A Simple Conceptual Model for Rainfall Over Flat Tropical Islands

Observations and simulations show that rainfall is enhanced over flat tropical islands relative to surrounding ocean areas. Previous work has explained this island rainfall enhancement as a consequence of rectification of the diurnal cycle of convection in response to periodic heating from the surface. A quantitative model, however, is lacking for the diurnal timing of rainfall, as well as the magnitude of time-mean rainfall enhancement over the island and its sensitivity to island size and background temperature. In this work, we present a conceptual model of rainfall over flat tropical islands as a consequence of an inward-propagating density current – representing the combined effects of the sea breeze and cold pools from earlier precipitation over the island – periodically sweeping out and lifting mass from the subcloud layer over the island. This model predicts that rainfall peaks later in the day for larger islands, that daily-mean island rainfall scales with the integrated water vapor content of the subcloud layer, and that an optimal island size for time-mean rainfall enhancement occurs when ventilation by the density current is most in phase with buoyancy of air over the island. We compare the model to simulations across a range of surface temperatures, island sizes, and surface evaporative efficiencies over the island, and we discuss sensitivity to two key factors: the radial distribution of mass fluxes and the efficiency with which lifted boundary layer moisture is converted to surface rain.