Toward Improved Short-Term Forecasting for Lake Victoria Basin. Part II: Preconvective Environment Analysis with ERA5

Abstract Lake Victoria is the largest freshwater lake in Africa, with around 30 million people living on its coastline, and it serves as one of the largest natural resources for East African communities due to its prosperous fishing industry. However, around 1000 fishermen die annually on the lake due to severe weather-related accidents. Radar-based research from the “High Impact Weather Lake System” (HIGHWAY) project in 2019 confirmed the marked diurnal cycle on Lake Victoria, studied over decades, where organized, intense convective systems pose a major risk to the fishermen operating overnight. Building upon the results from Part I of this study, we investigate the preconvective environment over the lake for the modes that have been previously identified with a radar-based classification for the two wet seasons in 2019. ERA5 reanalysis data show that in 2019, instability and steeper low-level lapse rates were higher during season I [March–May (MAM)], allowing unorganized storms overnight to have stronger downdrafts, increasing the potential for strong and damaging winds over the lake. Second, the multicell linear mode in season II [October–December (OND)] and at nighttime presents significantly low RH 700–500hPa , which might indicate potential strong winds at the surface (evaporative cooling). Third, bulk shear was higher in season I 2019 for almost all modes, with some modes indicating the capacity to organize into multicell systems and even some to have rotating updrafts. Finally, some modes in season I, at nighttime and early morning, present high storm-relative helicity values in midlevels, which, combined with high bulk shear, may lead to embedded rotations in dynamically complex systems. Significance Statement In the present work, we use ERA5 reanalysis retrieved soundings over Lake Victoria in Africa to investigate the preconvective environments for convective modes during the wet seasons in 2019, previously identified with weather radar. The lake is a world hotspot for severe weather phenomena with a high yearly death toll, especially for the fishing community. The results of our analysis provide updated information about convective environments on the lake, with a focus on operational marine forecasts.