Optimizing physical scheme selection in RegCM5 for improved air–sea fluxes over Southeast Asia

Abstract. This study evaluates the performance of RegCM5 in simulating air–sea fluxes over Southeast Asia through a set of 36 sensitivity experiments testing different physical scheme combinations. Scheme choices vary across five model aspects: radiative transfer, planetary boundary layer, cumulus convection, parameterized microphysics and cloud fraction. A multi-criteria decision-making framework is applied to rank model configurations based on their ability to reproduce spatiotemporal patterns of sea surface wind, latent and sensible heat fluxes, precipitation, and radiative heat fluxes, using mostly satellite-based reference products. No configuration performs consistently best across all criteria: scores assessing latent and shortwave radiative heat fluxes are generally conflicting, with each other and with the scores for precipitation and sea surface wind which instead tend to agree. The choice of cumulus convection scheme drives the performance in simulating the latter two variables, with Tiedtke outperforming and Kain–Fritsch underperforming. In contrast, the best shortwave radiative heat flux simulations are obtained with MIT cumulus convection, in combination with CCM3 radiative transfer. Overall, RRTM/UW-PBL/Tiedtke/SUBEX/Xu–Randall – using the same order of model aspects as listed in the beginning – stands out by maintaining relatively high scores across all assessed variables. Nonetheless, a stronger dissensus in precipitation outputs suggests that reliable rainfall patterns may be a higher priority for decision makers, highlighting CCM3/UW-PBL/Tiedtke/NoTo/Xu–Randall and RRTM/Holtslag/Tiedtke/NoTo/Xu–Randall as the best configurations for this variable. Beyond statistics, further analysis reveals key monsoon-related biases: Indian Summer Monsoon rainfall is generally underestimated, Western North Pacific Summer Monsoon features are overestimated and shifted northward, near-equatorial regions exhibit excessive boreal summer rainfall in most Tiedtke experiments, and austral summer monsoonal sea surface wind and precipitation only impact areas directly north of Australia without inducing the rainfall annual maximum observed in that season over equatorial seas. These findings provide a basis for selecting optimal physics in RegCM5 over Southeast Asia and offer guidance for future applications, including air–sea coupled regional climate modeling.