Understanding the regionality and diurnal cycles of precipitation in the Lake Victoria Basin during Boreal fall

The diurnal cycle of rainfall in the Lake Victoria Basin of East Africa results from the super positioning of regional circulations driven by lake/land temperature differences and topography in the presence of the large-scale flow. Analysis of a triple-nested regional model simulation with a convective-permitting inner domain shows how these elements combine to produce the observed regionality of precipitation, including diurnal cycling, in boreal fall. A single diurnal rainfall peak occurs throughout the basin, but the time of maximum rainfall varies within the basin. The rainy period over the lake begins with precipitation over the northern part of the lake near 02Z (0500 LT), so it is not simply nighttime rain driven by lake/land breezes. Onset of the rainy period occurs only when low geopotential heights over the relatively warm lake cause a southward branch of the Turkana Jet to form. The formation of the jet depends on nighttime cooling over the Eastern Rift Mountains, which acts to direct the large-scale, moist flow around the topography. Topography also plays a role in the daytime rainy period over land in the Lake Victoria Basin. Moist divergence over the lake supports convergence and precipitation over the shore regions. Precipitation rates are twice the magnitude over the eastern shore compared with the western shore because daytime warming of the high elevations of the Eastern Rift Mountains allows the large-scale easterly flow to go over the mountains. This easterly flow converges with the lake/land circulation and doubles precipitation rates over the eastern shore.