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The purpose of this article is to determine the meteorological factors controlling the lake-effect rains over Lake Victoria. Winds, divergence, vertical motion, specific humidity, convective available potential energy (CAPE), and convective inhibition (CIN) were examined. The local wind regime and associated divergence/convergence are the major factors determining the diurnal cycle of rainfall over the lake and catchment. The major contrast between overlake rainfall in the wet- and dry-season months is the vertical profile of omega. This appears to be a result of seasonal contrasts in CAPE, CIN, and specific humidity, parameters that play a critical role in vertical motion and convective development. Significance Statement: The results of this study suggest that local factors (the anabatic–katabatic flow and lake–land breezes) do not play a role in the seasonal contrast in the lake-effect rains. The major contrast between the dry and wet seasons is evident in the vertical profile of omega, which is stronger and extends higher in the atmosphere during the wet season. Atmospheric humidity is higher during the wet season, modifying the values of CAPE and CIN. By modulating omega, these determine the seasonal cycle of lake-effect rains. These results have implications for predicting changes in the lake that will occur under global warming. [ABSTRACT FROM AUTHOR]

This article examines the diurnal cycle of lake-effect rains over Lake Victoria and of rainfall in the surrounding catchment. The analysis focuses on four months, which represent the two wet seasons (April and November) and the two dry seasons (February and July). Lake-effect rains are strongest in April and weakest in July. In all cases there is a nocturnal rainfall maximum over the lake and a daytime maximum over the catchment, with the transition between rainfall over the lake and over the catchment occurring between 1200 and 1500 local standard time (LST). During the night the surrounding catchment is mostly dry. Conversely, little to no rain falls over the lake during the afternoon and early evening. In most cases the maximum over the lake occurs at either 0600 or 0900 LST and the maximum over the catchment occurs around 1500–1800 LST. The diurnal cycle of mesoscale convective systems (MCSs) parallels that of overlake rainfall. MCS initiation generally begins over the catchment around 1500 LST and increases at 1800 LST. MCS initiation over the lake begins around 0300 LST and continues until 1200 LST. While some MCSs originate over the highlands to the east of the lake, most originate in situ over the lake. Maximum MCS activity over the lake occurs at 0600 LST and is associated with the systems that initiate in situ. Significance Statement: This article presents two new conclusions about rainfall over Lake Victoria. One is that rainfall is enhanced over the lake in all months, even during the dry seasons. The other is that most nocturnal mesoscale systems actually develop in situ over the lake. The results enhance our understanding of the Lake Victoria system, which helps to sustain some 30 million people in East Africa and some 300 million living in the Nile basin. With the threat of climate change looming large, increased knowledge of the lake system will allow for improved estimation of its impact. [ABSTRACT FROM AUTHOR]