Mesoscale precipitation and cloud patterns over the Rift Valley lakes and implications for regional hydrology.

The large lakes of the African Rift Valley evoke a mesoscale pattern of circulation that modify both the intensity and diurnal cycle of rainfall and cloudiness. Over Lake Victoria, for example, rainfall is some 30% greater than over the surrounding land catchment. Cloudy skies and rainfall occur at night on the western half, but during the afternoon on the eastern half. This markedly influences both the lake-rainfall and evaporation terms of the water balance. This paper presents the results of a study of rainfall and cloudiness over Lakes Victoria, Tanzania and Malawi, based on high resolution satellite data. The results are used to produce a reliable estimate of water balance of each of the lakes and sensitivity studies are performed. Relatively small changes in cloudiness can have a significant influence on lake levels. An accurate assessment of water balance requires consideration of both the diurnal cycle and spatial variability of rainfall and evaporation over the lake.