Role of cumulus parameterization on the seasonal and diurnal precipitation over Southeast Asia in RegCM4

This study examines the sensitivity of the seasonal mean and diurnal precipitation simulated by the Regional Climate Model version 4 (RegCM4) to cumulus parameterization in the Southeast Asia (SEA) domain. Based on the same lateral boundary conditions from the interim European Centre for Medium-Range Weather Forecast reanalysis data (ERA-interim), RegCM4 integrations using the Emanuel cumulus convection scheme over all grid boxes in the model (EE), and those using the Emanuel (Grell) scheme in ocean (land) areas (referred to as the mixed scheme, or MC) were carried out at a 50km x 50km horizontal resolution in the period of 2000-2010. It was found that both MC and EE have comparable performance in capturing the mean circulation features in SEA during boreal summer. Simulations based on EE tend to produce a seasonal wet bias over South China Sea (SCS). In comparison, while the mean rainfall over SCS and east of the Philippines is improved by the use of MC, conditions in western coastal IndoChina become too wet. For the diurnal cycle (DC) of precipitation, it was reasonably well captured by both cumulus schemes; however, in comparison to EE, MC consistently underestimates the DC amplitude. Empirical Orthogonal Function (EOF) analyses revealed that, while the first leading mode of diurnal rainfall was reproduced by both schemes, the second mode was suppressed in the MC simulations. In particular, this mode corresponds to the afternoon rainfall over inland locations including western IndoChina and southeastern China. MC tends to produce more cloudy conditions, cooler surface air temperature and hence a more stable environment and weaker convection at 1200-1500 local time in these locations. Over the Maritime Continent, the second mode is associated with evening-to-midnight rainfall peaks in the mountain ranges of Sumatra, Borneo and New Guinea. There is weaker orographic precipitation at 1800-0000 local time by MC compared to EE, associated with weaker diurnal convergence and upward motion as well as a drier environment in the MC simulations.