Identifying key controls on storm formation over the lake Victoria basin

Abstract

The Lake Victoria region in East Africa is a hot spot for intense convective storms that are responsible forthe deaths of thousands of fishermen each year. The processes responsible for the initiation, development, andpropagation of the storms are poorly understood and forecast skill is limited. Key processes for the life cycle oftwo storms are investigated using Met Office Unified Model convection-permitting simulations with 1.5 kmhorizontal grid spacing. The two cases are analyzed alongside a simulation of a period with no storms to assessthe roles of the lake-land breeze, downslope mountain winds, prevailing large-scale winds, and moistureavailability. While 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 ofconvection. In the dry season, opposing offshore winds form a bulge of moist air above the lake surfaceovernight 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 modeled in previous literature. Storms over landon the preceding day are shown to alter the local atmospheric moisture and circulation to promote stormformation over the lake. The variety of initiation processes and differing characteristics of just two stormsanalyzed here show that the mean diurnal cycle over Lake Victoria alone is inadequate to fully understandstorm formation. Knowledge of daily changes in local-scale moisture variability and circulations are keys forskillful forecasts over the lake.