Assessment of the Diurnal Cycle of Precipitation over the Maritime Continent Simulated by a 20 km Mesh GCM Using TRMM PR Data

Abstract

This study investigates precipitation over the Maritime Continent, comparing the precipitation simulated by a 20 km-grid Meteorological Research Institute General Circulation Model (MRI-GCM) and the near-surface rain data of TRMM 2A25. The focus is particularly the diurnal cycle and its phase distribution of precipitation. The features of the simulated precipitation mostly agree well with observations made over islands and straits having horizontal scales smaller than 200 km. However, these are quite different around larger islands, such as Sumatra and Borneo, particularly in the phase of the diurnal cycle. The MRI-GCM indicates maximum precipitation in the early afternoon on these islands, while the observed precipitation has its maximum at night. In particular, over the inland areas of the larger islands, the simulated diurnal cycle has almost a reversed phase. The simulated precipitation is remarkably weaker than the observation around the western coast of Sumatra Island, where a large discrepancy is also found in the phase distribution along a line perpendicular to the coast. A higher-resolution simulation using a non-hydrostatic model without convective parameterizations substantially improves the phase distribution over Borneo Island. The non-hydrostatic model simulates well the migration of the precipitation zone and the daily maximum at night in the inland areas. In contrast, the GCM fails to simulate the diurnal cycle over islands whose horizontal scale is larger than 200 km, although the 20 km grid spacing is small enough to reproduce the major aspects of the local circulations. The cause for this seems to be the cumulus convective parameterization, which may not adequately represent the coupling of convection and local circulations.