Rain on small tropical islands

We are interested in understanding what controls precipitating convection. One question of interest is: on what scale a given forcing must act in order to produce a measurable convective response? Islands are useful to investigate this as they have very well-defined sizes and provide inhomogeneities in the lower boundary condition that are unchanging in time (although their influence on convection depends on season, time of day etc.). We examine the annual mean climatological precipitation over of a large set of tropical islands in the Indo-Pacific maritime continent region, using data from the NASA TRMM Precipitation Radar at a resolution of 0.05x0.05 degrees (around 5 km). The precipitation over each island is compared to that over the oceanic region immediately adjacent to it. There is a systematic enhancement of rainfall over islands compared to the surrounding ocean, on the order of 20-50%, for islands with areas of a few hundred square km or larger. Stratifying the results by topography does not make a large systematic difference, suggesting that island height is less important than island area in determining the island-averaged rainfall enhancement in this region. A preliminary analysis of the Caribbean, however, suggests a larger role for topography there.