Numerical simulations of the precipitation along the coastal areas of Sumatera Island

The satellite observations have revealed that significant amounts of precipitation concentrate in the coastal areas in the tropics (e.g., Ogino et al.2016). The western coast of Sumatera Island is one of the remarkable regions with the rainfall concentration(e.g., the left panel of the figure), since amounts of the climatological precipitation are especially large around that area. However, the mechanism still remains unclear.

The present study conducted numerical simulations in order to reproduce precipitation patterns off the coast of Sumatera Island, making use of a cloud(-system)-resolving model SCALE (Scalable Computing for Advanced Library and Environment). SCALE is a basic library for weather and climate model of the earth and planets, and has been developed by RIKEN AICS, Japan.

In the control experiment, the rainfall concentration along the western coast of Sumatera Island is obscure, although the small peak of the precipitation is located (the right panel of the figure), and dry biases are found in the corresponding region. Therefore, focusing on moisture budgets, we conducted 4 sensitivity experiments, in which (1) the terminal velocity of cloud ice is reduced (ICE experiment), (2) sea surface temperatures are raised along the western coast of Sumatera Island (SST experiment), (3) the terminal velocities of snow and graupel are reduced (WW experiment), and (4) the terminal velocity of rain is reduced (Nr0 experiment).

While precipitation is significantly inhibited and enhanced over the entire regions of the ocean in WW and Nr0 experiments, respectively, the precipitation peak along the western coast of Sumatera Island is more distinct in ICE and SST experiments than in the control, WW and Nr0 experiments. However, the concentration of the precipitation is still insufficient, compared with the observation. Accordingly, we further conducted an additional numerical simulation, combining the sensitivity tests of ICE and SST. It is found that the combination of the reduced terminal velocity of cloud ice and higher sea surface temperatures along the coast results in higher fidelity of the precipitation patterns (especially the precipitation peak along the western coast of Sumatera Island).

Based on the results obtained from these simulations, we speculate that the lateral transport of cloud ice from the Sumatera Island to the ocean, which is associated with the strong diurnal cycle of the inland precipitation, plays an essential role in the concentration of the precipitation along the coastal region.