Assessment of coupled regional climate model (RegCM4.6–CLM4.5) for Indian summer monsoon

A model experiment with a regional climate model (RegCM4.6) coupled with an updated land–surface scheme (Community Land Model; CLM4.5) is integrated for 34 years to study the Indian summer monsoon (ISM). An eastward shift (towards Bay of Bengal; BoB) in simulated monsoon features due to the secular shift of the moist convective systems is observed. Further diagnostics show lower temperature over India as compared to BoB; while the Findlater jet is significantly weakened over the Arabian Sea. The low-pressure drags the southwesterly towards BoB through India. The Bowen ratio and evaporation reflect less soil moisture availability over the land. The analysis of simulated omega, moist static energy and Hadley circulation illustrates that an atmospheric condition dominates over land which further suppresses the convection and hence the rainfall. However, favorable atmospheric condition supporting convection and moisture availability leads to heavy precipitation over BoB and hence causes the eastward shift in simulated ISM rainfall. The convective fraction contributes the major share to the total precipitation. An evaluation of appropriate land surface spin-up period in order to investigate the optimum stabilization period for CLM4.5, indicates a possible weak coupling between land and atmosphere components in model simulation. This led to the improper energy and moisture exchanges between land and atmosphere in the simulation. The present work also suggests that, the simulated mean rainfall is not predominantly influenced by the length of different spin-up periods (up to 25 years) in case of weak coupling of land and atmosphere.