A numerical study on precipitation over the western coastal area of
Sumatra Island
Abstract
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. We 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. Based on the
budget equation of the column-integrated frozen moist static energy
(which is nearly equivalent to the column-integrated moisture under the
weak temperature gradient assumption), we formulated 9 sensitivity
experiments. When the terminal velocity and effective radius of ice
clouds were reduced in addition to incorporating the warmer sea surface
temperature (SST) around the coast, the model showed better rainfall
peak fidelity. On the other hand, similar diurnal cycles of
precipitation were simulated in the sensitivity tests without any
conspicuous peaks in the coastal region, which indicated that the
precipitation concentration fidelity was irrelevant to that of the
diurnal cycle. An analysis in the time-mean fields 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 advection. Comparing the simulation results with
in situ observations, we 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.