Simulated Physical Mechanisms Associated with Climate Variability over Lake Victoria Basin in East Africa

Abstract

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.

Variability of East African rainfall based on multiyear Regcm3 simulations

“…Also the 60-km spatial resolution used in the present study is possibly not fine enough to effectively capture the spatial and temporal structure of precipitation over the complex and steep escarpments over CKE highlands. A recent case study by Anyah et al, 2006 show that RegCM3 coupled to 3D lake model at a spatial resolution of 20 km represents the observed rainfall over CKE highlands fairly well. However, because of computational limitations it was not possible to perform multiyear simulations at that spatial resolution in the present study.…”

“…Lake Victoria lies astride the equator and is also sandwiched by a chain of high mountains, oriented in north-south direction on both sides. This provides an enabling environment for frequent development of active convection and precipitation throughout the season sustained by the diurnally driven lake/land-breeze and mountain/valley circulations (Anyah et al, 2006). We examined the 'potential' contribution of the Lake Victoria induced precipitation to the persistent wet conditions simulated over the equatorial belt by performing a separate 4-month simulation of the 1988 short rains season, without activating the 1D lake submodel (for Lake Victoria).…”

“…In Figure 6(b) it is noticed that the tongue of wet conditions along the equatorial belt, shown in Figure 6(a), is significantly suppressed. Since the present study is not focused on the contribution of Lake Victoria, we refer readers to our recent paper, Anyah et al (2006) that discusses in details the simulated physical mechanisms associated with the interactions between local topography and Lake Victoria and their influence on regional rainfall variability. 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 Years 19821984198619881990199219941998Years 19821984198619881990199219941998 2000 Years …”

Understanding the performance of the FLake model over two African Great Lakes

“…However, to investigate the two-way interactions between climate and lake processes over East Africa, a correct representation of lakes within regional climate models (RCMs) and general circulation models (GCMs) is essential (Stepanenko et al, 2013; see Appendix for a list of all acronyms, variables and simulation names). For now, the high computational expense of complex hydrodynamic lake models limits the applicability of coupled lake-atmosphere model systems to process studies (Anyah et al, 2006;Thiery et al, 2014). To overcome this issue, the Freshwater Lake model (FLake) was recently developed (Mironov, 2008;Mironov et al, 2010).…”

Impact of soil moisture initialisation and lateral boundary conditions on regional climate model simulations of the West African Monsoon

“…The main idea behind this approach is to combine the large-scale response of GCMs to global forcing with the ability of RCMs to resolve smaller scale climate features (Dickinson et al 1989;Giorgi 1990;Jones et al 1995;Christensen et al 1998;Giorgi and Mearns 1999;Hong and Leetma 1999;Denis et al 2002). RCMs can also serve as experimental tools to understand regional climate processes and perform sensitivity studies (Gochis et al 2002;Seneviratne et al 2002;Wang et al 2004;Vannitsem and Chomé 2005;Anya et al 2006;Rowell and Jones 2006;Cuadra and da Rocha 2007;Yhang and Hong 2008). Although significant progress has been made in improving the physical formulation of RCMs, the most common remaining problem is the error inherited from the driving conditions, which are provided either by global analyses of observations or by GCM integrations (Noguer et al 1998;Giorgi and Bi 2000;Seth and Rojas 2003;Rojas and Seth 2003).…”