Mesoscale Patterns of Rainfall, Cloudiness and Evaporation over the Great Lakes of East Africa

Abstract

By employing remote sensing techniques, the characteristics of rainfall, cloudiness and evaporation over Lakes Victoria, Tanganyika and Malawi are studied. There exist diurnal cycles induced by the interaction between lake/land breeze and the lower level southeasterlies over each lake, particularly Lake Victoria. Generally, maximum convection/rainfall is in the night to early morning on the west side of the lake while on the other side of the lake it is in the afternoon to early evening. The mean annual over-lake rainfall is greater than mean annual catchment rainfall by 30% and 20% for Lakes Victoria and Tanganyika, respectively. Lake Victoria is divided into four quadrants based on the fact that convection is stronger in the northern quadrants and that the diurnal cycles in the east and west quadrants are quite different. Convective activity is compared among the four quadrants and the catchment. Temporal and spatial distribution of cloud cover resembles that of convection. Annual mean cloudiness is 0.5, 0.45 and 0.38 for the above three lakes, respectively. For Lake Victoria both the monthly and annual mean cloudiness maps demonstrated a daytime cloudiness minimum and a nighttime cloudiness maximum over the lake. Evaporation is calculated for each lake by energy-budget and combined-Penman methods. Both a fixed cloudiness scenario and a varied cloudiness scenario are used as cloud cover input. Calculated values range from 1537 to 1669 mm/year for Lake Victoria, 1559 to 1721 mm/year for Lake Tanganyika and 1698 to 1901 mm/year for Lake Malawi. In order to investigate the influence of cloudiness, evaporation calculation is also carried out in each quadrant and in each pixel of Lake Victoria. The results show that evaporation rate is highest in the middle of the lake and decreases in both the east and the west directions.