The Influence of Synoptics on Mesoscale Convective Systems and Associated Diurnal Cycle over Coastal West Africa

This study investigates the variability of diurnal rainfall over coastal West Africa and the corresponding changes in mesoscale convective systems (MCSs) and synoptic environment. Coastal West Africa is where African easterly waves (AEWs) propagate offshore and transition from continental to marine environment, which can determine its probability to grow into severe weather systems in the western Atlantic. As AEWs propagate offshore, they interact with MCSs that are strongly tied to diurnal rainfall variability over the coastal area, which can lead to different development of AEWs. Therefore, investigating what factors influence diurnal rainfall activity over the West African coast can advance our understanding of what conditions are conducive to strong diurnal rainfall activity over this coastal region. We use IMERG rainfall retrievals, ERA5 reanalysis, and the Tracked IMERG Mesoscale Precipitation Systems dataset to investigate how the strong diurnal rainfall activity varies over the West African coast and relates to different types of MCS passage and the overall synoptics. Over coastal West Africa, the maximum amplitude of the diurnal rainfall cycle occurs offshore between 7-10°N, rather than coastal inland. The climatological rainfall maximum also occurs in this offshore region, suggesting that the diurnal cycle explains a large fraction of rainfall variability over coastal West Africa. Rainfall over coastal water is predominantly attributed to MCSs that initiate over the ocean or over land and propagate offshore. Among those two types of MCSs, the greater number and stronger intensity of offshore propagating MCSs lead to the greater amplitude of the diurnal rainfall over West Africa. Synoptic variability, such as AEWs, strongly modulates the diurnal rainfall by influencing the number and intensity of offshore propagating MCSs. The diurnal rainfall activity tends to be stronger when the AEWs impose higher moisture and stronger near-surface convergence over this offshore area. These results indicate that representation of the synoptic scale environment and MCS activity is crucial to capturing the localized diurnal rainfall over the West African coast. More analyses will be presented in the conference.