West African Sahel and Soudan extreme rainfall events are impactful when strong mesoscale convective systems (MCSs) produce large amounts of rainfall in short periods. NASA IMERG rainfall estimates and the ERA5 reanalysis are examined to understand where the top 100 highest 12Z – 12Z 24-h rainfall totals and MCS storm genesis occur, and to assess the relative importance of environmental conditions in their generation including the influence of atmospheric moisture and vertical wind shear.
Most of the top 100 events are located south of 14°N over the Soudan. Events cluster over three regions, namely, Mali, Burkina Faso, and northern Nigeria. The associated MCSs are typically not locally generated, forming instead at distances greater than 100 km upstream. Composites reveal that a significant increase in atmospheric moisture content occurs prior to development, but there is no evidence of significant changes in the 600 – 925 hPa vertical wind shear. This indicates that changes in vertical wind shear are less influential in extreme storm development than atmospheric moisture preconditioning.
The top 10 events are further evaluated. A change in these storms’ direction and speed near the maximum rainfall location is common, suggesting the MCSs are reorganizing around peak rainfall intensity time. Three atmospheric conditions are associated with these events. They are (1) moisture preconditioning of the atmosphere, (2) interaction of the storm in the wake of a region of anticyclonic flow, and (3) interaction of the storm in the wake of a region of anticyclonic flow and the Sahel/tropical dryline boundary.
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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.
more » « less- Award ID(s):
- 1929074
- NSF-PAR ID:
- 10419799
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Quarterly Journal of the Royal Meteorological Society
- Volume:
- 149
- Issue:
- 752
- ISSN:
- 0035-9009
- Page Range / eLocation ID:
- p. 959-983
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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