Synoptic Modulation of the West African Coastal Atmosphere and Mesoscale Convective Systems

This study uses high-resolution airborne data from the NASA Convective Process Experiments–Cabo Verde field campaign to examine the effects of synoptic conditions on mesoscale convective systems (MCSs) over the western coastal waters of West Africa. African easterly waves (AEWs) are the dominant source of synoptic variability over the region, but their influences on coastal areas remain unclear. This study compares airborne data from 2 days with contrasting synoptic conditions and MCS evolution over the coastal water. On one day, large MCSs with high rain rates persisted over the coastal water when the AEW trough was near the coastline. The flow modulation by the AEW strengthened the near-surface onshore flow and warmed and moistened the boundary layer over the coastal water. These combined effects increased the equivalent potential temperature of inflow air into the coastal MCSs, likely supporting their development. On the other day, an AEW was absent or weak, and coastal MCSs remained small and short lived without the effects of the AEW. The observed modulation of the coastal atmosphere and MCSs by AEWs during the field campaign is analogous to the historical relationship of AEW and coastal MCSs obtained by satellite and reanalysis data. Although not represented by the two field campaign cases, the historical analysis suggests the enhancement of coastal vertical wind shear by AEWs can also intensify coastal MCSs. These results highlight the importance of synoptic modulation in understanding the variability and mechanism of coastal MCSs, which differ from those over land or open ocean.