Identifying Key Controls on Storm Formation over the Lake Victoria Basin

• Published in: Monthly weather review 2019-09, Vol.147 (9), p.3365-3390
• Main Authors: Woodhams, Beth J., Birch, Cathryn E., Marsham, John H., Lane, Todd P., Bain, Caroline L., Webster, Stuart
• Format: Article
• Language: English
• Subjects: Air circulation; Atmospheric circulation; Atmospheric moisture; Availability; Breeze circulation; Bulging; Computer simulation; Convection; Convective storms; Diurnal; Diurnal cycle; Diurnal variations; Dry season; Fishers; Lake basins; Lakes; Land breezes; Life cycle; Life cycles; Mathematical models; Moisture; Moisture availability; Mountain winds; Mountains; Offshore; Propagation; Rain; Seasonal variation; Seasonal variations; Simulation; Storm development; Storms; Winds
• ISSN: 0027-0644; 1520-0493
• DOI: 10.1175/MWR-D-19-0069.1

The Lake Victoria region in East Africa is a hot spot for intense convective storms that are responsible for the deaths of thousands of fishermen 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 life cycle of two storms are investigated using Met Office Unified Model convection-permitting simulations with 1.5 km horizontal grid spacing. The two cases are analyzed 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. 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 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 modeled 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 analyzed 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 keys for skillful forecasts over the lake.