A numerical study on precipitation over the western coastal area of Sumatra Island

It has been documented that significant amounts of tropical precipitation are concentrated in coastal areas, which is especially prominent over the western coast of Sumatra Island. The present study conducted 14-day-long numerical simulations, focusing on precipitation patterns around Sumatra Island. In the control experiment, the rainfall concentration was obscure, and dry biases were found. However, the model showed improvements in the rainfall peak fidelity, when the terminal velocity and effective radius of ice clouds were reduced in the cloud microphysics parameterization in addition to incorporating the warmer sea surface temperature (SST) around the coast. An analysis in the time-mean fields (such as water vapor, ice clouds, radiative heating etc.) demonstrated that inhibition of radiative cooling (associated with the upper-level ice clouds) and warmer SSTs induced anomalous updrafts, and thus, more moisture and precipitation were brought to the coast by enhancement of the vertical moisture transport. Comparing the simulation results with in situ observations, it was speculated that together with the climatologically warmer SSTs, the SST diurnal cycle and lateral transport of ice clouds from inland convection, which was strictly regulated by solar insolation, significantly contributed to the precipitation concentration around the coastal region.