J. Geophys. Res., 116, D08102, doi:10.1029/2010JD014695.

Rain on small tropical islands

Adam H. Sobel
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Columbia University, New York, NY.

Casey D. Burleyson and Sandra E. Yuter
Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC.

Abstract

A high-resolution rainfall climatology based on observations from the Tropical Rainfall Measuring Mission's Precipitation Radar (PR) instrument is used to evaluate the influence of small tropical islands on climatological rainfall. Annual mean climatological (1997-2007) rainfall over islands in the Indo-Pacific Maritime Continent and Caribbean regions is compared with that over the surrounding ocean regions. In addition to total rainfall, rain frequency and intensity are also analyzed. Results are stratified into two 12-hour halves of the diurnal cycle, and also by a measure of each island's topographic relief.

In both regions, there is a clear difference between larger islands (areas of a few hundred km^2 or greater) and smaller ones. Rain frequency and total rainfall are significantly enhanced over larger islands compared to the surrounding ocean. For smaller islands the enhancement is either negligibly small, statistically insignificant, or negative. The enhancement in total rainfall over larger islands is partly attributable to greater frequency and partly to greater intensity. A diurnal cycle in island enhancement is evident in frequency but not intensity, except over small Caribbean islands where the converse is true.

For the larger islands, higher orography is associated with greater rainfall enhancements. The orographic effect is larger (percentagewise) in the Caribbean than in the Maritime Continent. Orographic precipitation enhancement manifests more strongly in frequency than intensity, and is present during both day and night, suggesting that much of the orographic rainfall enhancement is attributable to mechanically forced upslope flow rather than elevated surface heating.