Understanding extreme diurnal convection over Lake Victoria from a convection permitting regional climate simulation

The Lake Victoria Basin is home to largest freshwater lake (Lake Victoria; LV) in Africa and second largest in the world. Each year hundreds of fisherman are lost on LV during intense night-time thunderstorms. LV is an essential component of the local economy while at the same time being one of the most hazardous lakes in the world. Despite this, until recently, understanding of the processes contributing to heavy rainfall events was very limited. In this study we present a 10-year (2005-2015) convection permitting (3km grid-spacing) simulation (CPS) of the Lake Victoria Basin using the RegCM version 4.7.0. A lake model is utilized in order to couple the lake regions with RegCM, which has been shown to be of great importance for simulating a realistic lake surface temperature (LST) and precipitation over LV. Mesoscale circulations associated with the diurnal cycle over LV are an important driver of intense night-time thunderstorms. An analysis of the diurnal rainfall cycle over LV shows that the CPS well represents the timing of nocturnal rainfall over the lake which is associated with a strong landbreeze, however the daytime peak in rainfall over the land surrounding the lake is too early relative to observed data. The temporal spectrum of lake rainfall shows a dominance in diurnal frequencies while intraseasonal timescales show only a very weak signal. Extreme nocturnal rainfall events exceeding 2STD of the lake rainfall timeseries are composited and separated for further analysis. These events show a clear migration from the previous daytime peak in rainfall over land, westward onto the lake during the night. To understand these diurnal precipitation events we explore mechanisms which may play a role in the events such as, the strength of the lake-landbreeze circulation, and convective instability.