Lethal night-storms over Lake Victoria

Lake Victoria, situated among Uganda, Kenya, and Tanzania, is a vital food source for around 30 million people living at its shore. With a surface of close to 70,000 km², it is the second-largest lake in the world, and arguably also the most lethal: The lake is a notoriously dangerous place for the 200,000 people who go fishing there at night. The International Red Cross estimates that between 3,000 and 5,000 fishermen per year lose their lives in violent storms on the lake [1]. But despite Lake Victoria¡¯s long-standing bad reputation, the potential consequences of climate change for this massive lake remained unknown until very recently.

Converging breezes brew a stormy cocktail

Lake Victoria can be so stormy at night because of the circulation in the atmosphere above its enormous water surface. During the day, a breeze develops that flows from the cool water towards the warm land. At night, the opposite happens: the land breeze flows away from the cooling land towards the warmer lake. As the lake is shaped like a circle, these land breezes converge above the lake from all directions. Add evaporation to this cocktail and you get a lot of storms, rain, wind, and waves.

In a new study [2] that combined the efforts of researchers from ETH Zurich and the University of Leuven, Belgium, we were able to provide scientific evidence for this pattern. In collaboration with the American space agency NASA, we developed a new satellite product which maps the number of severe thunderstorms and their locations in East Africa ¨C every 15 minutes for a period ranging from 2005 to 2013. During the day, most storms rage over the surrounding land, especially the typical afternoon thunderstorms that are caused by local upsurges of warm air. At night, these storms concentrate above Lake Victoria.

Intensifying nightly storms

To predict the impact of climate change on this process, our research team also ran climate simulations using an advanced computer model. In a business-as-usual scenario, where the emission of greenhouse gases continues to increase, the extreme amounts of rainfall over Lake Victoria will increase by twice as much as the rainfall over the surrounding land. As a result, the lake will become even more of a hotspot for nighttime storms. Superstorms that today only occur once every 15 years will occur almost every year by the end of the century. Lake Victoria is therefore likely to remain the most dangerous stretch of water in the world.

Developing an early warning system for the region

By now several organisations realise the danger associated with these lake storms, and they have stepped in to help develop warning systems for local fishermen. One promising development in this respect is the effort of the World Meteorological Organisation together with the UK Met Office and local telecommunication companies to provide weather alerts on mobile phones [3]. Fishermen who join the service receive a text message based on a simple traffic light system, warning them in case severe thunderstorms are forecast over the lake.

The results of our new study [2] have the potential to further optimize such warning systems, as they make it possible to improve the forecasting of these extreme storms. In particular, we found that afternoon conditions on land influence how strong nighttime storms on the lake will be, suggesting greater predictability than has been assumed so far. Based on these new insights, we already developed a prototype of a new storm-prediction system. Supporting ongoing efforts, this new system may help to reduce the vulnerability of the local communities that live, work, and depend on Lake Victoria.