Land-Ocean Transitions in the Tropics: What Makes MCSs Persist?

Mesoscale convective systems (MCSs), long-lived convective clusters spanning more than 100 km horizontally, are known to be the dominant source of rainfall in the tropics, and the longest-lived MCSs are shown to be largely responsible for tropical extreme precipitation [Roca and Fiolleau, 2020]. Globally, the most extreme storms tend to be located over land, and the most intense storms over oceans tend to be adjacent to land, where motion is favored from land to ocean, e.g. tropical West Africa and the adjacent Eastern Atlantic Ocean [Zipser et al., 2006]. These systems are organized and maintained by the atmospheric characteristics needed for deep convection (moisture, instability, and lift), and the presence of vertical wind shear [Schumacher and Rasmussen, 2020]. Dry soils seem to have a large influence on strengthening organized convection [Klein and Taylor, 2020]. However, the mechanisms behind the intensification or dissipation of MCSs advected from land to sea are not well established yet.
To address this shortcoming, we investigate the evolution of MCSs emerging from satellite data over tropical Africa and the Eastern Atlantic Ocean. We use a database of tracked MCSs from infrared satellite data, TOOCAN [Fioellau and Roca, 2013]. Using these data we built a lagrangian tracker by which groups of MCSs - occurring in spatial proximity of each other with a 15 deg x 15 deg patch - are followed. We study the evolution of the cloud field within the patch, initiated at the time and latitude of maximum convective activity in the season. We superimpose a collocated satellite precipitation dataset, IMERG, to gain insight into the precipitation field related to the tracked MCSs, and study the environmental properties (temperature, wind profiles) using ERA5 reanalysis datasets. Over land, we find (i) that the MCS cover exhibits a clear diurnal cycle with peaks in the late afternoon and (ii) the lagrangian patch moves with a near constant velocity. Over ocean, we find a (i) decrease of the MCS cover which does not correspond to a decrease in precipitation and that (ii) at times the MCS evolution becomes stationary, corresponding to near-zero wind profiles. By generalizing these results to five years of tracked MCSs, we aim to gain insight into what environmental conditions are necessary for the development of strongly organized MCS fields over the coastal regions and the ocean, which, if persistent in time, could eventually evolve into tropical cyclones.