Over eastern Africa a monumental geological process is occurring that will eventually split the countries of Somalia, Kenya, Ethiopia, Tanzania, and Mozambique from the rest of Africa. The African tectonic plate is spreading along a line from the Red Sea in the north to Mozambique in the south, forming an enormous valley surrounded by some of the tallest mountains in Africa. At the centre of this Great Rift Valley sits the second largest freshwater lake in the world, Lake Victoria.
Lake Victoria from the ISS, https://en.wikipedia.org/wiki/Lake_Victoria
Lake Victoria also happens to be situated on the equator and is subject to year-round thunderstorms which have an extraordinarily distinct diurnal cycle. During the daytime, solar heating warms the land surrounding the lake at a faster rate than the lake itself creating a local lake breeze circulation which focuses thunderstorms around the periphery of the lake. Once solar heating recedes in the evening this circulation begins to reverse due to the thermal inertia of the lake and thunderstorm activity migrates to the lake itself in connection with a land breeze. These nocturnal and morning thunderstorms are often hazardous to fishers on the lake with annual reports of about 1000 fatalities from weather related accidents on the lake each year (Watkiss et al. 2020). With around 30 million people living in and around Lake Victoria’s shores, and over 200,000 fishers operating on the lake (LVFO 2022), there is a clear need for efficient monitoring and communication of meteorological hazards in the Lake Victoria basin (LVB).
Waterbus catamaran capsized near Kenyan coast 2 May 2020, Roberts et al. (2022)
The recent multinational High Impact Weather Lake System (HIGHWAY) program (Roberts et al. 2022) sought to address these needs by developing new weather warning systems for the region and fostering collaboration with local weather service agencies. The project included field campaigns and the installation of a weather radar in Entebbe, Uganda along the northern coast of Lake Victoria which augments another weather radar along the southern coast operated by Tanzania. These radars enhance the ability of forecasters to see hazardous weather over the lake and provide better warnings to fishers.
Lightning strike density during the afternoon (a) and night-time (b) in the Lake Victoria region from the Earth Networks Global Lightning Network dataset. Figure 3 from Roberts et al. (2022).
Recent research efforts have also been focused on modelling studies of hazardous thunderstorms over the LVB such as Thiery et al. (2016), which used high resolution model simulations to show that strong daytime thunderstorm activity over land is related to subsequent nighttime strong storm activity. Strong daytime thunderstorms will cool the land surrounding the lake, most notably through cold pools, thereby weakening the daytime temperature gradient between the lake and land. However, during the subsequent nighttime, the cooler land surface leads to an enhanced gradient toward the lake, and this leads to an enhanced local land breeze.
As part of the Climate Extremes over Lake Victoria Basic (ELVIC; van Lipzig et al. 2023) project, high-resolution (3km) regional climate simulations were conducted with the RegCM version 4.7.1 at the International Centre for Theoretical Physics (ICTP) in Trieste, Italy (Glazer et al. 2023). This 10-year simulation of the LVB provided an opportunity to study hazardous nocturnal thunderstorms over the lake with a high-resolution model that can resolve individual thunderstorms and convection. In the simulations, convection is explicitly produced without a large-scale convection scheme, and the lake is coupled to the atmosphere through a lake model. In the study by Glazer et al. (2023) the mechanisms leading to extreme precipitation events over the lake at night were analyzed by compositing extreme, normal and dry events. Cold pools appear to play larger role in the propagation or triggering of storms in the extreme composite compared to normal precipitation events. Interestingly, convective instability appears similar in each of the extreme and normal composites, however the extreme composite shows greater dynamical convergence over the lake which could be the result of a stronger land breeze or the effects of cold pools. This stronger forcing for triggering storms may be a key ingredient for strong convection at night over Lake Victoria.
References:
Glazer, R., E. Coppola, F. Giorgi, (2023) Understanding nocturnally-driven extreme precipitation events over Lake Victoria in a convection-permitting model. Mon. Wea. Rev. (In review)
Lake Victoria Fisheries Organization. (2022) STATUS OF FISHING EFFORT ON LAKE VICTORIA UP TO 2016. Downloaded from https://www.lvfo.org/content/documents-0.
Lipzig, N.P.M.v., Walle, J.V.d., Belusic, D. et al. (2023) Representation of precipitation and top-of-atmosphere radiation in a multi-model convection permitting ensemble for the Lake Victoria Basin (East-Africa). Clim. Dyn. https://doi.org/10.1007/s00382-022-06541-5.
Roberts, R. D., and Coauthors, 2022: Taking the HIGHWAY to Save Lives on Lake Victoria. Bull. Amer. Meteor. Soc., 103, E485–E510, https://doi.org/10.1175/BAMS-D-20-0290.1.
Thiery W., E. L. Davin, S. I. Seneviratne, K. Bedka, S. Lhermitte, and N. P. van Lipzig, 2016: Hazardous thunderstorm intensification over Lake Victoria. Nat. Commun., 7, 12786, https://doi.org/10.1038/ncomms12786.
Watkiss, P., R. Powell, A. Hunt, F. Cimato 2020: The Socio-Economic Benefits of the HIGHWAY project. Technical Report (UK Met Office, World Meteorological Organization, UKaid).
