[1] Using a coupled regional climate-hydrologic modeling system, RAMS-Hydro, we investigate the role of the water table dynamics in controlling soil moisture, evapotranspiration (ET), boundary layer dynamics, and precipitation. In an earlier study we showed that a shallow water table can primarily exist in two types of hydrologic settings in North America: the humid river valleys and coastal regions in the east and the arid or semiarid intermountain valleys in the west. We also showed that the shallow water table in these settings can lead to significantly wetter soils than would exist without the presence of the water table. Here, we show that the water table-induced wetter soil directly maps into enhanced ET in the western setting, where soil water is a strong limiting factor of ET flux, but it is less likely to be the case in the more humid eastern setting where soil water is not limiting in general. We also ask whether any resulting enhanced ET will directly map into enhanced precipitation. Our hypothesis is that this can occur through two primary mechanisms: local, ET-driven enhancement of convective precipitation and enhanced regional or lateral moisture convergence caused by altered soil moisture fields, and hence altered ET, far from the region of concern. We find that, indeed, water table-induced higher ET in the arid west results in greater convective precipitation and that ET-precipitation coupling is primarily through local feedback pathways and precipitation recycling, with the main role of large-scale moisture convergence as an initiator of convection following dry periods. Transitioning to the more humid regions farther east, the greater atmospheric (relative to surface) control of precipitation progressively obscures any potential effects of the water table, and the effects of largescale moisture convergence tend to dominate.Citation: Anyah, R. O., C. P. Weaver, G. Miguez-Macho, Y. Fan, and A. Robock (2008), Incorporating water table dynamics in climate modeling: 3. Simulated groundwater influence on coupled land-atmosphere variability,

A fully coupled regional climate, 3D lake modeling system is used to investigate the physical mechanisms associated with the multiscale variability of the Lake Victoria basin climate. To examine the relative influence of different processes on the lake basin climate, a suite of model experiments were performed by smoothing topography around the lake basin, altering lake surface characteristics, and reducing or increasing the amount of large-scale moisture advected into the lake region through the four lateral boundaries of the model domain. Simulated monthly mean rainfall over the basin is comparable to the satellite (Tropical Rainfall Measuring Mission) estimates. Peaks between midnight and early morning hours characterize the simulated diurnal variability of rainfall over the four quadrants of the lake, consistent with satellite estimates, although the simulated peaks occur a little earlier. It is evident in the simulations with smoothed topography that the upslope/downslope flow generated by the mountains east of the lake and the land–lake breeze circulations play important roles in influencing the intensity, the location of lake/land breeze fronts, and the horizontal extent of the land–lake breeze circulation, as well as lake basin precipitation. When the lake surface is replaced with marsh (water hyacinth), the late night and early morning rainfall maximum located over the western sector of the lake is dramatically reduced. Our simulations also indicate that large-scale moisture transported via the prevailing easterly trades enhances lake basin precipitation significantly. This is in contrast to the notion advanced in some of the previous studies that Lake Victoria generates its own climate (rainfall) through precipitation–evaporation–reprecipitation recycling only.

Statistical downscaling can be used to efficiently downscale a large number of General Circulation Model (GCM) outputs to a fine temporal and spatial scale. To facilitate regional impact assessments, this study statistically downscales (to 1∕8°spatial resolution) and corrects the bias of daily maximum and minimum temperature and daily precipitation data from six GCMs and four Regional Climate Models (RCMs) for the northeast United States (US) using the Statistical Downscaling and Bias Correction (SDBC) approach. Based on these downscaled data from multiple models, five extreme indices were analyzed for the future climate to quantify future changes of climate extremes. For a subset of models and indices, results based on raw and bias corrected model outputs for the present-day climate were compared with observations, which demonstrated that bias correction is important not only for GCM outputs, but also for RCM outputs. For future climate, bias correction led to a higher level of agreements among the models in predicting the magnitude and capturing the spatial pattern of the extreme climate indices. We found that the incorporation of dynamical downscaling as an intermediate step does not lead to considerable differences in the results of statistical downscaling for the study domain.

We analyze the potential effect of global warming levels (GWLs) of 1.5 • C and 2 • C above pre-industrial levels (1861−1890) on mean temperature and precipitation as well as intra-seasonal precipitation extremes over the Greater Horn of Africa. We used a large, 25-member regional climate model ensemble from the Coordinated Regional Downscaling Experiment and show that, compared to the control period of 1971−2000, annual mean near-surface temperature is projected to increase by more than 1 • C and 1.5 • C over most parts of the Greater Horn of Africa, under GWLs of 1.5 • C and 2 • C respectively. The highest temperature increases are projected in the northern region, covering most parts of Sudan and northern parts of Ethiopia, and the lowest temperature increases are projected over the coastal belt of Tanzania. However, the projected mean surface temperature difference between 2 • C and 1. 5 • C GWLs is higher than 0.5 • C over nearly all land points, reaching 0.8 • C over Sudan and northern Ethiopia. This implies that the Greater Horn of Africa will warm faster than the global mean.While projected changes in precipitation are mostly uncertain across the Greater Horn of Africa, there is a substantial decrease over the central and northern parts of Ethiopia. Additionally, the length of dry and wet spells is projected to increase and decrease respectively. The combined effect of a reduction in rainfall and the changes in the wet and dry spells will likely impact negatively on the livelihoods of people within the coastal cities, lake regions, highlands as well as arid and semi-arid lands of Kenya, Tanzania, Somalia, Ethiopia and Sudan. The probable impacts of these changes on key sectors such as agriculture, water, energy and health sectors, will likely call for formulation of actionable policies geared towards adaptation and mitigation of the impacts of 1.5 • C and 2 • C warming.

Abstract:The International Center for Theoretical Physics (ICTP) regional climate model version 3 (ICTP-RegCM3) multiyear simulations of East Africa rainfall during the October-December, short rains season are evaluated. Two parallel runs; based on NCEP reanalysis and NASA FvGCM lateral boundary conditions are performed. The simulated monthly and seasonal rainfall climatology as well as the interannual variability are found to be fairly consistent with the observations. The model climatology over specific homogeneous climate subregions, except central Kenya (CKE) highlands, also reasonably agrees with the observed. The latitude-time evolution (intraseasonal variability) of the simulated seasonal rainfall exhibits two distinct modes of behavior. The first is a quasistationary mode associated with high rainfall throughout the season within the equatorial belt between 1°S and 2°N. The second mode is associated with the intertropical convergence zone (ITCZ)-driven southward migration of regions of rainfall maxima as the season progresses, which is also consistent with the observed. Furthermore, observed rainfall variability over distinct homogeneous climate subregions is also fairly reproduced by the model, except over central Kenya highlands and northeastern parts of Kenya. The spatial correlation between the simulated seasonal rainfall and some of the global teleconnections (DMI and Nino3.4 indices) shows that the regional model conserves some of the observed regional 'hot spots' where rainfall-ENSO/DMI associations are strong. At the same time, unlike observations, the model reveals that along the East Africa Rift Valley and over western parts of the Lake Victoria Basin, the association is weak, perhaps an indication that nonlinear interactions between local forcing (captured by the model) and large-scale systems either suppress or obscur the dominant influence of the teleconnections on rainfall over certain parts.

Characteristic patterns and changes in precipitation and temperature over the Greater Horn of Africa during the 20th and 21st century are analysed based on a sample of Coupled Model Intercomparison Project version 3 (CMIP3) models output. Analysis of the 11 CMIP3 models indicates that the equatorial eastern Africa region (including the entire Greater Horn of Africa (GHA)) have been experiencing a significant increase in temperature beginning in the early 1980s, in both A1B and A2 scenarios. All the Atmosphere Ocean Global Circulation Models (AOGCMs) analysed represent the correct mean annual cycle of precipitation, but there is a fairly large spread among the models in capturing the dominant bimodal peaks. In particular, all the models tend to overestimate the peak of the October-November-December (OND) season, while at the same time the peak of the March-April-May (MAM) season tends to be centered on May in the models instead of April as observed. The projected changes and probability distribution of minimum (T min ) and maximum (T max ) temperatures over the GHA sub-region based on PDFs constructed from daily values showed very diverse distributions for the present (1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000) and future (2046-2065; 2081-2100) periods. Whereas in the reference (1981-2000) the probability distribution functions (PDFs) constructed for both T min and T max , and during all the seasons had a near normal (but narrow) distribution, those of the future periods were quite diverse but generally very elongated, with significant shifts toward the positive tail. This generally implies that there is consensus among models and the ensemble mean about high likelihood of increase in extreme warmer T min and T max (more so T min ) in the future over the GHA region. Our results also show significant increase in the number of days with T min and T max greater the 2°C (above 1981-2000 average) by the middle as well as the end of 21st century in both the A1B and A1 scenarios. This is especially so during the June, July, and August (JJA) season where all the 92 days of the season indicate projected minimum temperature to increase by more than 2°C above the 1981-2000 average by the end of 21st century in both scenarios.

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[1] The changing climatic patterns and increasing human population within the Lake Victoria Basin (LVB), together with overexploitation of water for economic activities call for assessment of water management for the entire basin. This study focused on the analysis of a combination of available in situ climate data, Gravity Recovery And Climate Experiment (GRACE), Tropical Rainfall Measuring Mission (TRMM) observations, and high resolution Regional Climate simulations during recent decade(s) to assess the water storage changes within LVB that may be linked to recent climatic variability/changes and anomalies. We employed trend analysis, principal component analysis (PCA), and temporal/spatial correlations to explore the associations and covariability among LVB stored water, rainfall variability, and large-scale forcings associated with El-Niño/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). Potential economic impacts of human and climate-induced changes in LVB stored water are also explored. Overall, observed in situ rainfall from lake-shore stations showed a modest increasing trend during the recent decades. The dominant patterns of rainfall data from the TRMM satellite estimates suggest that the spatial and temporal distribution of precipitation have not changed much during the period of 1998-2012 over the basin consistent with in situ observations. However, GRACEderived water storage changes over LVB indicate an average decline of 38.2 mm/yr for [2003][2004][2005][2006], likely due to the extension of the Owen Fall/Nalubale dam, and an increase of 4.5 mm/yr over 2007-2013, likely due to two massive rainfalls in 2006-2007 and 2010-2011. The temporal correlations between rainfall and ENSO/IOD indices during the study period, based on TRMM and model simulations, suggest significant influence of large-scale forcing on LVB rainfall, and thus stored water. The contributions of ENSO and IOD on the amplitude of TRMM-rainfall and GRACE-derived water storage changes, for the period of [2003][2004][2005][2006][2007][2008][2009][2010][2011][2012][2013], are estimated to be 2.5 cm and 1.5 cm, respectively.