Nile catchment). The model also succeeds in reproducing the observed negative correlation between Pacific SST and rain- fall anomalies over the Blue Nile catchment, and in particular the association of droughts over the Blue Nile with El Niño events that start in April–June. We thus propose that observa- tions and model forecasts of Pacific SST during this season could be used in seasonal forecasting of summer rainfall over the upper Blue Nile region. 

VL - 18 UR - http://www.hydrol-earth-syst-sci.net/18/4311/2014/hess-18-4311-2014.pdf IS - 11 ER - TY - JOUR T1 - Epidemiology of malaria in an area of seasonal transmission in Niger and implications for the design of a seasonal malaria chemoprevention strategy JF - Malaria Journal Y1 - 2013 A1 - Julia Guillebaud A1 - Aboubacar Mahamadou A1 - Halima Zamanka A1 - Mariama Katzelma A1 - Ibrahim Arzika A1 - Maman L Ibrahim A1 - Eltahir, Elfatih A B A1 - Rabiou Labbo A1 - Pierre Druilhe A1 - Duchemin, Jean-Bernard A1 - Thierry Fandeur KW - Chemoprevention KW - Incidence KW - Malaria; Niger KW - Prevalence KW - Seasonality KW - Slide positivity rate AB -

BACKGROUND:

Few data are available about malaria epidemiological situation in Niger. However, implementation of new strategies such as vaccination or seasonal treatment of a target population requires the knowledge of baseline epidemiological features of malaria. A population-based study was conducted to provide better characterization of malaria seasonal variations and population groups the most at risk in this particular area.

METHODS:

From July 2007 to December 2009, presumptive cases of malaria among a study population living in a typical Sahelian village of Nigerwere recorded, and confirmed by microscopic examination. In parallel, asymptomatic carriers were actively detected at the end of each dry season in 2007, 2008 and 2009.

RESULTS:

Among the 965 presumptive malaria cases recorded, 29% were confirmed by microscopic examination. The incidence of malaria was found to decrease significantly with age (p < 0.01). The mean annual incidence was 0.254. The results show that the risk of malaria was higher in children under ten years (p < 0.0001). The number of malaria episodes generally followed the temporal pattern of changes in precipitation levels, with a peak of transmission in August and September. One-thousand and ninety subjects were submitted to an active detection of asymptomatic carriage of whom 16% tested positive; asymptomatic carriage decreased with increasing age. A higher prevalence of gametocyte carriage among asymptomatic population was recorded in children aged two to ten years, though it did not reach significance.

CONCLUSIONS:

In Southern Nigermalaria transmission mostly occurs from July to October. Children aged two to ten years are the most at risk ofmalaria, and may also represent the main reservoir for gametocytes. Strategies such as intermittent preventive treatment in children (IPTc) could be of interest in this area, where malaria transmission is highly seasonal. Based on these preliminary data, a pilot study could be implemented in Zindarou using IPTc targeting children aged two to ten years, during the three months of malaria transmission, together with an accurate monitoring of drug resistance.

VL - 12 UR - http://www.malariajournal.com/content/12/1/379 IS - 379 ER - TY - JOUR T1 - Hydrological Cycles Over the Congo and Upper Blue Nile Basins: Evaluation of General Circulation Models Simulations JF - Journal of Climate Y1 - 2013 A1 - Siam, M.S A1 - Marie Estelle Demory A1 - Eltahir, Elfatih A B AB -

The simulations and predictions of the hydrological cycle by general circulation models (GCMs) are characterized by a significant degree of uncertainty. This uncertainty is reflected in the range of Intergovernmental Panel on Climate Change (IPCC) GCM predictions of future changes in the hydrological cycle, particularly over major African basins. The confidence in GCM predictions can be increased by evaluating different GCMs, identifying those models that succeed in simulating the hydrological cycle under current climate conditions, and using them for climate change studies. Reanalyses are often used to validate GCMs, but they also suffer from an inaccurate representation of the hydrological cycle. In this study, the aim is to identify GCMs and reanalyses' products that provide a realistic representation of the hydrological cycle over the Congo and upper Blue Nile (UBN) basins. Atmospheric and soil water balance constraints are employed to evaluate the models' ability to reproduce the observed streamflow, which is the most accurate measurement of the hydrological cycle. Among the ECMWF Interim Re-Analysis (ERA-Interim), NCEP–NCAR reanalysis, and 40-yr ECWMF Re-Analysis (ERA-40), ERA-Interim shows the best performance over these basins: it balances the water budgets and accurately represents the seasonal cycle of the hydrological variables. The authors find that most GCMs used by the IPCC overestimate the hydrological cycle compared to observations. They observe some improvement in the simulated hydrological cycle with increased horizontal resolution, which suggests that some of the high-resolution GCMs are better suited for climate change studies over Africa.

UR - http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-12-00404.1 ER - TY - JOUR T1 - Incorporating the effects of humidity in a mechanistic model of Anopheles gambiae mosquito population dynamics in the Sahel region of Africa. JF - Parasit Vectors Y1 - 2013 A1 - Yamana, Teresa K. A1 - Eltahir, Elfatih A B AB -

BACKGROUND: Low levels of relative humidity are known to decrease the lifespan of mosquitoes. However, most current models of malaria transmission do not account for the effects of relative humidity on mosquito survival. In the Sahel, where relative humidity drops to levels <20% for several months of the year, we expect relative humidity to play a significant role in shaping the seasonal profile of mosquito populations. Here, we present a new formulation for Anopheles gambiae sensu lato (s.l.) mosquito survival as a function of temperature and relative humidity and investigate the effect of humidity on simulated mosquito populations. METHODS: Using existing observations on relationships between temperature, relative humidity and mosquito longevity, we developed a new equation for mosquito survival as a function of temperature and relative humidity. We collected simultaneous field observations on temperature, wind, relative humidity, and anopheline mosquito populations for two villages from the Sahel region of Africa, which are presented in this paper. We apply this equation to the environmental data and conduct numerical simulations of mosquito populations using the Hydrology, Entomology and Malaria Transmission Simulator (HYDREMATS). RESULTS: Relative humidity drops to levels that are uncomfortable for mosquitoes at the end of the rainy season. In one village, Banizoumbou, water pools dried up and interrupted mosquito breeding shortly after the end of the rainy season. In this case, relative humidity had little effect on the mosquito population. However, in the other village, Zindarou, the relatively shallow water table led to water pools that persisted several months beyond the end of the rainy season. In this case, the decrease in mosquito survival due to relative humidity improved the model's ability to reproduce the seasonal pattern of observed mosquito abundance. CONCLUSIONS: We proposed a new equation to describe Anopheles gambiae s.l. mosquito survival as a function of temperature and relative humidity. We demonstrated that relative humidity can play a significant role in mosquito population and malaria transmission dynamics. Future modeling work should account for these effects of relative humidity.

VL - 6 ER - TY - JOUR T1 - Linking environmental variability to village-scale malaria transmission using a simple immunity model. JF - Parasit Vectors Y1 - 2013 A1 - Yamana, Teresa K. A1 - Bomblies, Arne A1 - Laminou, Ibrahim M A1 - Duchemin, Jean-Bernard A1 - Eltahir, Elfatih A B AB -

BACKGROUND: Individuals continuously exposed to malaria gradually acquire immunity that protects from severe disease and high levels of parasitization. Acquired immunity has been incorporated into numerous models of malaria transmission of varying levels of complexity (e.g. Bull World Health Organ 50:347, 1974; Am J Trop Med Hyg 75:19, 2006; Math Biosci 90:385-396, 1988). Most such models require prescribing inputs of mosquito biting rates or other entomological or epidemiological information. Here, we present a model with a novel structure that uses environmental controls of mosquito population dynamics to simulate the mosquito biting rates, malaria prevalence as well as variability in protective immunity of the population. METHODS: A simple model of acquired immunity to malaria is presented and tested within the framework of the Hydrology, Entomology and Malaria Transmission Simulator (HYDREMATS), a coupled hydrology and agent-based entomology model. The combined model uses environmental data including rainfall, temperature, and topography to simulate malaria prevalence and level of acquired immunity in the human population. The model is used to demonstrate the effect of acquired immunity on malaria prevalence in two Niger villages that are hydrologically and entomologically very different. Simulations are conducted for the year 2006 and compared to malaria prevalence observations collected from the two villages. RESULTS: Blood smear samples from children show no clear difference in malaria prevalence between the two villages despite pronounced differences in observed mosquito abundance. The similarity in prevalence is attributed to the moderating effect of acquired immunity, which depends on prior exposure to the parasite through infectious bites - and thus the hydrologically determined mosquito abundance. Modelling the level of acquired immunity can affect village vulnerability to climatic anomalies. CONCLUSIONS: The model presented has a novel structure constituting a mechanistic link between spatial and temporal environmental variability and village-scale malaria transmission. Incorporating acquired immunity into the model has allowed simulation of prevalence in the two villages, and isolation of the effects of acquired immunity in dampening the difference in prevalence between the two villages. Without these effects, the difference in prevalence between the two villages would have been significantly larger in response to the large differences in mosquito populations and the associated biting rates.

VL - 6 UR - http://www.parasitesandvectors.com/content/pdf/1756-3305-6-226.pdf ER - TY - JOUR T1 - Projected Impacts of Climate Change on Environmental Suitability for Malaria Transmission in West Africa. JF - Environ Health Perspect Y1 - 2013 A1 - Yamana, Teresa K. A1 - Eltahir, Elfatih A B AB -

BACKGROUND: Climate change is expected to impact the distribution of environmental suitability for malaria transmission by altering temperature and rainfall patterns, but the local and global impacts of climate change on malaria transmission are uncertain. OBJECTIVE: To assess the effect of climate change on malaria transmission in West Africa. METHODS: We coupled a detailed mechanistic hydrology and entomology model with climate projections from general circulation models (GCMs) to predict changes in vectorial capacity, an indication of the risk of human malaria infections, resulting from changes in the availability of mosquito breeding sites and temperature-dependent development rates. Since there is strong disagreement in climate predictions from different GCMs, we focused on the GCM projections that produced the best and worst conditions for malaria transmission in each zone of the study area. RESULTS: Simulation-based estimates suggest that in the desert fringes of the Sahara, vectorial capacity would increase under the worst-case scenario, but not enough to sustain transmission. In the transitional zone of the Sahel, climate change is predicted to decrease vectorial capacity. In the wetter regions to the south, our estimates suggest an increase in vectorial capacity under all scenarios. However, because malaria is already highly endemic among human populations in these regions, we expect that changes in malaria incidence would be small. CONCLUSION: Our findings highlight the importance of rainfall in shaping the impact of climate change on malaria transmission in future climates. Even under the GCM predictions most conducive to malaria transmission, we do not expect to see a significant increase in malaria prevalence in this region.

ER - TY - JOUR T1 - A sensitivity study on the role of the swamps of southern Sudan in the summer climate of North Africa using a regional climate model JF - Theoretical and Applied Climatology Y1 - 2013 A1 - Zaroug, Modathir AH A1 - Sylla, MB A1 - Giorgi, F A1 - Eltahir, Elfatih A B A1 - Aggarwal, Pradeep K ER - TY - JOUR T1 - Assessment of the Regional Climate Model Version 3 over the Maritime Continent Using Different Cumulus Parameterization and Land Surface Schemes JF - Journal of Climate Y1 - 2012 A1 - Gianotti, Rebecca L. A1 - Dongfeng Zhang A1 - Eltahir, Elfatih A B KW - Atmosphere-land interaction KW - Diurnal effects KW - Maritime Continent KW - Rainfall KW - Regional models AB -

This paper describes an assessment of the Regional Climate Model, version 3 (RegCM3), coupled to two land surface schemes: the Biosphere–Atmosphere Transfer System, version 1e (BATS1e), and the Integrated Biosphere Simulator (IBIS). The model’s performance in simulating precipitation over the Maritime Continent was evaluated against the 3-hourly Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis 3B42 product. It is found that the model suffers from three major errors in reproducing the observed rainfall histogram: underestimation of the frequency of dry periods, overestimation of the frequency of low-intensity rainfall, and underestimation of the frequency of high-intensity rainfall. Additionally, the model does not accurately reproduce the observed timing of the diurnal rainfall peak, particularly over land. These four errors persisted regardless of the choice of lateral boundary conditions, convective parameterization scheme, or land surface scheme. The magnitude of the wet–dry bias in the simulated volumes of rainfall was, however, strongly dependent on the choice of the convection scheme and lateral boundary conditions. The Grell convection scheme with Fritsch–Chappell closure was the best performing of the convection schemes, having the smallest error magnitudes in both the rainfall histogram and average diurnal cycle, and also having good representation of the land surface energy and evapotranspiration components. The 40-yr ECMWF Re-Analysis (ERA-40) was found to produce better simulations of observed rainfall when used as lateral boundary conditions than did the NCEP–NCAR reanalysis. Discussion of the nature of the major model errors is provided, along with some suggestions for improvement.

VL - 25 ER - TY - JOUR T1 - Modeling the Summertime Climate of Southwest Asia: The Role of Land Surface Processes in Shaping the Climate of Semiarid Regions JF - Journal of Climate Y1 - 2012 A1 - Marcella, Marc P. ED - Eltahir, Elfatih A B AB -

 

Presented is a study on the role of land surface processes in determining the summertime climate over the
semiarid region of southwest Asia. In this region, a warm surface air temperature bias of 3.58C is simulated in
the summer by using the standard configuration of Regional Climate Model version 3 (RegCM3). Biases are
also simulated in surface albedo (underestimation), shortwave incident radiation (overestimation), and vapor
pressure (underestimation). Based on satellite measurements documented in NASA’s surface radiation
budget (SRB) dataset, a correction in surface albedo by 4% is introduced in RegCM3 to match the observed
SRB data. Increasing albedo values results in a nearly 18C cooling over the region. In addition, by incorporating
RegCM3’s dust module and including subgrid variability for surface wind, shortwave incident
radiation bias originally of about 45 W m22 is reduced by 30 W m22. As a result, the reduction of shortwave
incident radiation cools the surface by 0.68C. Finally, including a representation for the irrigation and
marshlands of Mesopotamia produces surface relative humidity values closer to observations, thus eliminating
a nearly 5-mb vapor pressure dry bias over some of the region. Consequently, the representation of
irrigation and marshlands results in cooling of nearly 18C in areas downwind of the actual land-cover change.
Along with identified biases in observational datasets, these combined processes explain the 3.58C warm bias
in RegCM3 simulations. Therefore, it is found that accurate representations of surface albedo, dust emissions,
and irrigation are important in correctly modeling summertime climates of semiarid regions.
VL - 25 UR - http://journals.ametsoc.org/doi/pdf/10.1175/2011JCLI4080.1 ER - TY - Generic T1 - "Coupling of a Regional Atmospheric Model (RegCM3) and a Regional Oceanic Model (FVCOM) Over the Maritime Continent." submitted to Climate Dynamics. Y1 - 2011 A1 - Wei, J. Zhange D., ED - P. Malanotte-Rizzoli ED - Eltahir, Elfatih A B ER - TY - JOUR T1 - Modeling the hydroclimatology of the midwestern United States. Part 2: future climate JF - Climate Dynamics Y1 - 2011 A1 - Winter, Jonathan A1 - Eltahir, Elfatih AB -

An ensemble of six 22-year numerical experiments was conducted to quantify the response of soil moisture to multiple climate change scenarios over the American Midwest. Regional Climate Model version 3 (RegCM3) was run using two surface physics schemes: Integrated Biosphere Simulator (IBIS) and Biosphere-Atmosphere Transfer Scheme 1e (BATS1e); and two convective closure assumptions: Fritsch and Chappell and Arakawa and Schubert. Experiments were forced with a surrogate climate change scenario constructed using the National Centers for Environmental Prediction-Department of Energy Reanalysis 2 dataset and the ECHAM5 A1B climate change scenario. RegCM3-IBIS and RegCM3-BATS1e simulate increased two-meter air temperature and downward longwave radiation throughout the year under both climate change scenarios. While differences in shortwave radiation are relatively small; some model configurations and climate change scenarios produce additional precipitation, evapotranspiration, and total runoff during the spring and summer. Soil moisture is unchanged or increased throughout the growing season as enhanced rainfall offsets greater evaporative demand. Negligible drying in root zone soil moisture is found in all climate change experiments conducted, regardless of surface physics scheme, boundary conditions, or convective closure assumption.

PB - Springer Berlin / Heidelberg VL - 38 UR - http://dx.doi.org/10.1007/s00382-011-1183-1 N1 - 10.1007/s00382-011-1183-1 ER - TY - JOUR T1 - Modeling the hydroclimatology of the midwestern United States. Part 1: current climate JF - Climate Dynamics Y1 - 2011 A1 - Winter, J.M. A1 - Eltahir, Elfatih A B AB -

A climate model must include an accurate surface physics scheme in order to examine the interactions between the land and atmosphere. Given an increase in the surface radiative forcing, the sensitivity of latent heat flux to available energy plays an important role in determining the energy budget and has a significant impact on the response of surface temperature. The Penman–Monteith equation is used to construct a theoretical framework for evaluating the climatology of evapotranspiration and the sensitivity of latent heat flux to available energy. Regional Climate Model version 3 coupled to Integrated Biosphere Simulator (RegCM3–IBIS); RegCM3 with its native land surface model, Biosphere–Atmosphere Transfer Scheme 1e (RegCM3–BATS1e); and Flux Network (FLUXNET) micrometeorological tower observations are compared and contrasted using the developed methodology. RegCM3–IBIS and RegCM3–BATS1e simulate the observed sensitivity of latent heat flux to available energy reasonably well during the summer on average; however, there are significant variations in the monthly values. Additional information provided by the physically based Penman–Monteith framework is employed for identifying deficiencies and guiding improvements in models, allowing calibration of both the climatology of evapotranspiration and the sensitivity of latent heat flux to available energy.

PB - Springer ER - TY - JOUR T1 - The role of lateral boundary conditions in simulations of mineral aerosols by a regional climate model of Southwest Asia JF - Climate Dynamics Y1 - 2011 A1 - Marcella, M.P. A1 - Eltahir, Elfatih A B AB - The importance of specifying realistic lateral boundary conditions in the regional modeling of mineral aerosols has not been examined previously. This study examines the impact of assigning values for mineral aerosol (dust) concentrations at the lateral boundaries of Regional Climate Model version 3 (RegCM3) and its aerosol model over Southwest Asia. Currently, the dust emission module of RegCM3 operates over the interior of the domain, allowing dust to be transported to the boundaries, but neglecting any dust emitted at these points or from outside the domain. To account for possible dust occurring at, or entering from the boundaries, mixing ratios of dust concentrations from a larger domain RegCM3 simulation are specified at the boundaries of a smaller domain over Southwest Asia. The lateral boundary conditions are monthly averaged concentration values (μg of dust per kg of dry air) resolved in the vertical for all four dust bin sizes within RegCM3’s aerosol model. RegCM3 simulations with the aerosol/dust model including lateral boundary conditions for dust are performed for a five year period and compared to model simulations without prescribed dust concentrations at the boundaries. Results indicate that specifying boundary conditions has a significant impact on dust loading across the entire domain over Southwest Asia. More specifically, a nearly 30% increase in aerosol optical depth occurs during the summer months from specifying realistic dust boundary conditions, bringing model results closer to observations such as MISR. In addition, smaller dust particles at the boundaries have a more important impact than large particles in affecting the dust loading within the interior of this domain. Moreover, increases in aerosol optical depth and dust concentrations within the interior domain are not entirely caused by inflow from the boundaries; results indicate that an increase in the gradient of concentration at the boundaries causes an increase of diffusion from the boundaries. Lastly, experiments performed using a climatology of dust concentrations yield similar results to those using actual monthly values. Therefore, using a climatology of dust mixing ratios is sufficient in implementing lateral boundary conditions for mineral aerosols. In short, this work concludes that realistic specification of lateral boundary conditions for mineral aerosols can be important in modeling the dust loading over arid regional climates such as Southwest Asia. PB - Springer ER - TY - JOUR T1 - On the use of satellite-based estimates of rainfall temporal distribution to simulate the potential for malaria transmission in rural Africa JF - Water Resources Research Y1 - 2011 A1 - Yamana, Teresa K. A1 - Eltahir, Elfatih A B AB - This paper describes the use of satellite-based estimates of rainfall to force the Hydrology, Entomology and Malaria Transmission Simulator (HYDREMATS), a hydrology-based mechanistic model of malaria transmission. We first examined the temporal resolution of rainfall input required by HYDREMATS. Simulations conducted over Banizoumbou village in Niger showed that for reasonably accurate simulation of mosquito populations, the model requires rainfall data with at least 1 h resolution. We then investigated whether HYDREMATS could be effectively forced by satellite-based estimates of rainfall instead of ground-based observations. The Climate Prediction Center morphing technique (CMORPH) precipitation estimates distributed by the National Oceanic and Atmospheric Administration are available at a 30 min temporal resolution and 8 km spatial resolution. We compared mosquito populations simulated by HYDREMATS when the model is forced by adjusted CMORPH estimates and by ground observations. The results demonstrate that adjusted rainfall estimates from satellites can be used with a mechanistic model to accurately simulate the dynamics of mosquito populations. PB - American Geophysical Union VL - 47 ER - TY - JOUR T1 - Early warnings of the potential for malaria transmission in rural Africa using the hydrology, entomology and malaria transmission simulator (HYDREMATS) JF - Malaria journal Y1 - 2010 A1 - Yamana, Teresa K. A1 - Eltahir, Elfatih A B KW - Animals KW - Computer Simulation KW - Culicidae KW - Epidemiologic Methods KW - Humans KW - Malaria KW - Niger KW - Rain KW - Rural Population AB -

BACKGROUND: Early warnings of malaria transmission allow health officials to better prepare for future epidemics. Monitoring rainfall is recognized as an important part of malaria early warning systems. The Hydrology, Entomology and Malaria Simulator (HYDREMATS) is a mechanistic model that relates rainfall to malaria transmission, and could be used to provide early warnings of malaria epidemics. METHODS: HYDREMATS is used to make predictions of mosquito populations and vectorial capacity for 2005, 2006, and 2007 in Banizoumbou village in western Niger. HYDREMATS is forced by observed rainfall, followed by a rainfall prediction based on the seasonal mean rainfall for a period two or four weeks into the future. RESULTS: Predictions made using this method provided reasonable estimates of mosquito populations and vectorial capacity, two to four weeks in advance. The predictions were significantly improved compared to those made when HYDREMATS was forced with seasonal mean rainfall alone. CONCLUSIONS: HYDREMATS can be used to make reasonable predictions of mosquito populations and vectorial capacity, and provide early warnings of the potential for malaria epidemics in Africa.

VL - 9 U1 - http://www.ncbi.nlm.nih.gov/pubmed/21073726?dopt=Abstract ER - TY - JOUR T1 - Effects of mineral aerosols on the summertime climate of southwest Asia: Incorporating subgrid variability in a dust emission scheme JF - Journal of Geophysical Research Y1 - 2010 A1 - Marcella, M.P. A1 - Eltahir, Elfatih A B AB -

 

Improvements in modeling mineral aerosols over southwest Asia are made to the dust
scheme in a regional climate model by representing subgrid variability of both wind
speed and surface roughness length. The new module quantifies wind variability by using
model meteorology while assuming that wind speed follows a Gaussian distribution. More
specifically, wind variability is approximated by dry convective eddies within the
planetary boundary layer, forced by sensible heat fluxes at the surface. Incorporating
subgrid variability of wind increases aerosol optical depth (AOD) over the region by
nearly 35% while reducing incoming shortwave radiation by an additional 5–10 W/m2.
Likewise, the dust scheme is modified to include the variability of surface roughness
length over southwest Asia. Here an empirical distribution of roughness length for each
grid cell is calculated based on the USGS’s 4 km resolution land cover data set. However,
incorporating roughness length variability does not significantly alter dust emissions over
the region due to the relatively homogeneous land cover conditions. Nevertheless,
including spatial variability for wind results in aerosol optical depth values closer to
observational data sets, particularly MISR, which performs better than MODIS over this
region. However, RegCM3’s dust model still underestimates AOD over southwest Asia.
In addition to improvements made in RegCM3’s dust model, this work examines the
effects of mineral aerosols on the mean monthly, surface summertime climate of southwest
Asia. It is shown that dust emissions reduce average summertime surface temperatures
by approximately 0.5°C while attenuating shortwave incident radiation by nearly
25 W/m2. Thus, the emission of dust is an important surface process in shaping the
summertime climate over southwest Asia. However, both a warm bias in surface
temperatures and overestimation of incoming shortwave radiation still exist in RegCM3
and need to be further addressed.
PB - American Geophysical Union VL - 115 ER - TY - JOUR T1 - The Sensitivity of Latent Heat Flux to Changes in the Radiative Forcing: A Framework for Comparing Models and Observations JF - Journal of Climate Y1 - 2010 A1 - Winter, Jonathan M. A1 - Eltahir, Elfatih A B KW - BIOSPHERE KW - CLIMATE KW - INTEGRATED KW - SIMULATOR KW - VARIABILITY AB -

A climate model must include an accurate surface physics scheme in order to examine the interactions between the land and atmosphere. Given an increase in the surface radiative forcing, the sensitivity of latent heat flux to available energy plays an important role in determining the energy budget and has a significant impact on the response of surface temperature. The Penman-Monteith equation is used to construct a theoretical framework for evaluating the climatology of evapotranspiration and the sensitivity of latent heat flux to available energy. Regional Climate Model version 3 coupled to Integrated Biosphere Simulator (RegCM3-IBIS); RegCM3 with its native land surface model, Biosphere-Atmosphere Transfer Scheme 1e (RegCM3-BATS1e); and Flux Network (FLUXNET) micrometeorological tower observations are compared and contrasted using the developed methodology. RegCM3-IBIS and RegCM3-BATS1e simulate the observed sensitivity of latent heat flux to available energy reasonably well during the summer on average; however, there are significant variations in the monthly values. Additional information provided by the physically based Penman-Monteith framework is employed for identifying deficiencies and guiding improvements in models, allowing calibration of both the climatology of evapotranspiration and the sensitivity of latent heat flux to available energy.

PB - AMERICAN METEOROLOGICAL SOCIETY, "Journal of Climate" CY - Boston VL - 23 IS - 9 ER - TY - JOUR T1 - Assessment of the impact of climate shifts on malaria transmission in the Sahel. JF - EcoHealth Y1 - 2009 A1 - Bomblies, Arne A1 - Eltahir, Elfatih A B KW - Animals KW - Anopheles KW - Climate Change KW - Disease Vectors KW - Ecosystem KW - Humans KW - Malaria KW - Niger KW - Risk Assessment KW - Temperature AB -

Climate affects malaria transmission through a complex network of causative pathways. We seek to evaluate the impact of hypothetical climate change scenarios on malaria transmission in the Sahel by using a novel mechanistic, high spatial- and temporal-resolution coupled hydrology and agent-based entomology model. The hydrology model component resolves individual precipitation events and individual breeding pools. The impact of future potential climate shifts on the representative Sahel village of Banizoumbou, Niger, is estimated by forcing the model of Banizoumbou environment with meteorological data from two locations along the north-south climatological gradient observed in the Sahel--both for warmer, drier scenarios from the north and cooler, wetter scenarios from the south. These shifts in climate represent hypothetical but historically realistic climate change scenarios. For Banizoumbou climatic conditions (latitude 13.54 N), a shift toward cooler, wetter conditions may dramatically increase mosquito abundance; however, our modeling results indicate that the increased malaria transmissibility is not simply proportional to the precipitation increase. The cooler, wetter conditions increase the length of the sporogonic cycle, dampening a large vectorial capacity increase otherwise brought about by increased mosquito survival and greater overall abundance. Furthermore, simulations varying rainfall event frequency demonstrate the importance of precipitation patterns, rather than simply average or time-integrated precipitation, as a controlling factor of these dynamics. Modeling results suggest that in addition to changes in temperature and total precipitation, changes in rainfall patterns are very important to predict changes in disease susceptibility resulting from climate shifts. The combined effect of these climate-shift-induced perturbations can be represented with the aid of a detailed mechanistic model.

VL - 6 IS - 3 U1 - http://www.ncbi.nlm.nih.gov/pubmed/20111887?dopt=Abstract ER - TY - JOUR T1 - Coupling of Integrated Biosphere Simulator to Regional Climate Model Version 3 JF - JOURNAL OF CLIMATE Y1 - 2009 A1 - Winter, Jonathan M. A1 - Pal, J. S. A1 - Eltahir, Elfatih A B KW - Evaluating Climate Regional Models Over the American Midwest AB -

A description of the coupling of Integrated Biosphere Simulator ( IBIS) to Regional Climate Model version 3 (RegCM3) is presented. IBIS introduces several key advantages to RegCM3, most notably vegetation dynamics, the coexistence of multiple plant functional types in the same grid cell, more sophisticated plant phenology, plant competition, explicit modeling of soil/plant biogeochemistry, and additional soil and snow layers. A single subroutine was created that allows RegCM3 to use IBIS for surface physics calculations. A revised initialization scheme was implemented for RegCM3-IBIS, including an IBIS-specific prescription of vegetation and soil properties. To illustrate the relative strengths and weaknesses of RegCM3-IBIS, one 4-yr numerical experiment was completed to assess ability of both RegCM3-IBIS ( with static vegetation) and RegCM3 with its native land surface model, Biosphere-Atmosphere Transfer Scheme 1e (RegCM3-BATS1e), to simulate the energy and water budgets. Each model was evaluated using the NASA Surface Radiation Budget, FLUXNET micro-meteorological tower observations, and Climate Research Unit Time Series 2.0. RegCM3-IBIS and RegCM3-BATS1e simulate excess shortwave radiation incident and absorbed at the surface, especially during the summer months. RegCM3-IBIS limits evapotranspiration, which allows for the correct estimation of latent heat flux, but increases surface temperature, sensible heat flux, and net longwave radiation. RegCM3-BATS1e better simulates temperature, net longwave radiation, and sensible heat flux, but systematically overestimates latent heat flux. This objective comparison of two different land surface models will help guide future adjustments to surface physics schemes within RegCM3.

PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 22 ER - TY - JOUR T1 - Hydrologic modeling to screen potential environmental management methods for malaria vector control in Niger JF - WATER RESOURCES RESEARCH Y1 - 2009 A1 - Gianotti, Rebecca L. A1 - Bomblies, Arne A1 - Eltahir, Elfatih A B KW - AFRICA KW - ARABIENSIS KW - DYNAMICS KW - INTERVENTIONS KW - Monsoons, Mosquitoes, and Malaria in Africa KW - MOSQUITO LARVAL HABITATS; ANOPHELES-GAMBIAE KW - POPULATIONS KW - RESISTANCE KW - TRANSMISSION AB -

This paper describes the first use of Hydrology-Entomology and Malaria Transmission Simulator (HYDREMATS), a physically based distributed hydrology model, to investigate environmental management methods for malaria vector control in the Sahelian village of Banizoumbou, Niger. The investigation showed that leveling of topographic depressions where temporary breeding habitats form during the rainy season, by altering pool basin microtopography, could reduce the pool persistence time to less than the time needed for establishment of mosquito breeding, approximately 7 days. Undertaking soil surface plowing can also reduce pool persistence time by increasing the infiltration rate through an existing pool basin. Reduction of the pool persistence time to less than the rainfall interstorm period increases the frequency of pool drying events, removing habitat for subadult mosquitoes. Both management approaches could potentially be considered within a given context. This investigation demonstrates that management methods that modify the hydrologic environment have significant potential to contribute to malaria vector control in water-limited, Sahelian Africa.

PB - {AMER GEOPHYSICAL UNION} VL - 45 SN - 1-56396-066-4 IS - WO8438 ER - TY - JOUR T1 - A Mechanistic Approach for Accurate Simulation of Village Scale Malaria Transmission JF - MALARIA JOURNAL Y1 - 2009 A1 - Bomblies, Arne A1 - Duchemin, Jean-Bernard A1 - Eltahir, Elfatih A B KW - ANOPHELES-GAMBIAE COMPLEX; WESTERN KENYA; MODEL; MOSQUITOS; AFRICA; HOST; VECTORS; LARVAE; WATER KW - Monsoons, Mosquitoes, and Malaria in Africa AB -

Background: Malaria transmission models commonly incorporate spatial environmental and climate variability for making regional predictions of disease risk. However, a mismatch of these models' typical spatial resolutions and the characteristic scale of malaria vector population dynamics may confound disease risk predictions in areas of high spatial hydrological variability such as the Sahel region of Africa. Methods: Field observations spanning two years from two Niger villages are compared. The two villages are separated by only 30 km but exhibit a ten-fold difference in anopheles mosquito density. These two villages would be covered by a single grid cell in many malaria models, yet their entomological activity differs greatly. Environmental conditions and associated entomological activity are simulated at high spatial- and temporal resolution using a mechanistic approach that couples a distributed hydrology scheme and an entomological model. Model results are compared to regular field observations of Anopheles gambiae sensu lato mosquito populations and local hydrology. The model resolves the formation and persistence of individual pools that facilitate mosquito breeding and predicts spatio-temporal mosquito population variability at high resolution using an agent-based modeling approach. Results: Observations of soil moisture, pool size, and pool persistence are reproduced by the model. The resulting breeding of mosquitoes in the simulated pools yields time-integrated seasonal mosquito population dynamics that closely follow observations from captured mosquito abundance. Interannual difference in mosquito abundance is simulated, and the inter-village difference in mosquito population is reproduced for two years of observations. These modeling results emulate the known focal nature of malaria in Niger Sahel villages. Conclusion: Hydrological variability must be represented at high spatial and temporal resolution to achieve accurate predictive ability of malaria risk at the village scale, which can then be integrated appropriately to regional spatial scales and seasonal temporal scales. These results have important implications for models seeking to link the impacts of climate change and climate variability to malaria transmission. The highly focal nature of malaria in the Sahel makes detailed representation necessary to evaluate village-level risks associated with hydrology-related vector population variability.

VL - 8 UR - http://www.malariajournal.com/content/8/1/223 ER - TY - JOUR T1 - A satellite-based Daily Actual Evapotranspiration estimation algorithm over South Florida JF - GLOBAL AND PLANETARY CHANGE Y1 - 2009 A1 - Jiang, Le A1 - Islam, Shafiqul A1 - Guo, Wei A1 - Jutla, Antarpreet Singh A1 - Senarath, Sharika U. S. A1 - Ramsay, Bruce H. A1 - Eltahir, Elfatih A B KW - AVHRR KW - Evaporative Fraction KW - evapotranspiration KW - Priestley-Taylor KW - remote sensing KW - Role of Biosphere in the Climate of Tropical Regions AB -

Water resources and agricultural applications require the knowledge of evapotranspiration (ET) over a range of spatial and temporal scales. Due to paucity of surface based hydro-meteorological stations, the spatial resolution of ET estimates is fairly coarse and is not particularly suitable or reliable for hydrologic modeling, water resources planning and decision making. An ET estimation algorithm has been developed by combining data from satellite and ground observations. The method extends the applicability of a commonly used energy balance formulation of ET and utilizes the contextual relationship between remotely sensed surface temperature and vegetation index. The required parameters are derived from the Advanced Very High Resolution Radiometer (AVHRR) aboard NOAA-14 satellite. First, the Evaporative Fraction (EF) is estimated by utilizing the relationship between a vegetation index and radiometric surface temperature observed from AVHRR for each day. Then spatio-temporal interpolation and filtering techniques are applied to obtain daily EF values for cloudy pixels to produce the EF map for the entire region. Daily Actual ET (DAET) maps are derived from these EF maps and net radiation maps obtained from ground-based observations. The comparisons between satellite derived DAFT and ground measurements showed overall low bias and root-mean-square-error for both clear and cloudy days at South Florida in 1998 and 1999. The proposed satellite-based DAFT (SatDAET) algorithm has its EF component primarily estimated from satellite data and the resulting DAET has been validated using multi-year ground observations over the South Florida region. The SatDAET algorithm appears to be robust and has the potential to provide near real-time land surface evapotranspiration monitoring over large heterogeneous areas at a very fine spatial and temporal resolution.

PB - {Amer Geophys Union} CY - {PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS} VL - 67 N1 - {Fall Annual Meeting of the American-Geophysical-Union, San Francisco, CA, 2005} ER - TY - JOUR T1 - Efficacy of local neem extracts for sustainable malaria vector control in an African village JF - MALARIA JOURNAL Y1 - 2008 A1 - Gianotti, Rebecca L. A1 - Bomblies, Arne A1 - Dafalla, Mustafa A1 - Issa-Arzika, Ibrahim A1 - Duchemin, Jean-Bernard A1 - Eltahir, Elfatih A B KW - Monsoons, Mosquitoes, and Malaria in Africa KW - Niger Project AB -

Background: Larval control of malaria vectors has been historically successful in reducing malaria transmission, but largely fell out of favour with the introduction of synthetic insecticides and bed nets. However, an integrated approach to malaria control, including larval control methods, continues to be the best chance for success, in view of insecticide resistance, the behavioural adaptation of the vectors to changing environments and the difficulties of reaching the poorest populations most at risk,. Laboratory studies investigating the effects of neem seed (Azadirachta indica) extracts on Anopheles larvae have shown high rates of larval mortality and reductions in adult longevity, as well as low potential for resistance development. Methods: This paper describes a method whereby seeds of the neem tree can be used to reduce adult Anopheles gambiae s. l. abundance in a way that is low cost and can be implemented by residents of rural villages in western Niger. The study was conducted in Banizoumbou village, western Niger. Neem seeds were collected from around the village. Dried seeds were ground into a coarse powder, which was then sprinkled onto known Anopheles larvae breeding habitats twice weekly during the rainy season 2007. Adult mosquitoes were captured on a weekly basis in the village and captures compared to those from 2005 and 2006 over the same period. Adult mosquitoes were also captured in a nearby village, Zindarou, as a control data set and compared to those from Banizoumbou. Results: It was found that twice-weekly applications of the powder to known breeding habitats of Anopheles larvae in 2007 resulted in 49% fewer adult female Anopheles gambiae s.l. mosquitoes in Banizoumbou, compared with previous captures under similar environmental conditions and with similar habitat characteristics in 2005 and 2006. The productivity of the system in 2007 was found to be suppressed compared to the mean behaviour of 2005 and 2006 in Banizoumbou, whereas no change was found in Zindarou. Conclusion: With a high abundance of neem plants in many villages in this area, the results of this study suggest that larval control using neem seed powder offers a sustainable additional tool for malaria vector control in the Sahel region of Niger.

PB - BIOMED CENTRAL LTD CY - CURRENT SCIENCE GROUP, MIDDLESEX HOUSE, 34-42 CLEVELAND ST, LONDON W1T 4LB, ENGLAND VL - 7 UR - http://www.malariajournal.com/content/7/1/138 ER - TY - JOUR T1 - The Hydroclimatology of Kuwait: Explaining the Variability of Rainfall at Seasonal and Interannual Time Scales JF - JOURNAL OF HYDROMETEOROLOGY Y1 - 2008 A1 - Marcella, Marc P. A1 - Eltahir, Elfatih A B KW - Modeling summertime temperatures over desert regions AB - This paper presents an analysis of the spatial, seasonal, and interannual variabilities of Kuwaiti rainfall. Based on an analysis of rain gauge, as well as satellite, datasets, it is estimated that about 110-190 mm of rainfall occurs annually in Kuwait, depending on the dataset sampled. The corresponding estimates for the standard deviations of the annual rainfall are about 40-70 mm. Discrepancies between values arise from the different techniques used in constructing each dataset. Moreover, the spatial distribution of annual rainfall features a gradual increase from the southwest to the northeast. A distinct rainy season occurs from November to April, with double peaks in January and March. In addition, the seasonal variability of rainfall is associated with shifts in patterns of midlatitude storm tracks, which propagate southward toward the Middle East during the winter and spring season. These trends are characterized using estimates of the spatial correlations of rainfall in Kuwait with the surrounding region. At the interannual time scale, significant correlation is found between the tropical El Nino-Southern Oscillation (ENSO) and annual rainfall anomalies. Similar weak correlations are found between midlatitude rainfall in Europe and rainfall in Kuwait. The weak connections observed with both tropical and midlatitude atmospheric systems are consistent with the fact that Kuwait is located in the transitional zone between the tropics and midlatitudes. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 9 UR - sites/default/files/pubs/2008 Marcella Eltahir.pdf ER - TY - JOUR T1 - Hydrology of malaria: Model development and application to a Sahelian village JF - WATER RESOURCES RESEARCH Y1 - 2008 A1 - Bomblies, Arne A1 - Duchemin, Jean-Bernard A1 - Eltahir, Elfatih A B KW - Monsoons, Mosquitoes, and Malaria in Africa KW - Niger Project AB - We present a coupled hydrology and entomology model for the mechanistic simulation of local-scale response of malaria transmission to hydrological and climatological determinants in semiarid, desert fringe environments. The model is applied to the Sahel village of Banizoumbou, Niger, to predict interannual variability in malaria vector mosquito populations that lead to variations in malaria transmission. Using a highresolution, small-scale distributed hydrology model that incorporates remotely sensed data for land cover and topography, we simulate the formation and persistence of the pools constituting the primary breeding habitat of Anopheles gambiae s.l. mosquitoes, the principal regional malaria vector mosquitoes. An agent-based mosquito population model is coupled to the distributed hydrology model, with aquatic-stage and adult-stage components. Through a dependence of aquatic-stage mosquito development and adult emergence on pool persistence, we model small-scale hydrology as a dominant control of mosquito abundance. For each individual adult mosquito, the model tracks attributes relevant to population dynamics and malaria transmission, which are updated as mosquitoes interact with their environment, humans, and animals. Weekly field observations were made in 2005 and 2006. A 16% increase in rainfall between the two years was accompanied by a 132% increase in mosquito abundance between 2005 and 2006. The model reproduces mosquito population variability at seasonal and interannual timescales and highlights individual pool persistence as a dominant control. Future developments of the presented model can be used in the evaluation of impacts of climate change on malaria, as well as the a priori evaluation of environmental management-based interventions. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 44 UR - sites/default/files/pubs/2008 Bomblies Duchemin Eltahir.pdf ER - TY - JOUR T1 - Hypsometric Control on Surface and Subsurface Runoff JF - WATER RESOURCES RESEARCH Y1 - 2008 A1 - Vivoni, Enrique R. A1 - Di Benedetto, Francesco A1 - Grimaldi, Salvatore A1 - Eltahir, Elfatih A B AB - A fundamental problem in hydrology is relating the basin hydrological response to the geomorphologic properties of a catchment. In this technical note, we show that the hypsometric distribution exerts control on surface and subsurface runoff partitioning by isolating its effect with respect to other basin characteristics. We conduct simulations using a distributed watershed model for hypsometric realizations developed by modifying the contour line values of a real basin. Results indicate that the runoff components are a function of the basin hypsometric form. In general, a relatively less eroded (convex) basin exhibits higher total runoff that is more dominated by subsurface processes, while a relatively more eroded (concave) basin shows less total runoff with a higher fraction of surface response. Hypsometric differences are also observed in the relations between base flow discharge and the mean groundwater depth and the variable source area. Furthermore, the hypsometric form reveals clear signatures on the spatial distribution of soil moisture and runoff response mechanisms. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 44 UR - sites/default/files/pubs/2008_Vivoni.pdf ER - TY - JOUR T1 - Modeling the hydroclimatology of Kuwait: The role of subcloud evaporation in semiarid climates JF - JOURNAL OF CLIMATE Y1 - 2008 A1 - Marcella, Marc P. A1 - Eltahir, Elfatih A B KW - Modeling summertime temperatures over desert regions AB - A new subcloud layer evaporation scheme is incorporated into Regional Climate Model, version 3 (RegCM3), to better simulate the rainfall distribution over a semiarid region around Kuwait. The new scheme represents subcloud layer evaporation of convective as well as large-scale rainfall. Model results are compared to observations from rain gauge data networks and satellites. The simulations show significant response to the incorporation of subcloud layer evaporation as a reduction by as much as 20% in annual rainfall occurs over the region. As a result, the new model simulations of annual rainfall are within 15% of observations. In addition, results indicate that the interannual variability of rainfall simulated by RegCM3 is sensitive to the specification of boundary conditions. For example, forcing RegCM3's lateral boundary conditions with the 40-yr ECMWF Re-Analysis (ERA-40) data, instead of NCEP-NCAR's Reanalysis Project 2 (NNRP2), reduces interannual variability by over 25%. Moreover, with subcloud layer evaporation incorporated and ERA-40 boundary conditions implemented, the model's bias and root-mean-square error are significantly reduced. Therefore, the model's ability to reproduce observed annual rainfall and the year-to-year variation of rainfall is greatly improved. Thus, these results elucidate the critical role of this natural process in simulating the hydroclimatology of semiarid climates. Last, a large discrepancy between observation datasets over the region is observed. It is believed that the inherent characteristics that are used to construct these datasets explain the differences observed in the annual and interannual variability of Kuwait's rainfall. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 21 UR - sites/default/files/pubs/2008 Marcella, Eltahir_2.pdf ER - TY - JOUR T1 - Using a Horizontal Precipitation Model to Investigate the Role of Turbulent Cloud Deposition in Survival of a seasonal Cloud Forest in Dhofar JF - JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES Y1 - 2008 A1 - Hildebrandt, Anke A1 - Eltahir, Elfatih A B AB - This paper investigates a feedback between vegetation height and cloud interception in a seasonal semiarid cloud forest in Dhofar, Oman. In this forest, cloud interception by tree canopies (horizontal precipitation) constitutes a substantial fraction of available water. Owing to cattle browsing on tree canopies the forest is gradually degrading to grassland. We investigated if tree removal could reduce cloud interception to the extent that natural reestablishment of trees is inhibited. For this, we included a model describing turbulent cloud deposition as a module into a dynamic vegetation model. The model allows for estimation of cloud deposition based on cloud properties and dynamically changing vegetation structure. Cloud properties were estimated, using an inverse solution of the cloud deposition model, based on measured precipitation and meteorological data. When applying the model to the Dhofar region, we found that equilibrium vegetation depended on the initial vegetation condition. For most of the range of assumed cloud properties, equilibrium vegetation tended toward grassland, when the initial condition was grassland, and to forest, when the model was initialized with forest. However, the difference between the equilibrium vegetation condition emerging from different initial vegetation types depended on the assumed cloud properties. According to these modeling results, land degradation in this semiarid cloud forest might lead to irreversible destruction of the forest biome. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 113 UR - sites/default/files/pubs/2008 Hildebrandt Eltahir.pdf ER - TY - JOUR T1 - Ecohydrology of a seasonal cloud forest in Dhofar: 1. Field experiment JF - WATER RESOURCES RESEARCH Y1 - 2007 A1 - Hildebrandt, Anke A1 - Al Aufi, Mohammed A1 - Amerjeed, Mansoor A1 - Shammas, Mahaad A1 - Eltahir, Elfatih A B AB - {[}1] We describe the ecohydrology of a unique semiarid broadleaf deciduous forest in Dhofar ( Oman). The forest is surrounded by desert and is confined to a coastal area, where the summer wet season is characterized by a persistent dense cloud immersion. Using field observations, we show how clouds render the ecosystem particularly water conserving and therefore create a niche for a moist forest biome in a semiarid area in three ways. First, horizontal precipitation ( collection of cloud droplets on tree canopies) added valuable water, such that about two times as much water was received below the canopy ( net precipitation) compared to above ( rainfall). Second, high stemflow, of about 30% to net precipitation, led to concentrated water input around the stems. Third, transpiration was suppressed during the cloudy summer season, which allowed for storage of the received water. It was only used after the end of the wet season and lasted for the following 3 months, which roughly doubled the length of the growing season. Our results demonstrate that cloud immersion may shape ecosystem hydrology in significant ways, particularly in semiarid environments. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 43 UR - sites/default/files/pubs/2007 Hildebrandt_1 .pdf ER - TY - JOUR T1 - Ecohydrology of a seasonal cloud forest in Dhofar: 2. Role of clouds, soil type, and rooting depth in tree-grass competition JF - WATER RESOURCES RESEARCH Y1 - 2007 A1 - Hildebrandt, Anke A1 - Eltahir, Elfatih A B AB - Using a dynamic ecosystem model, we investigate the role of summer cloud immersion in the ecohydrology of a seasonal deciduous forest in Oman. This is a semiarid region where vegetation is immersed in dense cloud during a 3-month-long monsoon season. The simulated vegetation at equilibrium depends strongly on cloud cover during the wet season, with trees predicted under cloudy conditions and grasses when assuming a cloud-free monsoon. By varying soil type and rooting depth, we identify a rooting depth at which tree performance is optimal. This is the depth at which transpiration is maximized and the sum of all other fluxes from the soil is minimized. Our analysis shows that cloud cover creates a favorable seasonality in this ecosystem that is crucial for maintaining trees. This is achieved by (1) prolonging the growing season from 3 months to 6 months and (2) allowing deeper infiltration, which assures competitiveness of trees in an otherwise too dry environment. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 43 UR - sites/default/files/pubs/2007 Hildebrandt Eltahir_2.pdf ER - TY - JOUR T1 - Forest on the Edge: Seasonal Cloud Forest in Oman Creates Its Own Ecological Niche JF - GEOPHYSICAL RESEARCH LETTERS Y1 - 2006 A1 - Hildebrandt, Anke A1 - Eltahir, Elfatih A B AB - Cloud forests usually grow in the moist tropics where water is not a limiting factor to plant growth. Here, for the first time, we describe the hydrology of a water limited seasonal cloud forest in the Dhofar mountains of Oman. This ecosystem is under significant stress from camels feeding on tree canopies. The Dhofar forests are the remnants of a moist vegetation belt, which once spread across the Arabian Peninsula. According to our investigation the process of cloud immersion during the summer season creates within this desert a niche for moist woodland vegetation. Woodland vegetation survives in this ecosystem, sustained through enhanced capture of cloud water by their canopies ( horizontal precipitation). Degraded land lacks this additional water source, which inhibits re-establishment of trees. Our modeling results suggest that cattle feeding may lead to irreversible destruction of one of the most diverse ecosystems in Arabia. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 33 UR - sites/default/files/pubs/2006 Hildebrandt Eltahir.pdf ER - TY - JOUR T1 - Representation of Water Table Dynamics in a Land Surface Scheme. Part I: Model Development JF - JOURNAL OF CLIMATE Y1 - 2005 A1 - Yeh, P. J. F. A1 - Eltahir, Elfatih A B AB -

Most of the current land surface parameterization schemes lack any representation of regional ground-water aquifers. Such a simplified representation of subsurface hydrological processes would result in significant errors in the predicted land surface states and fluxes especially for the shallow water table areas in humid regions. This study attempts to address this deficiency. To incorporate the water table dynamics into a land surface scheme, a lumped unconfined aquifer model is developed to represent the regional unconfined aquifer as a nonlinear reservoir, in which the aquifer simultaneously receives the recharge from the overlying soils and discharges runoff into streams. The aquifer model is linked to the soil model in the land surface scheme {[}Land Surface Transfer Scheme (LSX)] through the soil drainage flux. The total thickness of the unsaturated zone varies in response to the water table fluctuations, thereby interactively coupling the aquifer model with the soil model. The coupled model (called LSXGW) has been tested in Illinois for an 11-yr period from 1984 to 1994. The results show reasonable agreements with the observations. However, there are still secondary biases in the LSXGW simulation partially resulting from not accounting for the spatial variability of water table depth. The issue of subgrid variability of water table depth will be addressed in a companion paper.

PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 18 ER - TY - JOUR T1 - Representation of Water Table Dynamics in a Land Surface Scheme. Part II: Subgrid Variability JF - JOURNAL OF CLIMATE Y1 - 2005 A1 - Yeh, P. J. F. A1 - Eltahir, Elfatih A B AB - A lumped unconfined aquifer model has been developed and interactively coupled to a land surface scheme in a companion paper. Here, the issue of the representation of subgrid variability of water table depths (WTDs) is addressed. A statistical-dynamical (SD) approach is used to account for the effects of the unresolved subgrid variability of WTD in the grid-scale groundwater runoff. The dynamic probability distribution function (PDF) of WTD is specified as a two-parameter gamma distribution based on observations. The grid-scale groundwater rating curve (i.e., aquifer storage-discharge relationship) is derived statistically by integrating a point groundwater runoff model with respect to the PDF of WTD. Next, a mosaic approach is utilized to account for the effects of subgrid variability of WTD in the grid-scale groundwater recharge. A grid cell is categorized into different subgrids based on the PDF of WTD. The grid-scale hydrologic fluxes are computed by averaging all of the subgrid fluxes weighted by their fractions. This new methodology combines the strengths of the SD approach and the mosaic approach. The results of model testing in Illinois from 1984 to 1994 indicate that the simulated hydrologic variables (soil saturation and WTD) and fluxes (evaporation, runoff, and groundwater recharge) agree well with the observations. Because of the paucity of the large-scale observations on WTD, the development of a practical parameter estimation procedure is indispensable before the global implementation of the developed scheme of water table dynamics in climate models. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 18 UR - sites/default/files/pubs/2005 Yeh Eltahir.pdf ER - TY - JOUR T1 - Sensitivity of Regional Hydrology to Climate Changes, with Application to the Illinois River Basin JF - WATER RESOURCES RESEARCH Y1 - 2005 A1 - Niemann, J. D. A1 - Eltahir, Elfatih A B AB - This paper investigates the sensitivity of regional hydrology to climate change using a physically based model. The model partitions precipitation into surface runoff, groundwater runoff, and evapotranspiration by describing these fluxes first at the local instantaneous scale and then integrating over spatial and temporal distributions of soil saturation, precipitation, and wet environment evapotranspiration to calculate basin-wide climatic mean fluxes and soil saturation. The sensitivities of the mean fluxes are calculated by changing the mean precipitation and wet environment evapotranspiration. The model is applied to the Illinois River basin, and the impact of the basin's characteristics on the sensitivities is studied. For a relatively broad range of conditions the runoff processes tend to amplify climate change signals in precipitation and wet environment evapotranspiration, while evapotranspiration processes tend to dampen the same signals. These results indicate that it may be easier to detect climate changes in runoff measurements than in precipitation measurements. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 41 UR - sites/default/files/pubs/2005 Niemann Eltahir .pdf ER - TY - JOUR T1 - Decadal Variability of Rainfall in the Sahel: Results from the Coupled GENESIS-IBIS Atmosphere-biosphere Model JF - CLIMATE DYNAMICS Y1 - 2004 A1 - Wang, G. L. A1 - Eltahir, Elfatih A B A1 - Foley, J. A. A1 - Pollard, D. A1 - Levis, S. KW - west africa AB - In this study we investigate the impact of large-scale oceanic forcing and local vegetation feedback on the variability of the Sahel rainfall using a global biosphere-atmosphere model, the coupled GENESIS-IBIS model, running at two different resolutions. The observed global sea surface temperature in the twentieth century is used as the primary model forcing. Using this coupled global model.. we experiment on treating vegetation as a static boundary condition and as a dynamic component of the Earth climate system. When vegetation is dynamic, the R30-resolution model realistically reproduces the multi-decadal scale fluctuation of rainfall in the Sahel region; keeping vegetation static in the same model results in a rainfall regime characterized by fluctuations at much shorter time scales, indicating that vegetation dynamics act as a mechanism for persistence of the regional climate. Even when vegetation dynamics is included, the R15 model fails to capture the main characteristics of the long-term rainfall variability due to the exaggerated atmospheric internal variability in the coarse resolution model. Regardless how vegetation is treated and what model resolution is used, conditions in the last three decades of the twentieth century are always drier than normal in the Sahel, suggesting that global oceanic forcing during that period favors the occurrence of a drought. Vegetation dynamics is found to enhance the severity of this drought. However, with both the observed global SST forcing and feedback from dynamic vegetation in the model, the simulated drought is still not as persistent as that observed. This indicates that anthropogenic land cover changes, a mechanism missing in the model, may have contributed to the occurrence of the twentieth century drought in the Sahel. PB - SPRINGER CY - 233 SPRING STREET, NEW YORK, NY 10013 USA VL - 22 UR - sites/default/files/pubs/2004 Wang Eltahir.pdf ER - TY - JOUR T1 - Prediction of Regional Water Balance Components Based on Climate, Soil, and Vegetation Parameters, with Application to the Illinois River Basin JF - WATER RESOURCES RESEARCH Y1 - 2004 A1 - Niemann, J. D. A1 - Eltahir, Elfatih A B KW - illinois river KW - probabilistic methods KW - soil moisture KW - water balance AB - This paper presents a framework for studying regional water balance in which the physical processes are first described at the local instantaneous scale and then integrated to the annual, basin-wide scale. The integration treats the relative soil saturation (i.e., the soil moisture divided by the porosity) and precipitation intensities as stochastic variables in space and time. A statistical equilibrium characterizes the annual water balance, resulting in a specific relation that predicts the space-time average of soil saturation in terms of soil, climate, and vegetation parameters. Specific relationships are proposed to relate the space-time average soil saturation to runoff, groundwater recharge, and evapotranspiration. This framework is applied to the Illinois River Basin. The shape of the spatial and temporal distributions of soil saturation are determined from observations. The other parameters are determined from the physical characteristics of the basin and calibration procedures. The resulting model is able to reproduce an observed relation between the space-time average soil saturation and precipitation. It is also able to reproduce observed relations between space-time average soil saturation and space-time average evapotranspiration, surface runoff, and groundwater runoff. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 40 UR - sites/default/files/pubs/2004 Niemann Eltahir.pdf ER - TY - JOUR T1 - Role of topography in facilitating coexistence of trees and grasses within savannas JF - WATER RESOURCES RESEARCH Y1 - 2004 A1 - Kim, Y. A1 - Eltahir, Elfatih A B KW - coexistence KW - savannas KW - topography KW - west africa AB - The factors and processes that may explain the observed coexistence of trees and grasses in savannas are not well understood. Here we propose a new hypothesis that addresses this issue. We hypothesize that ``variations in elevation at relatively short horizontal scales of similar to1 km force similar variations in soil moisture and thus create significantly different hydrologic niches within any large area. Under water-limited conditions the relatively wet valleys favor trees, while the relatively dry hills favor grasses. This coexistence of trees and grasses is only possible for a window of climatic conditions that are characteristic of savannas.'' To test this hypothesis, numerical simulations are performed for the region of West Africa using a model that simulates vegetation dynamics, the Integrated Biosphere Simulator ( IBIS), and a distributed hydrologic model, Systeme Hydrologique Europeen ( SHE). IBIS is modified to include the groundwater table (GWT) as a lower boundary. The spatial distribution of GWT is simulated by SHE. At 9degreesN the model simulates trees even when the GWT is assumed to be infinitely deep; at 13degreesN the model simulates grasses even when the capillary fringe of the GWT reaches the surface. However, for the transitional climate, at 11degreesN, trees are simulated when the GWT is at similar to2.5 m from the surface, but grasses are simulated when the GWT is deeper than 2.5 m. These results suggest that the variability of soil moisture forced by topography can be a determinant factor of vegetation distribution within savannas. Furthermore, they confirm that this role of topography can be significant only in a certain climatic window characteristic of savannas. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 40 UR - sites/default/files/pubs/2004 Kim Eltahir.pdf ER - TY - JOUR T1 - A See-saw Oscillation Between the Amazon and Congo Basins JF - GEOPHYSICAL RESEARCH LETTERS Y1 - 2004 A1 - Eltahir, Elfatih A B A1 - Loux, B. A1 - Yamana, Teresa K. A1 - Bomblies, Arne KW - Amazon PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 31 UR - sites/default/files/pubs/2004 Eltahir Loux Yamana Bomblies.pdf ER - TY - JOUR T1 - Atmospheric Controls on Soil Moisture-Boundary Layer Interactions. Part I: Framework Development JF - JOURNAL OF HYDROMETEOROLOGY Y1 - 2003 A1 - Findell, K. L. A1 - Eltahir, Elfatih A B AB - This paper investigates the influence of soil moisture on the development and triggering of convection in different early-morning atmospheric conditions. A one-dimensional model of the atmospheric boundary layer (BL) is initialized with atmospheric sounding data from Illinois and with the soil moisture set to either extremely wet ( saturated) or extremely dry (20% of saturation) conditions. Two measures are developed to assess the low-level temperature and humidity structure of the early-morning atmosphere. These two measures are used to distinguish between four types of soundings, based on the likely outcome of the model: 1) those soundings favoring deep convection over dry soils, 2) those favoring deep convection over wet soils, 3) those unlikely to convect over any land surface, and 4) those likely to convect over any land surface. Examples of the first two cases are presented in detail. The early-morning atmosphere is characterized in this work by the newly developed convective triggering potential (CTP) and a low-level humidity index, HIlow. The CTP measures the departure from a moist adiabatic temperature lapse rate in the region between 100 and 300 mb ( about 1 - 3 km) above the ground surface (AGS). This region is the critical interface between the near-surface region, which is almost always incorporated into the growing BL, and free atmospheric air, which is almost never incorporated into the BL. Together, these two measures form the CTP-HIlow framework for analyzing atmospheric controls on soil moisture - boundary layer interactions. Results show that in Illinois deep convection is trigged in the model 22% of the time over wet soils and only 13% of the time over dry soils. Additional testing varying the radiative conditions in Illinois and also using the 1D model with soundings from four additional stations confirm that the CTP-HIlow framework is valid for regions far removed from Illinois. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 4 UR - sites/default/files/pubs/2003Findell-Eltahir.pdf ER - TY - JOUR T1 - Atmospheric Controls on Soil Moisture-Boundary Layer Interactions. Part II: Feedbacks Within the Continental United States JF - JOURNAL OF HYDROMETEOROLOGY Y1 - 2003 A1 - Findell, K. L. A1 - Eltahir, Elfatih A B AB - The CTP-HIlow framework for describing atmospheric controls on soil moisture - boundary layer interactions is described in a companion paper, Part I. In this paper, the framework is applied to the continental United States to investigate how differing atmospheric regimes influence local feedbacks between the land surface and the atmosphere. The framework was developed with a one-dimensional boundary layer model and is based on two measures of atmospheric thermodynamic properties: the convective triggering potential (CTP), a measure of the temperature lapse rate between approximately 1 and 3 km above the ground surface, and a low-level humidity index, HIlow. These two measures are used to distinguish between three types of early-morning atmospheric conditions: those favoring moist convection over dry soils, those favoring moist convection over wet soils, and those that will allow or prevent deep convective activity, independent of the surface flux partitioning. Analyses of multiyear CTP-HIlow scatterplots from radiosonde stations across the contiguous 48 United States reveal that during the summer months ( June, July, and August) positive feedbacks between soil moisture and moist convection are likely in much of the eastern half of the country. Over the western half of the country, atmospheric conditions and the likelihood of moist convection are largely determined by oceanic influences, and land surface conditions in the summer are unlikely to impact convective triggering. The only area showing a potential negative feedback is in the dryline and monsoon region of the arid Southwest. This potential arises because of the topography of this and surrounding regions. A relatively narrow band of stations lies in between the eastern and western portions of the country, in some years behaving like the stations to the west and in other years behaving like the stations to the east. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 4 UR - sites/default/files/pubs/2003 Findell-Eltahir.pdf ER - TY - JOUR T1 - Atmospheric Controls on Soil Moisture-boundary Layer Interactions: Three-dimensional Wind Effects JF - JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Y1 - 2003 A1 - Findell, K. L. A1 - Eltahir, Elfatih A B KW - boundary layer processes KW - convective processes KW - hydroclimatology KW - land/atmosphere interactions KW - precipitation KW - soil moisture AB - {[}1] This paper expands the one-dimensionally based CTP-HIlow framework for describing atmospheric controls on soil moisture-boundary layer interactions {[}Findell and Eltahir, 2003] to three dimensions by including low-level wind effects in the analysis. The framework is based on two measures of atmospheric thermodynamic properties: the convective triggering potential (CTP), a measure of the temperature lapse rate between approximately 1 and 3 km above the ground surface, and a low-level humidity index, HIlow. These two measures are used to distinguish between three types of early morning soundings: those favoring rainfall over dry soils, those favoring rainfall over wet soils, and those whose convective potential is unaffected by the partitioning of fluxes at the surface. The focus of this paper is the additional information gained by incorporating information about low-level winds into the CTP-HIlow framework. Three-dimensional simulations using MM5 and an analysis of observations from the FIFE experiment within this framework highlight the importance of the winds in determining the sensitivity of convection to fluxes from the land surface. A very important impact of the 3D winds is the potential for low-level backing or unidirectional winds with great shear to suppress convective potential. Because of this suppression of convection in certain wind conditions, far fewer simulations produced rain than would be anticipated based solely on the 1D framework of understanding. However, when the winds allowed, convection occurred in a manner consistent with the 1D-based expectations. Generally speaking, in the regime where dry soils were expected to have an advantage, convection was triggered over dry soils more often than over wet; in the regime where wet soils were expected to have an advantage, convection was more frequently triggered over wet soils than over dry. Additionally, when rainfall occurred in both simulations with wet soils and simulations with dry soils for a given day, rainfall depths were typically greater in the simulations with wet soils. Similarly, the FIFE data showed numerous days with convective potential but no rainfall: each of these days had low-level backing or strongly shearing winds. Four days with high humidity deficits and veering winds in the lowest 300 mbar did have rain, highlighting the enhanced buoyancy effects of low-level veering winds. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 108 UR - sites/default/files/pubs/2003 Findell Eltahir .pdf ER - TY - JOUR T1 - A Feedback Mechanism Between Soil-moisture Distribution and Storm Tracks JF - QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Y1 - 2003 A1 - Pal, J. S. A1 - Eltahir, Elfatih A B KW - hydro-climatology KW - precipitation KW - water resources AB - How the distribution of soil moisture impacts the location and persistence of the storm track and the resulting distribution of precipitation is investigated. Particular attention is given to North America where an extensive series of numerical experiments using a regional climate model are performed to investigate these issues. The findings suggest that soil-moisture distribution has a pronounced impact on the large-scale dynamics via a meridional displacement of the storm track. The displacement tends to enhance the local soil moisture-rainfall feedback over the region of the anomaly. The initial feedback is induced by the local effects of soil moisture on the boundary-layer energy budget and then propagated to the large-scale through modifications to the geopotential heights. Because of soil moisture's impact on the large-scale dynamics, a local soil-moisture anomaly can induce both flood- and drought-like conditions in surrounding regions. Experiments with a dry anomaly applied either over the upper Midwest or Great Plains display a northward shift in the storm track. This shift tends to result in drought-like conditions in the region of the anomaly and both flood- and drought-like conditions in surrounding regions. Anomalous wetting in the Southwest impacts the distribution of precipitation not only locally, but also over most of the United States; drought-like conditions are simulated to the north and east of the anomaly region while flood-like conditions are simulated locally and eastward. Overall, the impacts of soil-moisture distribution on the large-scale dynamics and the location and intensity of the storm track play a significant role in determining summer rainfall distribution. Soil-moisture anomalies over relatively small regions can induce floods and droughts not only locally, but also over remote regions. PB - ROYAL METEOROLOGICAL SOC CY - 104 OXFORD ROAD, READING RG1 7LJ, BERKS, ENGLAND VL - 129 UR - sites/default/files/pubs/2003 Pal-Eltahir.pdf ER - TY - JOUR T1 - Role of the Biosphere in the Mid-Holocene Climate of West Africa JF - JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Y1 - 2003 A1 - Irizarry, M. A1 - Wang, G. L. A1 - Eltahir, Elfatih A B KW - Climate Change KW - holocene KW - insolation KW - monsoon KW - vegetation dynamics KW - west africa AB - {[}1] In previous studies, a zonally symmetric, synchronously coupled biosphere-atmosphere model ( ZonalBAM), which includes explicit representation of ecosystem dynamics, has been developed and validated based on current conditions over the region of West Africa. Here, we use ZonalBAM to study the response of the coupled biosphere-atmosphere system to changes in the Earth's orbital forcing during the Middle Holocene (6K yrs BP) and the relative contribution of vegetation feedbacks. Simulations in which vegetation conditions were fixed to the current distribution, show that an orbitally induced increased seasonality in insolation for the Middle Holocene, by itself, results in a 1.1degrees northward shift in the location of the southern margin of the Sahara as compared to current solar forcings. When vegetation is allowed to be dynamic, a 2.4degrees northward shift is simulated. However, when dynamic vegetation is initialized to palaeovegetation, a 5.1degrees northward shift is simulated, bringing results more consistent with palaeoevidence. Based on previous studies on the role of the gradient of moist static energy on the dynamics of large-scale tropical circulations, a mechanism for the enhancement of the summer monsoon circulation has been developed. Our results suggest that multiple equilibria could have coexisted over the region of West Africa during the Middle Holocene. Furthermore, based on previous studies on the current climate over the region, we hypothesize that transitions between the different equilibria could have taken place during the Middle Holocene causing the southern desert margin to migrate between 18.1degreesN and 21.4degreesN and shaping climate variability. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 108 UR - sites/default/files/pubs/2003 Irizarry Wang Eltahir.pdf ER - TY - JOUR T1 - Impact of CO2 Concentration Changes on the Biosphere-Atmosphere System of West Africa JF - GLOBAL CHANGE BIOLOGY Y1 - 2002 A1 - Wang, G. L. A1 - Eltahir, Elfatih A B ER - TY - JOUR T1 - Summer dryness in a warmer climate: a process study with a regional climate model JF - CLIMATE DYNAMICS Y1 - 2002 A1 - Seneviratne, S. I. A1 - Pal, J. S. A1 - Eltahir, Elfatih A B A1 - Schar, C. AB - Earlier GCM studies have expressed the concern that an enhancement of greenhouse warming might increase the occurrence of summer droughts in mid-latitudes, especially in southern Europe and central North America. This could represent a severe threat for agriculture in the regions concerned, where summer is the main growing season. These predictions must however be considered as uncertain, since most studies featuring enhanced summer dryness in mid-latitudes use very simple representations of the land-surface processes ({''}bucket{''} models), despite their key importance for the issue considered. The current study uses a regional climate model including a land-surface scheme of intermediate complexity to investigate the sensitivity of the summer climate to enhanced greenhouse warming over the American Midwest. A surrogate climate change scenario is used for the simulation of a warmer climate. The control runs are driven at the lateral boundaries and the sea surface by reanalysis data and observations, respectively. The warmer climate experiments are forced by a modified set of initial and lateral boundary conditions. The modifications consist of a uniform 3 K temperature increase and an attendant increase of specific humidity (unchanged relative humidity). This strategy maintains a similar dynamical forcing in the warmer climate experiments, thus allowing to investigate thermodynamical impacts of climate change in comparative isolation. The atmospheric CO2 concentration of the sensitivity experiments is set to four times its pre-industrial value. The simulations are conducted from March 15 to October 1st, for 4 years corresponding to drought (1988), normal (1986, 1990) and flood (1993) conditions. The numerical experiments do not present any great enhancement of summer drying under warmer climatic conditions. First, the overall changes in the hydrological cycle (especially evapotranspiration) are of small magnitude despite the strong forcing applied. Second, precipitation increases in spring lead to higher soil water recharge during this season, compensating for the enhanced soil moisture depletion occurring later in the year. Additional simulations replacing the plant control on transpiration with a bucket-type formulation presented increased soil drying in 1988, the drought year. This suggests that vegetation control on transpiration might play an important part in counteracting an enhancement of summer drying when soil water gets limited. Though further aspects of this issue would need investigating, our results underline the importance of land-surface processes in climate integrations and suggest that the risk of enhanced summer dryness in the region studied might be less acute than previously assumed, provided the North American general circulation does not change markedly with global warming. PB - SPRINGER-VERLAG CY - 175 FIFTH AVE, NEW YORK, NY 10010 USA VL - 20 UR - sites/default/files/pubs/2002 Seneviratne Eltahir Schar.pdf ER - TY - JOUR T1 - Teleconnections of soil moisture and rainfall during the 1993 midwest summer flood JF - GEOPHYSICAL RESEARCH LETTERS Y1 - 2002 A1 - Pal, J. S. A1 - Eltahir, Elfatih A B AB - {[}1] Here, we investigate the role of spring soil moisture distribution during the 1993 summer flood in North America. A series of idealized numerical experiments are performed using a regional climate model with different soil moisture conditions. It is concluded that the abnormally dry conditions in the Southwest may be partially responsible for the meridional location of the intense flooding in the Upper Midwest. Furthermore, the abnormally wet soil moisture conditions in the Midwest are likely to be partially responsible for the flood's persistence and large magnitude, but not for its spatial location. These impacts are initiated via processes involving the local energy and water budgets and then propagated to the surrounding regions through the large scale dynamics. Depending on the location, soil moisture anomalies over relatively small regions can significantly alter rainfall both locally and in surrounding regions. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 29 UR - sites/default/files/pubs/2002 Pal Eltahir.pdf ER - TY - JOUR T1 - Geomorphic Controls on Regional Base Flow JF - Water Resources Research Y1 - 2001 A1 - Marco Marani A1 - Eltahir, Elfatih A B A1 - Andrea Rinaldo KW - BALANCE KW - CHANNEL NETWORKS KW - Environmental Sciences KW - EROSION THRESHOLDS KW - ILLINOIS KW - INSTANTANEOUS UNIT-HYDROGRAPH; GEOMORPHOLOGICAL DISPERSION KW - Limnology KW - MODEL KW - MORPHOLOGY KW - TERRAIN KW - water resources AB -

The subsurface hydrological response plays an important role in the hydrology of humid regions. In particular, the physical relationship between base flow dynamics and the fluctuations in spatially averaged water table depth, as described by the groundwater rating curve, determine to a significant extent the nature of statistical persistence of hydrological anomalies in the unconfined aquifers level and river flow. In this paper, we propose that the scale and shape of the groundwater rating curve reflect some of the geomorphological characteristics of the region such as relief, drainage density, and the hypsometric distribution of the elevation field. These connections between geomorphology and hydrology of river basins are investigated using a simple model of unconfined groundwater flow applied to synthetic basins as well as observed basins from Illinois.

VL - 37 IS - 10 ER - TY - JOUR T1 - Pathways relating soil moisture conditions to future summer rainfall within a model of the land-atmosphere system JF - JOURNAL OF CLIMATE Y1 - 2001 A1 - Pal, J. S. A1 - Eltahir, Elfatih A B AB - In this paper, the key pathways and mechanisms through which soil moisture conditions affect future rainfall over the U.S. Midwest are investigated using a regional climate model. A series of numerical experiments are performed to identify these pathways using the drought of 1988 and flood of 1993 as representative events. The results suggest that the soil moisture-rainfall feedback is an important mechanism for hydrologic persistence during the late spring and summer over the midwestern United States. They indicate that the feedback between soil moisture and subsequent rainfall played a significant role in enhancing the persistence of the drought of 1988 and the flood of 1993. It is found that there is a pronounced asymmetry in the sensitivity of simulated rainfall to specified initial soil moisture. The asymmetry acts to favor a stronger soil moisture-rainfall feedback during drought conditions as opposed to flood conditions. Detailed analyses of the simulations indicate that the impact of soil moisture on both the energy and water budgets is crucial in determining the strength of the soil moisture-rainfall feedback. Anomalously high soil moisture tends to 1) increase the flux of high moist static energy air into the planetary boundary layer from the surface via an increase in net surface radiation, 2) reduce the planetary boundary layer height thus increasing the moist static energy per unit mass of air, and 3) reduce the amount of entrained air of low moist static energy from above the planetary boundary layer. Each of these effects are additive and combine to increase the moist static energy per unit mass of air in the planetary boundary layer. This increase results in an increase in the frequency and magnitude of convective rainfall events and a positive feedback between soil moisture and subsequent rainfall. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 14 UR - sites/default/files/pubs/2001-Pal-Eltahir.pdf ER - TY - JOUR T1 - Biosphere-atmosphere interactions over West Africa. I: Development and validation of a coupled dynamic model JF - QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Y1 - 2000 A1 - Wang, G. L. A1 - Eltahir, Elfatih A B KW - biosphere-atmosphere interactions KW - drought KW - ecosystem dynamics KW - west africa AB - In this study we develop a zonally symmetric, synchronously coupled biosphere-atmosphere model including ecosystem dynamics, and apply this model to study biosphere-atmosphere interactions in the region of West Africa. The atmospheric model is zonally symmetric, and includes representation of atmospheric dynamics, a radiation scheme, a moist convection scheme, a boundary-layer scheme, and a cloud-parametrization scheme. The biospheric model is the Integrated BIosphere Simulator (IBIS), which includes representation of the water, energy, momentum, and carbon balance, vegetation phenology, and vegetation dynamics. We modified the representation of canopy hydrology in IBIS to account for the impact of rainfall subgrid variability. The biospheric model and atmospheric model are separately tested against observations. The synchronously coupled model is then used to simulate the biosphere-atmosphere: system of West Africa. A study on the role of biosphere-atmosphere interactions, including ecosystem dynamics, in the climate variability over West Africa using this model will be presented in a companion paper. PB - ROYAL METEOROLOGICAL SOC CY - 104 OXFORD ROAD, READING, BERKS, ENGLAND RG1 7LJ VL - 126 UR - sites/default/files/pubs/2000 Wang Eltahir_1.pdf ER - TY - JOUR T1 - Biosphere-atmosphere interactions over West Africa. II: Multiple climate equilibria JF - QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Y1 - 2000 A1 - Wang, G. L. A1 - Eltahir, Elfatih A B KW - biosphere-atmosphere interactions KW - climate transition KW - drought KW - ecosystem dynamics KW - multiple climate equilibria KW - west africa AB - This paper presents both theoretical and numerical analyses on the multiple-equilibrium nature of the regional climate system in West Africa. Based on simple analyses on how the coupled biosphere-atmosphere system responds to vegetation perturbations within the scope of a dynamic ecosystem, we propose that the regional climate system may have multiple equilibrium states coexisting under the same precessional forcing. Using a synchronously coupled biosphere-atmosphere model which includes explicit representation of ecosystem dynamics, we show that the equilibrium state of the model is sensitive to initial vegetation distribution. This modelling result supports the existence of multiple climate equilibria. Using the same model, further experiments are carried out to investigate how the coupled system responds to non-permanent vegetation perturbations. Our results demonstrate how transitions between different climate equilibria can take place when governed by the two-way biosphere-atmosphere feedback. These findings advance our understanding regarding the mechanisms of climate variability over West Africa. PB - ROYAL METEOROLOGICAL SOC CY - 104 OXFORD ROAD, READING, BERKS, ENGLAND RG1 7LJ VL - 126 UR - sites/default/files/pubs/2000 Wang Eltahir bio_2a.pdf ER - TY - JOUR T1 - Ecosystem dynamics and the Sahel drought JF - GEOPHYSICAL RESEARCH LETTERS Y1 - 2000 A1 - Wang, G. L. A1 - Eltahir, Elfatih A B KW - west africa AB - The Sahel region in West Africa has been experiencing a persistent drought throughout the last three decades. Here, we present a new perspective on the underlying physical mechanism behind this phenomenon. We use a coupled biosphere-atmosphere model including explicit representation of ecosystem dynamics to demonstrate that, regardless of the nature of the initial forcing, the natural response of the local grass ecosystem to the dry conditions of the late 1960s played a Critical role in maintaining the drought through the following decades. The onset of the drought has been marked by, a forced shift from a self-sustaining wet climate equilibrium to a similarly self-sustaining but dry climate equilibrium. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 27 UR - sites/default/files/pubs/2000 Wang Eltahir-3.pdf ER - TY - JOUR T1 - The impact of observed deforestation on the mesoscale distribution of rainfall and clouds in Amazonia JF - JOURNAL OF HYDROMETEOROLOGY Y1 - 2000 A1 - Wang, Jingfeng A1 - Bras, R. L. A1 - Eltahir, Elfatih A B KW - Amazon AB - A numerical mesoscale model has been used to investigate the impact of mesoscale circulations on the distribution of precipitation and cloudiness over a deforested area in Amazonia. Observed patterns of deforestation in Rondonia, Amazonia, with scales on the order of 10 km were used in this study to describe land surface conditions. Various simulations have been performed to identify the conditions under which the mesoscale circulations induced by the heterogeneous land surface could enhance cloudiness and local rainfall. The simulation results suggest that the synoptic forcing, in terms of atmospheric stability and background horizontal wind, dominates during the rainy season; synoptic conditions were so favorable to moist convection that the added effect of surface heterogeneity was negligible. During the dry season, a noticeable impact of mesoscale circulations resulting in enhancement of shallow clouds was simulated; the mesoscale circulations also triggered scattered deep convection that altered the spatial distribution of precipitation. During the break period, the transition from the rainy season to the dry season, the impact of mesoscale circulations on low-level clouds was evident only after reducing the magnitude of the background wind. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 1 UR - sites/default/files/pubs/2000 Wang Bras Eltahir.pdf ER - TY - JOUR T1 - Modeling the biosphere-atmosphere system: The impact of the subgrid variability in rainfall interception JF - JOURNAL OF CLIMATE Y1 - 2000 A1 - Wang, G. L. A1 - Eltahir, Elfatih A B KW - west africa AB - Subgrid variability in rainfall distribution has been widely recognized as an important factor to include in the representation of land surface hydrology within climate models. In this paper, using West Africa as a case study, the impact of the subgrid variability in rainfall interception on the modeling of the biosphere-atmosphere system is investigated. According to the authors' results, when neglecting the rainfall spatial variability, even if the impact on the total evapotranspiration is negligible, significant errors may result in the representation of surface hydrological processes and surface energy balance. These findings are consistent with the results of previous studies. However, in this paper, this issue is further explored and it is demonstrated that the extent of the resulting errors is not limited to the land surface processes. They extend to the atmosphere via the low-level cloud feedback to impact solar radiation, boundary layer energy, atmospheric circulation, and the distribution of precipitation. The same errors also propagate into the biosphere through vegetation dynamics and can eventually lead to a significantly different biosphere-atmosphere equilibrium state. This study provides a good example for the need to have physical realism in modeling the subgrid variability and most other details of the complex biosphere-atmosphere-ocean system. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 13 UR - sites/default/files/pubs/2000 Wang Eltahir_2.pdf ER - TY - JOUR T1 - Role of vegetation dynamics in enhancing the low-frequency variability of the Sahel rainfall JF - WATER RESOURCES RESEARCH Y1 - 2000 A1 - Wang, G. L. A1 - Eltahir, Elfatih A B KW - west africa AB - Rainfall observations in the Sahel region of West Africa show significant variability at the timescale of decades. Here we explore the mechanisms of this low-frequency variability using a coupled biosphere-atmosphere model which includes explicit representation of vegetation dynamics. By forcing the model with the observed sea surface temperature (SST) of the tropical Atlantic Ocean during the period 1898-1997, numerical experiments on the climate variability of West Africa have been carried out. The results of these experiments suggest that vegetation dynamics is a significant process in shaping the natural variability of the Sahel rainfall. The response of the regional climate system to large-scale forcings is significantly regulated by vegetation dynamics. The relatively slow response of vegetation to changes in the atmosphere acts to enhance the low-frequency rainfall variability. The regional climate system over West Africa has several climate regimes coexisting under the current precessional forcing. Climate transitions between different regimes act as another mechanism contributing to the low-frequency rainfall variability. Climate persistence at one regime and climate transition toward another collectively compose a distinct type of multidecadal variability. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 36 UR - sites/default/files/pubs/2000 WRR Wang Eltahir.pdf ER - TY - JOUR T1 - Simulation of regional-scale water and energy budgets: Representation of subgrid cloud and precipitation processes within RegCM JF - JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Y1 - 2000 A1 - Pal, J. S. A1 - Small, E. E. A1 - Eltahir, Elfatih A B AB - A new large-scale cloud and precipitation scheme, which accounts for the subgrid-scale variability of clouds, is coupled to NCAR's Regional Climate Model (RegCM). This scheme partitions each grid cell into a cloudy and noncloudy fraction related to the average grid cell relative humidity. Precipitation occurs, according to a specified autoconversion rate, when a cloud water threshold is exceeded. The specification of this threshold is based on empirical in-cloud observations of cloud liquid water amounts. Included in the scheme are simple formulations for raindrop accretion and evaporation. The results from RegCM using the new scheme, tested over North America, show significant improvements when compared to the old version. The outgoing longwave radiation, albedo, cloud water path, incident surface shortwave radiation, net surface radiation, and surface temperature fields display reasonable agreement with the observations from satellite and surface station data. Furthermore, the new model is able to better represent extreme precipitation events such as the Midwest flooding observed in the summer of 1993. Overall, RegCM with the new scheme provides for a more accurate representation of atmospheric and surface energy and water balances, including both the mean conditions and the variability at daily to interannual scales. The latter suggests that the new scheme improves the model's sensitivity, which is critical for both climate change and process studies. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 105 UR - sites/default/files/pubs/2000 -Pal et al.pdf ER - TY - JOUR T1 - Analysis of the pathways relating soil moisture and subsequent rainfall in Illinois JF - JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Y1 - 1999 A1 - Findell, K. L. A1 - Eltahir, Elfatih A B AB - This study is a continuation of an earlier work {[}Findell and Eltahir, 1997] on the soil moisture-rainfall feedback using a data set of biweekly neutron probe measurements of soil moisture at up to 19 stations throughout Illinois. Analyses in this earlier work showed a positive correlation between initial soil saturation and subsequent rainfall from early June to mid-August. This correlation was more significant than the serial correlation within precipitation, suggesting the likelihood of a physical mechanism linking soil moisture to subsequent rainfall. This paper probes the nature of such a physical pathway linking soil moisture to subsequent rainfall. The pathway is divided into two stages: soil moisture and near-surface air, and nearsurface air and rainfall. An analysis of the connections between an average daily soil saturation for the whole state of Illinois with statewide average near-surface air conditions did not yield the anticipated positive correlation between soil moisture and moist static energy (MSE). It is not clear if this is due to limitations of the data or of the theory. Other factors, such as clouds, could potentially be masking the impacts of soil moisture on the energy of the near-surface air. There was evidence, however, that moisture availability at the surface has a very strong impact on the wet-bulb depression of near-surface air, particularly from mid-May to the end of August, showing good correspondence to the period of significant soil moisture-rainfall association. The final set of analyses performed used hourly boundary layer and rainfall data. A link between high MSE and high rainfall was noted during some summer months, and a link between low wet-bulb depression and high rainfall was evident for all of the months analyzed (April through September). These analyses suggest that the significant but weak correlation between soil moisture and rainfall during Illinois summers is at least partially due to soil moisture controls on the wet-bulb depression of near-surface air. PB - Amer Geophys Union; Amer Meterol Soc; Mauna Lani Bay Hotel & Bungalows CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 104 UR - sites/default/files/pubs/1999 Findell Eltahir.pdf N1 - 6th International Conference of Precipitation on Predictability of Rainfall at the Various Scales, MAUNA LANI BAY, HAWAII, JUN 29-JUL 01, 1998 ER - TY - JOUR T1 - On the asymmetric response of aquifer water level to floods and droughts in Illinois JF - WATER RESOURCES RESEARCH Y1 - 1999 A1 - Eltahir, Elfatih A B A1 - Yeh, P. J. F. AB - Here we analyze observed characteristics of the natural variability in the regional-scale hydrological cycle of Illinois, including the soil and atmospheric branches. This analysis is based on a consistent data set that describes several hydrological variables: the flux of-atmospheric water vapor, incoming solar radiation, precipitation, soil moisture content, aquifer water level, and river flow. The climatology of the average regional hydrological cycle has been estimated. Variability in incoming solar radiation, not precipitation, is the main forcing of the seasonal variability in evaporation, soil moisture content, aquifer water level, and river flow. While precipitation plays a minor role in shaping the natural variability in the regional hydrological cycle at the seasonal timescale, variability in precipitation is the major factor in shaping the natural variability in the regional hydrological cycle at the interannual timescale. The anomalies in the different variables of the regional hydrological cycle have been computed and the persistence patterns of extreme floods and droughts have been compared. The 1988 drought left a signature in the aquifer water level that is significantly more persistent than the corresponding signature for the 1993 summer flood. The discharge from unconfined groundwater aquifers to streams (base flow) provides an efficient dissipation mechanism for the wet anomalies in aquifer water level. However, the nonlinear dependence of the groundwater discharge on aquifer water level (groundwater rating curve) may explain why droughts leave a significantly more persistent signature on groundwater hydrology, in comparison to the signature of floods. This nonlinearity has been attributed to the increasing degree by which the unconfined aquifers get connected to the channels network, as the aquifer water level rises leading to higher drainage density. The potential implications of these results regarding the impact on regional water resources due to any future climate change are discussed. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 35 UR - sites/default/files/pubs/1999_Eltahir_Yeh.pdf ER - TY - JOUR T1 - A mechanism relating tropical Atlantic spring sea surface temperature and west African rainfall JF - QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Y1 - 1999 A1 - Zheng, X. Y. A1 - Eltahir, Elfatih A B A1 - Emanuel, K. A. KW - air-sea interaction KW - monsoons KW - west africa AB - In this paper, we describe a mechanistic study on the role of tropical Atlantic sea surface temperature (SST) variability in the dynamics of the west African monsoon. A hypothesis that a warm spring (April-June) SST results in a wet monsoon is explored using a moist, zonally symmetric model. A positive spring rainfall anomaly has been simulated over the ocean, in response to the warm SST, which then propagates onto the land and persists two to three months, even after the SST anomaly vanishes. While the ocean-atmosphere interaction is crucial for the initial development of the rainfall anomaly over land, the interactions between the ocean, land, and atmosphere are found to be important for relating tropical Atlantic spring SST to west African rainfall. Furthermore, the positive feedback between rainfall and soil moisture is responsible for some of the persistence in the rainfall anomaly. We present a case-study for the wettest (1994) and driest (1992) years of the 1990s so far. The observations show that a warm spring SST anomaly in 1994 was associated with abundant summer rainfall over west Africa. These empirical observations are consistent with the proposed physical mechanism. PB - ROYAL METEOROLOGICAL SOC CY - 104 OXFORD ROAD, READING, BERKS, ENGLAND RG1 7LJ VL - 125 UR - sites/default/files/pubs/1999_Zheng_et_al.pdf ER - TY - JOUR T1 - Nilometers, El Nino, and climate variability JF - GEOPHYSICAL RESEARCH LETTERS Y1 - 1999 A1 - Eltahir, Elfatih A B A1 - Wang, G. L. AB - Nilometers have been used for gauging the level of water in the Mile river for more than five millennia. The written records describing some of these measurements represent the longest written records for any hydrological phenomenon. They describe interannual fluctuations in the Mile river flow which are closely associated with El Nino phenomenon. Here, we use information about long-term variability in El Nino occurrences that has been extracted from the Nilometers records to test the significance of the recent-trend in the frequency of El Nine years. We show that the observed frequency of EI Nino years during the last two decades is rather high compared to the long-term statistics that are computed from about a thousand years of Nilometers data; however similar levels of activity have been observed during the first millennium. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 26 UR - sites/default/files/pubs/1999 Eltahir Wang.pdf ER - TY - JOUR T1 - Role of ecosystem dynamics in biosphere-atmosphere interaction over the coastal region of West Africa JF - JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Y1 - 1999 A1 - Kiang, J. E. A1 - Eltahir, Elfatih A B KW - west africa AB - In this study, we develop a one-dimensional model of the tropics which includes two-way interaction between the biosphere and the atmosphere, including ecosystem dynamics. The model integrates an atmospheric model, a biospheric model, and a monsoon circulation model and is applied to coastal West Africa to test the sensitivity of the coupled system to changes in vegetation cover. We perform three sets of simulations: one with a fixed monsoon circulation, one with an interactive monsoon circulation, and one with modified boundary conditions at the northern edge of the domain. Our control simulations show that the model is able to reasonably approximate observed conditions. The model simulates a single stable forest equilibrium in the first two sets of simulations, those which correspond most readily with present conditions in West Africa. These simulations indicate that the monsoon plays an important role in modulating the climate of the region and in shaping the response of the system to vegetation changes. Changes in the monsoon which allowed hot and dry air to penetrate into the model domain from the north strongly modified the equilibrium climate toward drier conditions. This finding motivated further testing of the system assuming degraded conditions to the north, which revealed the possibility of two different equilbria: one forest and one grassland. The existence of multiple equilibria in the biosphere-atmosphere system depends not only on the magnitude of the vegetation-induced climate perturbation but also on whether or not the perturbation extends across the threshold in moisture conditions controlling competition between trees and grasses. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 104 UR - sites/default/files/pubs/1999 Kiang Eltahir.pdf ER - TY - JOUR T1 - Use of ENSO information in medium- and long-range forecasting of the Nile floods (vol 12, pg 1726, 1999)--CORRECTION JF - JOURNAL OF CLIMATE Y1 - 1999 A1 - Wang, G. L. A1 - Eltahir, Elfatih A B KW - Sustainability Considerations in the Design of Big Dams PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 12 ER - TY - JOUR T1 - Use of ENSO information in medium- and long-range forecasting of the Nile floods JF - JOURNAL OF CLIMATE Y1 - 1999 A1 - Wang, G. L. A1 - Eltahir, Elfatih A B KW - Sustainability Considerations in the Design of Big Dams AB - The natural variability in the annual flow of the Nile is significantly regulated by the El Nino-Southern Oscillation (ENSO). In this paper, several sources of information are combined, including ENSO, rainfall over Ethiopia, and the recent history of river flow in the Nile, in order to obtain accurate forecasts of the Nile hood at Aswan. The Bayesian theorem is used in developing the discriminant forecasting algorithm. Conditional categoric probabilities are used to describe the flood forecasts, and a synoptic index is defined to measure the forecasts' skill. The results presented show that ENSO information is the only valuable predictor for the long-range forecasts (lead time longer than the hydrological response timescale, which is 2-3 months in this study). However, the incorporation of the rainfall and river flow information in addition to the ENSO information significantly improves the quality of the medium-range forecasts (lead time shorter than the hydrological response timescale). PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 12 UR - sites/default/files/pubs/1999 Wang Eltahir.pdf ER - TY - JOUR T1 - Hydroclimatology of Illinois: A comparison of monthly evaporation estimates based on atmospheric water balance and soil water balance JF - JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Y1 - 1998 A1 - Yeh, P. J. F. A1 - Irizarry, M. A1 - Eltahir, Elfatih A B AB - Here we describe the regional-scale hydrological cycle of Illinois, including both the land and atmospheric branches, using a data set on most of the hydrological variables, i.e., precipitation, streamflow, soil water content, snow depth, groundwater level, and atmospheric flux of water vapor. Since direct observations of evaporation are not available, mio different approaches, soil water balance and atmospheric water balance, were applied to estimate the regional evaporation over Illinois from 1983 to 1994, The availability of a comprehensive hydrological data set covering the large area of Illinois facilitated a comparison between these two approaches for estimation of evaporation. To our knowledge, this is the first time such a comparison has been made. The climatologies of the monthly evaporation estimates from the two approaches agree reasonably well and within a 10% error; however, substantial differences exist between the two estimates of evaporation for individual months. The seasonal variability of the evaporation estimates based on soil water balance is largely balanced by the seasonal pattern of subsurface storage, whereas the seasonal variability of evaporation estimates from the atmospheric water balance is almost entirely balanced by the seasonal pattern of lateral fluxes of water vapor. This contrast reflects a fundamental difference in the hydrology of the land and atmospheric branches of the regional water cycle. In light of the fact that independent data sets were used in the two approaches, our results are encouraging: The atmospheric water balance approach has the potential for the accurate estimation of the climatology of regional evaporation, at least for humid regions at a scale similar to that of Illinois (similar to 10(5) km(2)). However, sensitivity analysis suggests that the accuracy of atmospheric water balance computations is rather poor for the scale smaller than 10(5) km(2). For the calculation of evaporation using the soil water balance approach in regions where the groundwater table is rather shallow, the incorporation of the change in groundwater storage is indispensable since groundwater aquifers provide a significant portion of water storage at the monthly timescale. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 103 UR - sites/default/files/pubs/1998 Yeh Irizarry Eltahir.pdf ER - TY - JOUR T1 - Numerical simulation of nonlinear mesoscale circulations induced by the thermal heterogeneities of land surface JF - JOURNAL OF THE ATMOSPHERIC SCIENCES Y1 - 1998 A1 - Wang, Jingfeng A1 - Eltahir, Elfatih A B A1 - Bras, R. L. KW - Amazon AB - Mesoscale circulations forced by a random distribution of surface sensible heat flux have been investigated using a three-dimensional numerical model. The complex land surface is modeled as a homogeneous random field characterized by a spectral distribution. Standard deviation and length scale of the sensible heal flux at the surface have been identified as the important parameters that describe the thermal variability of land surface. The form of the covariance of the random surface forcing is not critical in driving the mesoscale circulation. The thermally induced mesoscale circulation is significant and extends up to about 5 km when the atmosphere is neutral. It becomes weak and is suppressed when the atmosphere is stable. The mesoscale momentum flux is much stronger than the corresponding turbulent momentum flux in the neutral atmosphere, while the two are comparable in the stable atmosphere. The mesoscale heat flux has a different vertical profile than turbulent heat flux and may provide a major heat transport mechanism beyond the planetary boundary layer. The impact of synoptic wind on the mesoscale circulations is relatively weak. Nonlinear advection terms are responsible for momentum flux in the absence of synoptic wind. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 55 UR - sites/default/files/pubs/1998 Wang_Eltahir_Bras.pdf ER - TY - JOUR T1 - The role of clouds in the surface energy balance over the Amazon forest JF - INTERNATIONAL JOURNAL OF CLIMATOLOGY Y1 - 1998 A1 - Eltahir, Elfatih A B A1 - Humphries, E. J. KW - Amazon KW - cloud feedbacks KW - deforestation KW - surface energy balance AB - Deforestation in the Amazon region will initially impact the energy balance at the land surface through changes in land cover and surface hydrology. However, continuation of this human activity will eventually lead to atmospheric feedbacks, including changes in cloudiness which may play an important role in the final equilibrium of solar and terrestrial radiation at the surface. In this study, the different components of surface radiation over an undisturbed forest in the Amazon region are computed using data from the Amazon region micrometerological experiment (ARME). Several measures of cloudiness are defined: two estimated from the terrestrial radiation measurements, and one from the solar radiation measurements. The sensitivity of the surface fluxes of solar and terrestrial radiation to natural variability in cloudiness is investigated to infer the potential role of the cloudiness feedback in the surface energy balance. The results of this analysis indicate that a 1% decrease in cloudiness would increase net solar radiation by ca. 1.6 W/m(2). However, the overall magnitude of this feedback, due to total deforestation of the Amazon forest, is likely to be of the same order as the magnitude of the decrease in net solar radiation due to the observed increase in surface albedo following deforestation. Hence, the total change in net solar radiation is likely to have a negligible magnitude. In contrast to this conclusion, we find that terrestrial radiation is likely to be more strongly affected; reduced cloudiness will decrease net terrestrial radiation; a 1% decrease in cloudiness induces a reduction in net terrestrial radiation of ca. 0.7 W/m(2); this process augments the similar effects of the predicted warming and drying in the boundary layer. Due to the cloudiness feedback, the most significant effect of large-scale deforestation on the surface energy balance is likely to be in the modification of the terrestrial radiation field rather than the classical albedo effect on solar radiation fields. The net effect of clouds is to reduce net radiation; a 1% increase in cloudiness induces a reduction in net radiation of ca. 1 W/m(2). The implications of this negative feedback on large-scale land-atmosphere interactions over rainforests are discussed. (C) 1998 Royal Meteorological Society. PB - JOHN WILEY & SONS LTD CY - BAFFINS LANE CHICHESTER, W SUSSEX PO19 1UD, ENGLAND VL - 18 UR - sites/default/files/pubs/1998 Eltahir Humphries.pdf ER - TY - JOUR T1 - The role of vegetation in the dynamics of West African monsoons JF - JOURNAL OF CLIMATE Y1 - 1998 A1 - Zheng, X. Y. A1 - Eltahir, Elfatih A B AB - The focus of this paper is the role of meridional distribution of vegetation in the dynamics of monsoons and rainfall over West Africa. A moist zonally symmetric atmospheric model coupled with a simple land surface scheme is developed to investigate these processes. Four primary experiments have been carried out to examine the sensitivity of West African monsoons to perturbations in the meridional distribution of vegetation. In the control experiment, the authors assume a distribution of vegetation that resembles the natural vegetation cover in West Africa. Each perturbation experiment is identical to the control experiment except that a change in vegetation cover is imposed for a latitudinal belt that is 10 degrees in width. The results of the numerical experiments demonstrate that West African monsoons and therefore rainfall distribution depend critically on the location of the vegetation perturbations. Changes in vegetation cover along the border between the Sahara desert and West Africa (desertification) may have a minor impact on the simulated monsoon circulation. However, coastal deforestation may cause the collapse of the monsoon circulation and have a dramatic impact on the regional rainfall. The observed deforestation in West Africa is then likely to be a significant contributor to the observed drought. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 USA VL - 11 UR - sites/default/files/pubs/1998 Zheng Eltahir.pdf ER - TY - JOUR T1 - On the sensitivity of drainage density to climate change JF - WATER RESOURCES RESEARCH Y1 - 1998 A1 - Moglen, G. E. A1 - Eltahir, Elfatih A B A1 - Bras, R. L. AB - Drainage density reflects the signature of climate on the topography and dictates the boundary conditions for surface hydrology. Hence defining the relationship between drainage density and climate is important in assessing the sensitivity of water resources and hydrology to climate change. Here we analyze the equilibrium relationship between drainage density and climate and estimate the relative sensitivity of drainage density to climate change. We conclude that the sign of the resulting change in drainage density depends not only on the direction of the change in climate but also on the prevailing climatic regime. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 34 UR - sites/default/files/pubs/1998 Moglen Eltahir Bras.pdf ER - TY - JOUR T1 - A soil moisture rainfall feedback mechanism 1. Theory and observations JF - WATER RESOURCES RESEARCH Y1 - 1998 A1 - Eltahir, Elfatih A B AB - This paper presents a hypothesis regarding the fundamental role of soil moisture conditions in land-atmosphere interactions. We propose that wet soil moisture conditions over any large region should be associated with relatively large boundary layer moist static energy, which favors the occurrence of more rainfall. Since soil moisture conditions themselves reflect past occurrence of rainfall, the proposed hypothesis implies a positive feedback mechanism between soil moisture and rainfall. This mechanism is based on considerations of the energy balance at the land-atmosphere boundary, in contrast to similar mechanisms that were proposed in the past and that were based on the concepts of water balance and precipitation recycling. The control of soil moisture on surface albedo and Bowen ratio is the fundamental basis of the proposed soil moisture-rainfall feedback mechanism. The water content in the upper soil layer affects these two important properties of the land surface such that both variables decrease with any increase in the water content of the top soil layer. The direct effect of soil moisture on surface albedo implies that wet soil moisture conditions enhance net solar radiation. The direct effect of soil moisture on Bowen ratio dictates that wet soil moisture conditions would tend to enhance net terrestrial radiation at the surface through cooling of surface temperature, reduction of upwards emissions of terrestrial radiation, and simultaneous increase in atmospheric water vapor content and downwards flux of terrestrial radiation. Thus, under wet soil moisture conditions, both components of net radiation are enhanced, resulting in a larger total flux of heat from the surface into the boundary layer. This total flux represents the sum of the corresponding sensible and latent heat fluxes. Simultaneously, cooling of surface temperature should be associated with a smaller sensible heat flux and a smaller depth of the boundary layer. Whenever these processes occur over a large enough area, the enhanced flux of heat from the surface into the smaller reservoir of boundary layer air should favor a relatively large magnitude of moist static energy per unit mass of the boundary layer air. The dynamics of localized convective storms as well as the dynamics of large-scale atmospheric circulations have been shown to be sensitive to the distribution of boundary layer moist static energy by several previous studies. These theoretical concepts are tested using field observations from Kansas and explored further in a companion paper {[}Zheng and Eltahir, this issue] using a simple numerical model. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 34 UR - sites/default/files/pubs/1998 Eltahir.pdf ER - TY - JOUR T1 - A soil moisture rainfall feedback mechanism 2. Numerical experiments JF - WATER RESOURCES RESEARCH Y1 - 1998 A1 - Zheng, X. Y. A1 - Eltahir, Elfatih A B AB - Here we develop a numerical model to investigate the hypothesis proposed by a companion paper {[}Eltahir, this issue], which describes a soil moisture-rainfall feedback mechanism. The model is designed to describe the seasonal evolution of the West African monsoon rainfall and is used to perform numerical experiments that elucidate the mechanisms of the response of rainfall to soil moisture anomalies. A significant rainfall anomaly is simulated by the model in response to a hypothetical soil moisture anomaly that has been imposed during early summer. However, the magnitude of this anomaly almost vanishes when the net radiation at the surface is not allowed to respond to the soil moisture anomaly. Hence the results of the numerical experiments support the proposed hypothesis and highlight the crucial importance of the radiative and dynamical feedbacks in regulating the rainfall anomalies that result from the soil moisture anomalies. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 34 UR - sites/default/files/pubs/1998 Zheng Eltahir_1.pdf ER - TY - JOUR T1 - Stochastic analysis of the relationship between topography and the spatial distribution of soil moisture JF - WATER RESOURCES RESEARCH Y1 - 1998 A1 - Yeh, P. J. F. A1 - Eltahir, Elfatih A B AB - This paper deals with the issue of the spatial horizontal variability of soil moisture in the root zone of a shallow soil at the large scale. The problem of water flow in the unsaturated zone is formulated so that topography appears explicitly as a forcing for the movement and redistribution of soil moisture. This formulation emphasizes the role of the lateral redistribution of water that is induced by topography. A stochastic theory is developed to relate the statistical distribution of soil moisture to that of elevation. This approach will ultimately facilitate the use of the readily available data sets describing topography for the purpose of defining the large-scale distribution of soil moisture. The steady state horizontal distribution of soil moisture under homogeneous bare soil conditions is regulated by three distinct factors: topography, climate, and soil properties. First, topography, forces a distribution of soil moisture that tends to mimic the elevation field at large scales. The other two factors are the vertical divergence of water in response to the climate forcing (evaporation) and the capillary resistance to water movement. The climate forcing tends to smooth the spatial distribution of soil moisture. However, the capillary forces exerted by the soil matrix tend to resist displacement of water and hence exert adverse effects against the topography and climate forcings. The variance of the soil moisture distribution increases with the variance of the elevation held and decreases with the correlation scale of the elevation field and the magnitude of the climate forcing. The impact of capillary forces on the vertical fluxes of water is more significant than their impact on the topographically induced horizontal fluxes, owing to the larger hydraulic gradient in the vertical, direction resulting from the disparity in scale between the vertical and horizontal directions. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 34 UR - sites/default/files/pubs/1998 Yeh Eltahir .pdf ER - TY - JOUR T1 - Stochastic analysis of the relationship between topography and the spatial distribution of soil moisture (vol 34, pg 1251, 1998) JF - WATER RESOURCES RESEARCH Y1 - 1998 A1 - Yeh, P. J. F. A1 - Eltahir, Elfatih A B PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA VL - 34 UR - sites/default/files/pubs/1998 Yeh Eltahir .pdf ER - TY - JOUR T1 - An analysis of the soil moisture-rainfall feedback, based on direct observations from Illinois JF - WATER RESOURCES RESEARCH Y1 - 1997 A1 - Findell, K. L. A1 - Eltahir, Elfatih A B AB - Many global and regional climate modeling studies have demonstrated the importance of the initial soil water condition in their simulations of regional rainfall distribution. However, none of these modeling studies has been tested against directly observed data. This study tests the hypothesis that soil saturation is positively correlated with subsequent precipitation by analyzing a 14-year soil moisture data set from the state of Illinois. The linear correlation between an initial soil saturation condition and subsequent rainfall is significant during the summer months, reaching a peak of r(2)>0.4 in mid-June. This result is consistent with the hypothesis that knowledge of late spring/early summer soil moisture conditions can aid in the prediction of drought or flood years, but it does not necessarily prove that feedback from anomalous soil moisture reservoirs is the cause of anomalous summer conditions. Further analyses indicate that from early June to mid-August, persistence in rainfall cannot fully account for the observed correlations, suggesting the likelihood of a physical feedback mechanism linking early summer soil saturation with subsequent precipitation. However, spatial and temporal data limitations restrict the potential for drawing strong new conclusions from the Illinois study. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 VL - 33 UR - sites/default/files/pubs/1997 Findell Eltahir .pdf ER - TY - JOUR T1 - ENSO and the natural variability in the flow of tropical rivers JF - JOURNAL OF HYDROLOGY Y1 - 1997 A1 - Amarasekera, K. N. A1 - Lee, R. F. A1 - Williams, E. R. A1 - Eltahir, Elfatih A B KW - ei nino southern oscillation (enso) KW - flow variability KW - sea surface temperature (sst) KW - Sustainability Considerations in the Design of Big Dams KW - tropical rivers AB - This paper examines the relationship between the annual discharges of the Amazon, Congo, Paran ii, and Nile rivers and the sea surface temperature (SST) anomalies of the eastern and central equatorial Pacific Ocean, an index of El Nino-Southern Oscillation (ENSO). Since river systems are comprehensive integrators of rainfall over large areas, accurate characterization of the flow regimes in major rivers will increase our understanding of large-scale global atmospheric dynamics. Results of this study reveal that the annual discharges of two large equatorial tropical rivers, the Amazon and. the Cungo, are weakly and negatively correlated with the equatorial Pacific SST anomalies with 10% of the variance in annual discharge explained by ENSO. Two smaller subtropical rivers, the Nile and the Parana, show a correlation chat is stronger by about a factor of 2. The Nile discharge is negatively correlated with the SST anomaly, whereas the Parana river discharge shows a positive relation. The tendency for reduced rainfall/discharge over large tropical convection zones in the ENSO warm phase is attributed to global scale subsidence associated with major upwelling in the eastern Pacific Ocean. (C) 1997 Elsevier Science B.V. PB - ELSEVIER SCIENCE BV CY - PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS VL - 200 UR - sites/default/files/pubs/1997 Amarasekera .pdf ER - TY - JOUR T1 - The response to deforestation and desertification in a model of West African monsoons JF - GEOPHYSICAL RESEARCH LETTERS Y1 - 1997 A1 - Zheng, X. Y. A1 - Eltahir, Elfatih A B KW - west africa AB - Since Charney proposed his theory on the dynamics of deserts and droughts in the Sahel {[}Charney, 1975], there has been significant scientific interest in the interaction between vegetation and climate in this region. The essence of this interaction is that the atmospheric circulation, and therefore rainfall, over this region may be sensitive to changes in vegetation cover near the desert border. Here we describe simulations of the West African monsoons with a simple zonally-symmetric model. The results suggest that the potential impact of human induced change of land cover on regional climate depends critically on the location of the change in vegetation cover. That is, desertification along the border with the Sahara (e.g., in Chad, Niger, Mali and Mauritania) leaves a relatively minor impact on monsoon circulation and regional rainfall; deforestation along the southern coast of West Africa (e.g., in Nigeria, Ghana and Ivory Coast) may result in complete collapse of monsoon circulation, and a significant reduction of regional rainfall. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 VL - 24 UR - sites/default/files/pubs/1997 Zheng Eltahir .pdf ER - TY - JOUR T1 - Dynamics of wet and dry years in West Africa JF - JOURNAL OF CLIMATE Y1 - 1996 A1 - Eltahir, Elfatih A B A1 - Gong, C. L. KW - west africa AB - This paper proposes a theoretical framework for describing interannual climatic variability over West Africa. The dynamical theory of zonally symmetrical thermally direct circulations suggests that a meridional monsoon circulation must develop over any tropical region (off the equator) when the absolute vorticity near the tropopause reaches a threshold value of zero. However, for a moist atmosphere that satisfies a quasi-equilibrium balance between moist convection and the radiative forcing, the absolute vorticity at upper-tropospheric levels, is a function of both latitude and the meridional distribution of boundary-layer entropy. Hence, the onset of a monsoon circulation depends in a nonlinear fashion on these two factors. The theory predicts that a flat distribution of entropy does not drive any circulation and that a relatively large gradient of entropy should drive a strong monsoon circulation. The location of the region of West Africa, relatively close to the equator, dictates that the dynamics of a monsoon over that region are relatively sensitive to interannual fluctuations in the meridional gradient of boundary-layer entropy. Hence, we present observations on entropy and wind over West Africa during the monsoon seasons of 1958 and 1960. The following observations were consistent with the proposed relationship between boundary-layer entropy and the monsoon circulation: a large meridional gradient of boundary-layer entropy, a healthy monsoon, and wet conditions over the Sahel region were observed in 1958; and a nearly flat distribution of entropy, very weak circulation, and relatively dry conditions were observed in 1960. Moreover, the proposed theoretical relationship between the meridional gradient of boundary-layer entropy and the monsoon circulation over West Africa is consistent with the empirical observations of sea surface temperature anomalies (SSTAs) int he tropical Atlantic and rainfall in the Sahel region. Theoretically, a cold (warm) SSTA in the region located south of the West African coast should favor a large (small) meridional gradient of entropy, a strong (weak) monsoon circulation, and wet (dry) conditions in the Sahel. A large body of observations confirms that cold (warm) SSTAs off the southern coast of West Africa are associated with wet (dry) years in the Sahel region. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 VL - 9 UR - sites/default/files/pubs/1996 Eltahir Gong.pdf ER - TY - JOUR T1 - El Nino and the natural variability in the flow of the Nile river JF - WATER RESOURCES RESEARCH Y1 - 1996 A1 - Eltahir, Elfatih A B KW - Sustainability Considerations in the Design of Big Dams AB - Natural variability in the annual flow of the Nile River has been the subject of great interest to the civilizations that have historically occupied the banks of that river. Here we report results from analysis on two extensive data sets describing sea surface temperature of the Pacific Ocean, and the flow of water in the Nile River. The analysis suggests that 25% of the natural variability in the annual flow of the Nile is associated with El Nino oscillations, A procedure is developed for using this observed correlation to improve the predictability of the Nile flood. A simple hypothesis is presented to explain physically the occurrence of the Hurst phenomenon in the Nile flow. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 VL - 32 UR - sites/default/files/pubs/1996 Eltahir_2.pdf ER - TY - JOUR T1 - Precipitation recycling JF - REVIEWS OF GEOPHYSICS Y1 - 1996 A1 - Eltahir, Elfatih A B A1 - Bras, R. L. AB - The water cycle regulates and reflects natural variability in climate at the regional and global scales. Large-scale human activities that involve changes in land cover, such as tropical deforestation, are likely to modify climate through changes in the water cycle. In order to understand, and hopefully be able to predict, the extent of these potential global and regional changes, we need first to understand how the water cycle works. In the past, most of the research in hydrology focused on the land branch of the water cycle, with little attention given to the atmospheric branch. The study of precipitation recycling, which is defined as the contribution of local evaporation to local precipitation, aims at understanding hydrologic processes in the atmospheric branch of the water cycle. Simply stated, any study on precipitation recycling is about how the atmospheric branch of the water cycle works, namely, what happens to water vapor molecules after they evaporate from the surface, and where will they precipitate? Estimation of precipitation recycling over any large basin, such as the Mississippi or the Amazon, is a necessary step before developing a quantitative description of the regional water cycle. This paper reviews the research on the concept of precipitation recycling and emphasizes the basic role of this process in defining the different components of the atmospheric branch in any regional water cycle. To illustrate the assumptions and limitations involved in estimation of precipitation recycling, we present and discuss a general formula for estimation of precipitation recycling. The recent estimates of annual precipitation recycling ratio from different regions are reviewed and compared. Finally, the dependence of precipitation recycling over any region on the spatial scale is discussed and illustrated by the example of the Amazon basin. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 VL - 34 UR - sites/default/files/pubs/1996 Eltahir Bras.pdf ER - TY - JOUR T1 - Relationship between surface conditions and subsequent rainfall in convective storms JF - JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Y1 - 1996 A1 - Eltahir, Elfatih A B A1 - Pal, J. S. KW - Amazon AB - This paper describes the relationship between surface conditions (temperature and humidity) and subsequent rainfall. The focus is on convective storms that are forced and maintained locally due to conditional instability in the vertical distribution of atmospheric temperature. These storms are described using two probabilistic measures: (1) the probability of occurrence of storms given surface conditions and (2) the average storm rainfall. The surface conditions are described by a single variable: surface wet-bulb temperature. The proposed theoretical relationships are tested using an hourly data set on rainfall and wet-bulb temperature from the Amazon region. These observations confirm that both measures increase linearly with wet-bulb temperature. However, for the occurrence of anp storm the wet-bulb temperature has to exceed a threshold of about 22 degrees C. The sensitivity of the frequency of storms to changes in the climatology of surface wet-bulb temperature is larger than the corresponding sensitivity of the average storm rainfall. These general concepts are applied in discussing the potential impact of changes in land cover on rainfall patterns using two specific examples: deforestation in the Amazon region and development of irrigation projects in the Columbia River basin. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 VL - 101 UR - sites/default/files/pubs/1996 Eltahir Pal.pdf N1 - 5th International Conference on Precipitation on Space-Time Variability and Dynamics of Rainfall, ELOUNDA, GREECE, JUN 14-16, 1995 ER - TY - JOUR T1 - Role of vegetation in sustaining large-scale atmospheric circulations in the tropics JF - JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Y1 - 1996 A1 - Eltahir, Elfatih A B KW - west africa AB - The focus of this paper is the role of rain forests in large-scale atmospheric circulations. The significance of this role is investigated by studying the response of the tropical atmosphere to a perturbation in the state of vegetation (deforestation) over three regions: the Amazon, Congo, and Indonesia. A theory is developed to relate tropical deforestation and the resulting changes in the large-scale atmospheric circulation. Field observations and numerical simulations support the argument that tropical deforestation reduces the total net surface radiation, including terrestrial and solar forms. However, the energy balance at the land-atmosphere boundary dictates that for equilibrium conditions, any reduction in net surface radiation has to be balanced by a similar reduction in the total flux of heat, including sensible and latent forms. Since these fluxes supply heat as well as entropy from the forest into the atmospheric boundary layer, a reduction in the total flux of heat reduces the boundary layer entropy. In a moist atmosphere, that satisfies a quasi equilibrium between moist convection and radiative forcing, the equilibrium temperature profile is uniquely related to the boundary layer entropy. Under such conditions, large-scale deforestation reduces boundary layer entropy relative to the surroundings, cools the upper troposphere, and results in subsidence, divergent flow in the boundary layer, and weakening of the large-scale circulation. These changes are simulated using a simple linear model of atmospheric flow. The comparison of the model predictions with observations of atmospheric circulations over the Amazon, Congo, and Indonesia suggests a significant role for vegetation in maintaining large-scale atmospheric circulations in the tropics. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 VL - 101 UR - sites/default/files/pubs/1996 Eltahir_3.pdf ER - TY - JOUR T1 - Sources of Moisture for Rainfall in West Africa JF - WATER RESOURCES RESEARCH Y1 - 1996 A1 - Gong, C. L. A1 - Eltahir, Elfatih A B KW - west africa AB -

The objective of this study is to identify the sources of moisture for rainfall in west Africa. A model of precipitation recycling is developed and applied to the region of west Africa to obtain quantitative estimates of the moisture contributed by local evaporation as well as the moisture contributed by the zonal and meridional fluxes from the surrounding regions. We estimated the recycling ratio for the entire region by specifying three subregions where evaporation is treated as the source of moisture: west Africa, central Africa, and the tropical Atlantic Ocean. We find that evaporation from the tropical Atlantic Ocean, west Africa, and central Africa contribute about 23, 27, and 17% of rainfall in west Africa, respectively. Moisture fluxes from the tropical Atlantic are almost in phase with rainfall in west Africa. However, we find that moisture supply from central Africa is strongly regulated and limited by the westerly flow associated with the monsoon circulation. Hence the large-scale monsoon circulation is not only the main forcing of rainfall over west Africa, but the dynamics of this circulation exert significant control on where the moisture comes from.

PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 VL - 32 ER - TY - JOUR T1 - A stochastic linear theory of mesoscale circulation induced by the thermal heterogeneity of the land surface JF - JOURNAL OF THE ATMOSPHERIC SCIENCES Y1 - 1996 A1 - Wang, Jingfeng A1 - Bras, R. L. A1 - Eltahir, Elfatih A B KW - Amazon AB -

This paper presents a three-dimensional stochastic linear model of the mesoscale circulation induced by the variability of turbulent sensible heat flux over land surface. The primitive equations relating wind field, geopotential, and potential temperature are formulated as a system of stochastic partial differential equations and solved analytically. The solution is based on spectral representations of homogeneous random fields. The flow intensity is found to be proportional to the standard deviation of the turbulent sensible heat flux into the atmosphere. Large (small) scales of spatial variability in the surface heating preferably impact circulations at high (low) altitudes. The mesoscale fluxes associated with the atmospheric flow are related to explicit functions of atmospheric stability, variance of turbulent heat flux, and synoptic wind. The authors find that the vertical momentum flux is significant in the presence of synoptic wind and that the flow perpendicular to the direction of the synoptic wind is responsible for this momentum flux. The proposed linear theory identifies the synoptic conditions under which the land-surface heterogeneity may play a role in atmospheric circulations at the mesoscale.

PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 VL - 53 UR - sites/default/files/pubs/1996 Wang Bras Eltahir_1.pdf ER - TY - JOUR T1 - Stochastic modeling of the thermally induced atmospheric flow at mesoscale JF - MECCANICA Y1 - 1996 A1 - Wang, Jingfeng A1 - Bras, R. L. A1 - Eltahir, Elfatih A B KW - Amazon KW - hydrometeorology KW - mesoscale flow KW - stochastic approach KW - surface heating AB - This paper presents a three-dimensional stochastic linear model of the atmospheric flow induced by the variability of heat flux over land surface. The primitive equations relating perturbation terms of wind field, geopotential and buoyancy are formulated as a system of stochastic partial differential equations and solved analytically. The solution is based on spectral representations of the homogeneous random fields. The flow intensity is found to be proportional to the standard deviation of the heat flux into the atmosphere. The intensity of the vertical motion becomes more sensitive to the differential heating with a larger length scale as altitude goes higher. Stability and synoptic wind inhibit the development of the flow. The proposed theory improves the understanding of the role that heterogeneous land surface plays in atmospheric circulations at the mesoscale. PB - KLUWER ACADEMIC PUBL CY - SPUIBOULEVARD 50, PO BOX 17, 3300 AA DORDRECHT, NETHERLANDS VL - 31 UR - sites/default/files/pubs/1996 Wang Bras Eltahir_2.pdf ER - TY - JOUR T1 - AGGREGATION-DISAGGREGATION PROPERTIES OF A STOCHASTIC RAINFALL MODEL JF - WATER RESOURCES RESEARCH Y1 - 1994 A1 - Bo, Z. Q. A1 - Islam, S. A1 - Eltahir, Elfatih A B AB - A statistical approach based on the modified Bartlett-Lewis rectangular pulses model is presented to disaggregate rainfall statistics from daily data. Six model parameters are estimated from 24- and 48-hour accumulated rainfall data. Based on these estimated parameters, in addition to reproducing 24- and 48-hour statistics, the model is shown to infer 1-, 2-, 6-, and 12-hour historical statistics satisfactorily. An upper limit for disaggregation scale (about 2 days) for this model has been identified. This characteristic behavior of the model is related to the power law dependence of the power spectrum for timescales smaller than 2 days. A detailed comparison between observed and modeled statistics of rainfall data is presented for two rain gages, one from central Italy and the other from the midwestern United States. PB - AMER GEOPHYSICAL UNION CY - 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 VL - 30 UR - sites/default/files/pubs/1994 Bo Islam Eltahir.pdf ER - TY - JOUR T1 - THE PARAMETRIZATION OF RAINFALL INTERCEPTION IN GCMS - COMMENT JF - QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Y1 - 1994 A1 - Eltahir, Elfatih A B A1 - Bras, R. L. PB - ROYAL METEOROLOGICAL SOC CY - 104 OXFORD ROAD, READING, BERKS, ENGLAND RG1 7LJ VL - 120 ER - TY - JOUR T1 - PRECIPITATION RECYCLING IN THE AMAZON BASIN JF - QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Y1 - 1994 A1 - Eltahir, Elfatih A B A1 - Bras, R. L. KW - Amazon AB - Precipitation recycling is the contribution of evaporation within a region to precipitation in that same region. The recycling rate is a diagnostic measure of the potential for interactions between land surface hydrology and regional climate. In this paper we present a model for describing the seasonal and spatial variability of the recycling process. The precipitation recycling ratio, rho, is the basic variable in describing the recycling process. Rho is the fraction of precipitation at a certain location and time which is contributed by evaporation within the region under study. The recycling model is applied in studying the hydrologic cycle in the Amazon basin. It is estimated that about 25% of all the rain that falls in the Amazon basin is contributed by evaporation within the basin. This estimate is based on analysis of a data set supplied by the European Centre for Medium-range Weather Forecasts. The same analysis is repeated using a different data set from the Geophysical Fluid Dynamics Laboratory. Based on this data set, the recycling ratio is estimated to be 35%. The seasonal variability of the recycling ratio is small compared with the yearly average. The new estimates of the recycling ratio are compared with results of previous studies, and the differences are explained. PB - ROYAL METEOROLOGICAL SOC CY - 104 OXFORD ROAD, READING, BERKS, ENGLAND RG1 7LJ VL - 120 UR - sites/default/files/pubs/1994 Eltahir Bras_1.pdf ER - TY - JOUR T1 - SENSITIVITY OF REGIONAL CLIMATE TO DEFORESTATION IN THE AMAZON BASIN JF - ADVANCES IN WATER RESOURCES Y1 - 1994 A1 - Eltahir, Elfatih A B A1 - Bras, R. L. KW - Amazon AB - Deforestation results in several adverse effects on the natural environment. The focus of this paper is on the effects of deforestation on land-surface processes and regional climate of the Amazon basin. In general, the effects of deforestation on climate are likely to depend on the scale of the deforested area. In this study, we are interested in the effects due to deforestation of areas with a scale of about 250 km. Hence, a meso-scale climate model is used in performing numerical experiments on the sensitivity of regional climate to deforestation of areas with that size. It is found that deforestation results in less net surface radiation, less evaporation, less rainfall, and warmer surface temperature. The magnitude of the change in temperature is of the order of 0.5-degrees-C, the magnitudes of the changes in the other variables are of the order of 10%. In order to verify some of the results of the numerical experiments, the model simulations of net surface radiation are compared to recent observations of net radiation over cleared and undisturbed forest in the Amazon. The results of the model and the observations agree in the following conclusion: the difference in net surface radiation between cleared and undisturbed forest is, almost, equally partitioned between net solar radiation and net long-wave radiation. This finding contributes to our understanding of the basic physics in the deforestation problem. PB - MIT CY - THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD, OXON, ENGLAND OX5 1GB VL - 17 UR - sites/default/files/pubs/1994 Eltahir Bras_2.pdf N1 - MIT Colloquium on Hydroclimatology and Global Hydrology, CAMBRIDGE, MA, APR 07-08, 1993 ER - TY - JOUR T1 - A DESCRIPTION OF RAINFALL INTERCEPTION OVER LARGE AREAS JF - JOURNAL OF CLIMATE Y1 - 1993 A1 - Eltahir, Elfatih A B A1 - Bras, R. L. KW - Amazon AB - A new scheme is developed for describing interception at spatial scales comparable to the typical resolution of climate models. The scheme is based on the Rutter model of interception and statistical description of the subgrid-scale spatial variability of canopy storage and rainfall. The interception loss simulated by the new scheme is significantly smaller than those simulated by other schemes that do not include considerations for spatial variability. The explanation of this result is partly in the enhancement of spatially averaged canopy drainage due to the large local drainage from the few buckets of large canopy storage. The relative reduction in interception loss simulated by the new scheme may explain the overestimation of interception loss by climate models that do not include the effects of spatial variability on interception processes. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 VL - 6 UR - sites/default/files/pubs/1993 Eltahir Bras_4.pdf ER - TY - JOUR T1 - ESTIMATION OF THE FRACTIONAL COVERAGE OF RAINFALL IN CLIMATE MODELS JF - JOURNAL OF CLIMATE Y1 - 1993 A1 - Eltahir, Elfatih A B A1 - Bras, R. L. KW - Amazon AB - The fraction of the grid cell area covered by rainfall, mu, is a very important parameter in the descriptions of land surface hydrology in climate models. A simple procedure is presented for estimating this fraction, based on extensive observations of storm areas and rainfall volumes. It is often observed that storm area and rainfall volume are linearly related. This relation is utilized in rainfall measurement to compute rainfall volume from radar observations of the storm area. The authors suggest that the same relation be used to compute the storm area from the volume of rainfall simulated by a climate model. A formula is developed for computing mu, which describes the dependence of the fractional coverage of rainfall on the season of the year, the geographical region, rainfall volume, spatial resolution of the model, and the temporal resolution of the model. The new formula is applied in computing mu over the Amazon region. Significant temporal variability in the fractional coverage of rainfall is demonstrated. The implications of this variability for the modeling of land surface hydrology in climate models are discussed. PB - AMER METEOROLOGICAL SOC CY - 45 BEACON ST, BOSTON, MA 02108-3693 VL - 6 UR - sites/default/files/pubs/1993 Eltahir Bras_2.pdf ER - TY - JOUR T1 - ON THE RESPONSE OF THE TROPICAL ATMOSPHERE TO LARGE-SCALE DEFORESTATION JF - QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Y1 - 1993 A1 - Eltahir, Elfatih A B A1 - Bras, R. L. KW - Amazon AB - Recent studies on the Amazon deforestation problem predict that removal of the forest will result in a higher surface temperature, a significant reduction in evaporation and precipitation, and possibly significant changes in the tropical circulation. Here, we discuss the basic mechanisms contributing to the response of the tropical atmosphere to deforestation. A simple linear model of the tropical atmosphere is used in studying the effects of deforestation on climate. It is suggested that the impact of large-scale deforestation on the circulation of the tropical atmosphere consists of two components: the response of the tropical circulation to the negative change in precipitation (heating), and the response of the same circulation to the positive change in surface temperature. Owing to their different signs, the changes in predicted temperature and precipitation excite competing responses working in opposite directions. The predicted change in tropical circulation determines the change, if any, in atmospheric moisture convergence, which is equivalent to the change in run-off. The dependence of run-off predictions on the relative magnitudes of the predicted changes in precipitation and surface temperature implies that the predictions about run-off are highly sensitive, which explains, at least partly, the disagreement between the different models concerning the sign of the predicted change in Amazonian run-off. PB - ROYAL METEOROLOGICAL SOC CY - 104 OXFORD ROAD, READING, BERKS, ENGLAND RG1 7LJ VL - 119 UR - sites/default/files/pubs/1993 Eltahir Bras_3.pdf ER - TY - CONF T1 - SPATIAL-DISTRIBUTION OF PRECIPITATION RECYCLING IN THE AMAZON BASIN T2 - WORLD AT RISK : NATURAL HAZARDS AND CLIMATE CHANGEWORLD AT RISK : NATURAL HAZARDS AND CLIMATE CHANGE Y1 - 1993 A1 - Eltahir, Elfatih A B A1 - Bras, R. L. ED - Bras, R. L. KW - Applied; Physics KW - Multidisciplinary KW - Physics AB -

Precipitation recycling is the contribution of evaporation within a large region to precipitation in that same region. The rate of recycling is a diagnostic measure of the coupling of land surface hydrology and regional climate. Here we describe the spatial and seasonal variability of the precipitation recycling process over the Amazon basin. The results are based on data of evaporation and water vapor fluxes from the European Center for Medium Range Weather Forecast (ECMWF). We estimate that 25% of all the rain that falls in the Amazon basin is contributed by evaporation within the basin. The contribution of recycled water vapor increases westward and southward with significantly different spatial distributions in the different seasons.

JF - WORLD AT RISK : NATURAL HAZARDS AND CLIMATE CHANGEWORLD AT RISK : NATURAL HAZARDS AND CLIMATE CHANGE T3 - AIP CONFERENCE PROCEEDINGS PB - US NATL SCI FDN, ENGN DIRECTORATE, NAT HAZARDS & MAN MADE HAZARD MITIGAT PR; OGRAM CY - SYMP ON THE WORLD AT RISK : NATURAL HAZARDS AND CLIMATE CHANGE, MIT, CAMBRIDGE, MA, JAN 14-16, 1992 VL - 277 SN - 1-56396-066-4 N1 - SYMP ON THE WORLD AT RISK : NATURAL HAZARDS AND CLIMATE CHANGE, CAMBRIDGE, MA, JAN 14-16, 1992 ER - TY - JOUR T1 - Drought Frequency Analysis of Annual Rainfall Series in Central and Western Sudan JF - HYDROLOGICAL SCIENCES JOURNAL-JOURNAL DES SCIENCES HYDROLOGIQUES Y1 - 1992 A1 - Eltahir, Elfatih A B KW - west africa AB - Rainfall is the most important water resource in central and western Sudan, a region affected by the recent drought in Africa. A general methodology for studying the annual rainfall process is presented and applied to data from central and western Sudan. It is assumed that certain time series models adequately describe the annual rainfall process in the region. Based on this assumption, the drought frequencies are calculated in the subregions with stationary series. The theory of runs is applied in calculating drought frequencies using a data generation method. PB - IAHS PRESS, INST HYDROLOGY CY - C/O SARAH CAGE, WALLINGFORD, ENGLAND OX10 8BB VL - 37 UR - sites/default/files/pubs/1992_Eltahir.pdf ER - TY - JOUR T1 - Forecast of Winter Flow in the Blue Nile JF - The Sudan Engineering Society Journal Y1 - 1992 A1 - Eltahir, Elfatih A B VL - 32 ER - TY - JOUR T1 - CORRECTION JF - JOURNAL OF HYDROLOGY Y1 - 1990 A1 - Eltahir, Elfatih A B PB - ELSEVIER SCIENCE BV CY - PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS VL - 114 ER - TY - JOUR T1 - A FEEDBACK MECHANISM IN ANNUAL RAINFALL, CENTRAL SUDAN JF - JOURNAL OF HYDROLOGY Y1 - 1989 A1 - Eltahir, Elfatih A B PB - ELSEVIER SCIENCE BV CY - PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS VL - 110 UR - sites/default/files/pubs/1989_Eltahir.pdf ER - TY - JOUR T1 - TESTS FOR THE SUSPECTED TREND IN ANNUAL RAINFALL SERIES IN CENTRAL AND WESTERN SUDAN JF - The Sudan Engineering Society Journal Y1 - 1988 A1 - Eltahir, Elfatih A B KW - annual rainfall KW - annual rainfall amounts KW - central and wester Africa KW - rainfall in the western and central Sudan AB -

It is widely noticed that the annual rainfall amounts to the entire region of central and western Africa is showing a decline for the last decade. The significance of the decline in the annual rainfall series in the region of central and western Sudan is investigated to check whether it is the random variability or there are indications of a trend in annual rainfall amounts.

VL - 30 ER -