Extreme rainfall events in the West African Sahel: Understanding storm development over the Damergou gap using convection-permitting simulations in the Weather Research and Forecasting model

Extreme rainfall events in the West African Sahel can be impactful, yet we do not completely understand why such storms develop. Here, we utilize NASA long-term Integrated Multi-satellitE Retrievals for Global precipitation measurement (IMERG) rainfall estimates, various atmospheric reanalyses, and Weather Research and Forecasting (WRF) convection-permitting simulations to further examine the regional/local conditions that led to the development of two extreme events over the Damergou Gap of Niger/Nigeria identified in a prior study. The August 20, 2019 central Niger event is associated with the passage of a westward-moving convective line. A strong thermal low over eastern Niger preconditions the environment by increasing the atmospheric moisture and vertical wind shear. Cold-pool outflow boundaries generated from afternoon convection over the higher terrain ahead of the approaching line enhances convergence along the line while slowing down the system's movement, resulting in higher-intensity rainfall for a longer time over the region. The July 19, 2001 northern Nigerian event has rainfall developing over the Jos Plateau in the afternoon. Guinean Highland ridging combined with low pressure over Niger/Chad produces a strong low-level height gradient associated with the development of a strong southwesterly flow surge that transports tropical moisture into the region. This surge interacts with the equatorward migration of the Sahel-tropical Africa dryline, enhancing the convergence and convection north of the Jos Plateau. Our results indicate that while extreme rainfall in the Damergou Gap is likely to occur in anomalously moist environments, it is not necessarily associated with highly unstable environments (e.g., convective available potential energy [CAPE] > 2,500 J.kg(-1)). Furthermore, interactions with cold-pool outflow boundaries generated from other convective areas is important, and local terrain features are influential in the development of such convective areas.

Automated summary

We utilized NASA IMERG rainfall estimates, various atmospheric reanalyses, and WRF simulations to examine the regional/local conditions that led to extreme rainfall events in the Damergou Gap. Our results indicate that these events occur in anomalously moist environments, but are not necessarily associated with highly unstable conditions. Furthermore, interactions with cold-pool outflow boundaries generated from other convective areas and local terrain features are influential in the development of such convective areas.