Island Rainfall Enhancement in the Maritime Continent

The hypothesis that the islands of the Maritime Continent (MC) enhance total rainfall and time-mean upward motion is tested using a convection-permitting regional model. Sensitivity experiments with the islands removed greatly diminish both rainfall and upward motion, supporting the hypothesis. We examine the individual factors in this enhancement, isolating the impacts of the diurnal cycle from those of basic-state (i.e., constant) forcing of orography and the land surface. We find that the basic-state forcing by land is the only factor that substantially enhances total island rainfall, specifically through the enhancement of mean surface heat fluxes. The diurnal cycle and orographic forcing, however, substantially enhance rainfall in the seas surrounding the islands. Moreover, the diurnal cycle is found to be essential for promoting mesoscale circulations on the spatial scales of the islands (sea breezes), which are critical to both the upscale growth of deep convection and the most extreme rainfall rates.