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Island Rainfall Enhancement in the Maritime Continent

Presentation Date
Wednesday, December 9, 2020 at 4:00am



The hypothesis that the islands of the Maritime Continent (MC) enhance total regional 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, lending support to 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 related to both orography and the land surface. We find that the basic‐state forcing by land is the only factor that substantially enhances total island-mean 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, and also promote mesoscale rainfall organization within the islands. The diurnal cycle is found to be especially critical to driving both the strongest rainfall rates and greatest spatial organization of deep convection over the islands. This nonlinear diurnal time-scale feedback operates as follows. The diurnal heating/cooling cycle drives mesoscale circulations at the spatial scales of the islands, such as the land–sea breeze, which in turn drive both nocturnal upscale growth and intensification of deep convection, and hence explain the most extreme rainfall rates.

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