Understanding Nocturnally Driven Extreme Precipitation Events over Lake Victoria in a Convection-Permitting Model

Abstract Each year, on the order of 1000 fishers are lost on Lake Victoria (LV) due to hazardous weather and intense nighttime thunderstorms making LV one of the most hazardous lakes in the world. In this study, we present a 10-yr (2006–15) convection-permitting (3-km grid spacing) simulation of the LV basin (LVB) using the nonhydrostatic version of RegCM (RegCM-NH). We construct composites of three different types of events over the lake during nighttime and early morning hours: 1) Dry events in which no rain fell over the lake, 2) Normal events in which rainfall over LV matches that of the annual mean, and 3) Extreme events in which rainfall exceeded the 99.99th percentile. In Extreme, an enhanced land breeze takes place along the western side of LV, resulting from convection along the high terrain to the west of the lake in the early evening and nighttime hours which cools the boundary layer through evaporative cooling, cold pools, and prolonged radiatively driven cooling. Cold pools are identified as playing a role in the triggering of convection over the lake and/or the propagation of convection during Extreme events. Greater dynamical forcing in the lead up to an event exists in Extreme relative to Normal, while convective instability over LV is similar in Extreme and Normal. Dry events exhibit hazardous wind conditions over the lake occurring outside the main rainy season.