Identifying key controls on storm formation over the Lake Victoria Basin

The Lake Victoria region in East Africa is a hotspot for intense convective storms that are responsible for the deaths of thousands of fisherman each year. The processes responsible for the initiation, development and propagation of the storms are poorly understood and forecast skill is limited. Key processes for the lifecycle of two storms are investigated using Met Office Unified Model convection-permitting simulations with 1.5 km horizontal gridspacing. The two cases are analysed alongside a simulation of a period with no storms to assess the roles of the lake–land breeze, downslope mountain winds, prevailing large-scale winds and moisture availability. Whilst seasonal changes in large-scale moisture availability play a key role in storm development, the lake–land breeze circulation is a major control on the initiation location, timing and propagation of convection. In the dry season, opposing offshore winds form a bulge of moist air above the lake surface overnight that extends from the surface to ~1.5 km and may trigger storms in high CAPE/low CIN environments. Such a feature has not been explicitly observed or modelled in previous literature. Storms over land on the preceding day are shown to alter the local atmospheric moisture and circulation to promote storm formation over the lake. The variety of initiation processes and differing characteristics of just two storms analysed here show that the mean diurnal cycle over Lake Victoria alone is inadequate to fully understand storm formation. Knowledge of daily changes in local-scale moisture variability and circulations are key for skilful forecasts over the lake.