The origin of water-vapor rings in tropical oceanic cold pools

Abstract:

Tropical deep convection over the ocean is found to grow preferentially from thermodynamically pre-conditioned regions of high specific humidity and, thus, high moist static energy. For this reason, rings of enhanced specific humidity at the leading edges of evaporatively driven cold pools have recently received considerable attention. The prevailing theory explains these rings by the water-vapor source from the evaporation of rain drops below cloud base. Their origin is studied in this letter using large-eddy simulations of individual cumulus clouds that rise into a tropical atmosphere over ocean. It is demonstrated that -- in contrast to this theory -- water-vapor rings are primarily explained by surface latent-heat fluxes rather than by the evaporation of rain. This finding implies that conceptual models used in subgrid-scale parameterizations of deep convection should consider the formation of rings of increased specific humidity by the cold-pool-induced enhancement of surface fluxes.