Effects of Large-Scale Perturbations on the Evolution of Coastal Offshore Convection over West Africa

This study uses the high-resolution airborne data from NASA CPEX-CV field campaign data to examine the effects of monsoon and synoptic conditions on coastal offshore convection over West Africa. West Africa is one of the major monsoonal regions where climatologically high rainfall is observed immediately offshore of its coastline. This high rainfall is contributed by the frequent offshore initiation of convective systems and the offshore propagation of intense continental mesoscale convective systems. However, there is large day-to-day variability in the evolution of coastal offshore convection that remains challenging to predict. To understand what large-scale conditions are associated with the variability of coastal offshore convection, three research flights during the field campaign focused on observing the structure of the offshore convection and its environment. The airborne data obtained from those research flights reveal the detailed structures of monsoon flow, thermodynamics, and convective structure that have not been observed collectively before. The collected data show that offshore convection becomes more organized when synoptic and monsoon circulation constructively interfere to strengthen both mechanical and thermodynamic forcing for convection development. In particular, when the trough of the African Easterly Wave is over the West African coast, its associated circulation intensifies the monsoonal onshore flow to enhance boundary-layer convergence and deepen the moist layer near the coastline. Such a condition maintains and grows offshore convection, while scattered isolated convection becomes prominent otherwise. These results suggest that the influence of African Easterly Waves on the coastal monsoon environment strongly modulates coastal convection development and evolution.