Mechanisms Controlling Rainfall over Idealized Tropical Islands in Radiative–Convective Equilibrium

More rain falls over tropical islands than over the surrounding ocean regions. As part of an ongoing attempt to characterize the climatology of precipitation over tropical islands, we present results obtained from a set of three-dimensional radiative-convective equilibrium simulations performed with the System for Atmospheric Modeling, SAM, where we explore the effects of variations in sea-surface temperature and surface moisture on rainfall over highly idealized, low-heat-capacity circular islands of varying sizes, embedded in a slab-ocean domain. We find that daily averaged precipitation over the islands changes at rates varying from -3% to 9% per Kelvin of warming of the sea surface, depending on island size and SST, and is consistently highest over islands with radii in the 24-to-36-kilometer range. Additionally, islands with no surface moisture register higher precipitation averages than those with completely moist surfaces. We analyze how dynamical and thermodynamic aspects of the sea-breeze circulation, as well as the effects of precipitation-induced cold pools, explain some of the observed features of this daily cycle, such as rainfall distribution, time of highest precipitation, and changes in atmospheric moisture convergence with varying SST and surface moisture. In seeking explanations for this enhancement, we hope to gain understanding of localized atmospheric convection, and elucidate the connection between convection over the Maritime Continent and the Walker circulation.