Rainfall enhancement over tropical islands and rectification of the diurnal cycle

Tropical islands are observed to be rainier than nearby ocean areas, and convection over land, even modest-sized islands, is typically more intense than over open oceans. Observations indicate that convective heating over tropical islands is strongly modulated by the diurnal cycle of solar insolation and surface enthalpy fluxes. Here we explore the hypothesis that diurnally-driven variability in the surface energy balance and land/sea breezes plays a key role in increasing time-mean rainfall rate and convective intensity over tropical islands. We explore how a highly idealized island, which differs only in heat capacity and moisture availability from the surrounding ocean, could rectify the diurnal cycle and impact the spatial distribution of rainfall in the tropics.

We perform simulations of radiative-convective equilibrium with the System for Atmospheric Modeling cloud-system-resolving model, with interactive surface temperature and a varied surface heat capacity. For the case of relatively small-scale simulations, where a shallow (~5 cm) “slab-island” surface is embedded in a deeper (~1 m) slab-ocean domain, the precipitation rate over the island can be more than triple the domain average precipitation rate, with island rainfall occurring primarily in a strong regular convective event each afternoon. We discuss the importance of island size and surface moisture availability in determining the magnitude of simulated island rainfall enhancement, and discuss dynamical mechanisms for the difference in convective activity over the island and background ocean. We also find that the upper troposphere typically warms with the inclusion of an island, which may have implications for larger-scale overturning circulations in the tropics.