How the Large-Scale Environment and Mesoscale Convective System Modulates the Rainfall Activity over Coastal West Africa

Shun-Nan Wu

University of Oklahoma, School of Meteorology


Tuesday, Mar 14, 2023, 2:00 pm MT

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This study uses IMERG satellite measurements, ERA5 reanalysis, and the Tracked IMERG Mesoscale Precipitation Systems dataset to examine the rainfall activity and associated Mesoscale Convective Systems (MCSs) over coastal West Africa and its variability with the large-scale disturbances. West Africa is where African Easterly Waves (AEWs) and MCSs tend to grow, propagate offshore, and subsequently develop into tropical storms in the western Atlantic. Therefore, understanding the factors that conduce strong rainfall activity over the West African coast can advance our understanding of what conditions can potentially lead to the downstream development of severe weather systems.

Over the coastal water of West Africa, MCSs can initiate over land and propagate offshore or initiate over the ocean. Both types of MCSs are crucial to conducing climatological rainfall maximum at the offshore coastal area, and the lifecycle of these MCSs is strongly tied to the diurnal cycle. While ocean-initiated MCSs contribute more to the offshore accumulated rainfall, we find that offshore-propagating MCSs are the major modulator of the offshore diurnal rainfall activity over West Africa. As the diurnal rainfall activity is highly correlated with daily accumulated rainfall around this offshore area, it is of interest to investigate what situation can potentially lead to an active diurnal rainfall activity. The active diurnal rainfall activity results from a greater number of more intense offshore-propagating MCSs, while ocean-initiated MCSs demonstrate less difference during active and inactive diurnal rainfall days. In addition to the impact from MCSs, this diurnal rainfall activity is also strongly modulated by the large-scale environment, such as the AEWs. We find that the diurnal rainfall activity tends to be stronger within the large-scale environment with a wetter atmospheric column and stronger near-surface convergence over the offshore area where the climatological rainfall is observed. The results above suggest that the accurate representation of the large-scale environment around the coastline and its modulation of the coastal diurnal cycle and MCSs is critical to predicting convective systems that can potentially develop into severe weather systems.

SPEAKER BIO: Shun-Nan Wu is a postdoctoral research scientist at the School of Meteorology at the University of Oklahoma. His research centers on understanding the intensifying dynamics of tropical cyclones and the development process of tropical convective systems. He specializes in utilizing satellite measurements to identify key precursors of the evolving process and adopting numerical models to investigate critical physical mechanisms for system development.

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