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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, which are critical to both the upscale growth of deep convection and the most extreme rainfall rates.

Plain Language Summary Long-term mean rainfall is greater in the Maritime Continent (MC) than its neighboring oceans, yet an explanation for this has long eluded us. In this study, we have examined this issue through experiments with a high-resolution numerical model, wherein we remove either the islands or the daily (diurnal) cycle by imposing daily-mean solar heating to test its impact. The results indicate that the enhancement of total rainfall over the islands is the result of the low heat capacity of islands compared to oceans and is not impacted by the diurnal cycle. With such low heat capacity, solar energy received at the island surface is rapidly transferred to the atmosphere, in turn enhancing rainfall. Yet the findings also indicate that the diurnal cycle is vital for triggering stronger thunderstorms and higher local rain rates in the MC region.