Controls on the convective environment in the Lake Victoria region and their interactions with large-scale climate variability

The Lake Victoria region is inhabited by over 40 million people and is a major source of food, water and economic activity in East Africa. As the intertropical convergence zone passes by, this region experiences two rainy seasons that result in extreme precipitation events and flash flooding. Over 80% of extreme rainfall around Lake Victoria is produced by mesoscale convective systems (MCSs), which are characterised by organised convection spanning a few hundred kilometres, and often lasting several hours. Here, we tracked 4,811 MCSs between 2014 and 2019 that moved over Lake Victoria and lasted longer than 3 hours. A clustering algorithm was applied to identify different types of MCSs crossing this region: cross-lake storms that initiate overnight East Africa time, lake-to-land storms that initiate in the morning, and land-to-lake storms that initiate in the afternoon. We examined conditions of the local environment leading to the development of these storms to link them to larger scale climate variability, such as the Madden-Julian Oscillation (MJO). The MJO is an eastward-propagating envelope of suppressed and enhanced convection that originates over the western Indian Ocean. Active MJO convection over the western Indian Ocean (MJO phase 2) creates favourable zonal wind anomalies for the formation of lake-to-land and land-to-lake storms. In addition, MJO phase 2 is also likely to influence specific humidity and temperature anomalies prior to and during the formation of land-to-lake storms. Conditions over the Lake Victoria region during days when no storms occur are similar to the conditions created during active MJO phase 6, i.e., when active MJO convection is over the Maritime Continent. Our analysis is used to inform a machine learning model that will predict the probability of a given storm type occurring over this region in order to improve predictions of high-impact weather over the lake.