Nocturnal Offshore Convection: The Role of the West African Monsoon and Land Breeze during CPEX-CV

The land breeze-monsoon mechanism is well-documented over the Maritime Continent. In monsoonal offshore areas, the convergence of westerly monsoon and land breeze flows during late nights and early mornings can facilitate localized convergence, supporting the offshore propagation and development of nocturnal convection. While studied on the West African Guinea Coast, this mechanism remains unexplored in the West African coast, region of the offshore rainfall maximum. During Research Flight Nine (RF9; 22 September) of the 2022 NASA CPEX-CV field campaign, a mesoscale convective system (MCS) that was initially coupled to, and dynamically forced by an African easterly wave (AEW) was sampled by the instrumented DC-8 as it was propagating off the western coast of Africa. This MCS was categorized through TAMS (Tracking Algorithm for Mesoscale Convective Systems) as a long-lived Mesoscale Convective Complex.

Dropsonde observations documented evidence of a density current related to an active land breeze event at the coast while the DAWN Doppler wind lidar documented the West African Monsoon activity. ERA5 reanalysis confirms the existence of such density current associated with a land breeze. The recession of the monsoonal flow from the coastal shore based on ERA5 is indicative of offshore flow at the surface and lower boundary layer. The analysis of the MCS characteristics and local environment was done using flight observations, reanalysis, and combined TAMS and AEW tracks. This data reveals that the coastal mechanism partially supported the propagation and maintenance of the MCS as it interacted with the coast before it continued expanding and moving northward with the center of the AEW. Once the MCS recoupled to the AEW, it eventually developed into Tropical Storm Hermine on 23 September. The results motivate future modeling studies with the goal to improve the inclusion and representation of such coastal mechanisms over this region. The results as well as including the representation of such mechanisms in our models could further the understanding of how high-impact weather like AEWs, and MCSs interact and are maintained over the western coast of Africa.