Lake-Effect Rainfall over Three East African Lakes

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This project is an examination of lake effect rainfall over three of the African Great Lakes: Lake Victoria, Lake Tanganyika, and Lake Turkana. These lakes are an essential resource for East Africa, providing fisheries, fresh water, and hydroelectric power to Tanzania, Kenya, and Uganda, and Lake Victoria also contributes to the flow of the Nile. Lake Victoria has a round shape and much larger surface area than Tanganyika and Turkana, which are long narrow rift valley lakes. But despite their differences in size, shape, and setting, all three experience lake effect rainfall, or an enhancement of rainfall over the lake relative to its surroundings. The lake effect is particularly intense over Victoria, where it takes the form of nighttime thunderstorms accompanied by strong winds and high waves. The thunderstorms create a severe hazard for the fishing fleet, claiming between three and five thousand lives every year. In addition to the thunderstorm hazard, lake effect rain is of interest for its contribution to the water balance of the lakes, which have suffered declining water levels resulting in hardship for nearby populations. There have been few attempts to estimate the contribution of over-lake rainfall to variability and trends in lake water level, and existing studies have considerable uncertainty.The goal of this project is to provide a detailed climatology of the rainfall regimes over the lakes and to examine the meteorological factors that modulate these regimes, with particular attention to the relationship between over-lake rainfall and regional-scale winds. Specific objectives are to 1) develop quantitative statistics on over-lake rainfall, including magnitude, diurnal cycle, seasonality of occurrence, interannual variability, and relationship to rainfall in the catchment and/or surrounding land; 2) establish the meteorological controls on over-lake rainfall; 3) assess recent changes in the rainfall regime over the lakes, perhaps as a result of anthropogenic changes such as increased lake temperature; and 4) determine whether the changes are commensurate with changes in the surrounding catchment or land.Although lake effect precipitation is the common thread in the research, preliminary results suggest that the mechanisms of over-lake rainfall enhancement may differ from one lake to another. Over Lake Victoria, with its large surface area and siting within a topographic depression, rainfall enhancement could result from simple land breeze dynamics, in which winds converge over the lake at night when the hillslopes cool but the lake is still warm (up to 3.5C warmer than its shores). Rainfall enhancement over Lake Tanganyika is less pronounced and is limited to the wet season, with two distinct maxima over the lake. These characteristics lead the PI to hypothesize that over-lake rainfall is controlled primarily by the interaction of large-scale wind and topography rather than downslope winds and land breezes. The PI further hypothesizes that rainfall over Lake Turkana is influenced by both the slope winds that develop around Lake Victoria and by the strong low-level jet that flows through the Turkana gap. The implications of these mechanisms for the variability of over-lake rainfall and its likely response to climate change is examined as part of the project. The work is conducted using a variety of observational datasets including a rain gauge dataset compiled by the PI and others, satellite data from the Global Precipitation Measurement Mission, and surface winds from spaceborne scatterometers.The work is of societal interest due to the importance of the lakes for East Africa, the hazards posed by severe over-lake convection, and the relevance of the lake effect mechanisms for anticipating the future evolution of lake levels in a changing climate. Results of the work are communicated to stakeholders through local organizations, for instance ACLENet and the Nile Basin Initiative. The project also develops datasets of interest to researchers and decision makers including detailed climatologies of over-lake rainfall and surface winds at high spatial and temporal resolution. The project also provides support and training for a graduate student.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.