Satellite Estimation of Precipitation over Africa's Rift Valley Lakes

This presentation examines rainfall over numerous lakes of the Rift Valley and in their surrounding catchments. The focus in on Lake Victoria, the world's third largest lake, but rainfall over and around 26 other lakes is described. Both the seasonal cycle and diurnal cycle are examined. Lake Victoria precipitation was examined using TRMM 3B43, IMERG, CHIRPS2, and PERSIANN- CDR. There was excellent agreement among the satellite products for catchment rainfall but a large disparity among them for over-lake rainfall. IMERG, in particular was an outlier, overestimating over-lake rainfall by about 36%.

Two most important conclusion of this study is that over nearly all 27 lakes rainfall is enhanced compared to rainfall in the surrounding catchment. The enhancement of over-lake rainfall is 20 to 50% for Lake Victoria, depending on the season. It is strongest in April and weak in July, a particularly dry month. For annual rainfall the enhancement is 40%. For the remaining lakes the enhancement ranges from 10% to 210%, with no enhancement evident over Lakes Edward, Kyoga, and Mweru. Enhancement is generally evident in all months, even during the dry seasons and contrast between lake and catchment is generally greatest at night.

Over Lake Victoria the diurnal maximum is around 0600 or 0900 LST, but between 1200 and 1500 LST over the catchment. Skies are generally clear over the lake in the afternoon, when wind divergence prevails over the lake. The majority of lakes have a bimodal maximum in the diurnal cycle, with strong peaks at 06 to 09 LST and 18 to 21 LST. The daytime maximum over the lakes might represent a drift of land systems over the lake. Except for the large lakes there is strong uniformity in the times of maximum and minimum rainfall (18 LST and 12 LST, respectively) over both the lakes and the catchments. Nocturnal rainfall is common over both the lakes and the catchments. That over the land might be a drift of the systems generated over the lakes.