The Contribution of Mesoscale Convective Systems to the Coastal Rainfall Maximum over West Africa

This study examines the behaviors of coastal mesoscale convective systems (MCSs) to understand why the maximum climatological rainfall appears offshore of West Africa using satellite measurements. West Africa has a higher amount of rainfall offshore than inland during the monsoon season, but the reason for this offshore rainfall maximum remains unclear. MCSs that propagate offshore from coastal West Africa can develop into high-impact weather events in the eastern Atlantic, yet we know little about the MCS transitions across coastlines. Therefore, better understanding of how MCSs evolve during the coastal-to-maritime transition and contribute to the offshore climatological rainfall maximum over West Africa is key to advancing the knowledge of high-impact weather in this coastal region. The offshore rainfall can be contributed by either offshore-propagating MCSs generated over land or the MCSs initiated over the ocean. While continental MCSs are known to have more intense deep convection, their frequency of propagating offshore is found to be small. Instead, we find that the offshore environment supports the frequent formation of maritime MCSs, leading to their dominant contribution to offshore rainfall. However, an exception is found for the 88–108N offshore area, where the presence of high terrain generates MCSs with intense rainfall that more frequently propagate offshore and enhance offshore rainfall. These MCSs also often intensify their rainfall and expand in size as they move offshore. These results suggest that understanding the frequency, intensity, and transition of MCS generated over land and the ocean is key to better comprehending and simulating the rainfall activity over the coastal-to-marine transition zone.