SIMULATIONS OF THE INDIAN-SUMMER MONSOON USING A NESTED REGIONAL CLIMATE MODEL - DOMAIN SIZE EXPERIMENTS

Seasonal simulations of the Indian summer monsoon using a 50-km region al climate model (RCM) are described. Results from three versions of t he RCM distinguished by different domain sizes are compared against th ose of the driving global general circulation model (AGCM). Precipitat ion over land is 20% larger in the RCMs due to stronger vertical motio ns arising from finer horizontal resolution. The resulting increase in condensational heating helps to intensify the monsoon trough relative to the AGCM. The RCM precipitation distributions show a strong orogra phically forced mesoscale component (similar in each version). This co mponent is not present in the AGCM. The RCMs produce two qualitatively realistic intraseasonal oscillations (ISOs) associated respectively w ith monsoon depressions which propagate northwestward from the Bay of Bengal and repeated northward migrations of the regional tropical conv ergence zone. The RCM simulations are relatively insensitive to domain size in several respects: (1) the mean bias relative to the AGCM is s imilar for all three domains; (2) the variability simulated by the RCM is strongly correlated with that of the driving AGCM on both daily an d seasonal time scales, even for the largest domain; (3) the mesoscale features and ISOs are not damped by the relative proximity of the lat eral boundaries in the version with the smallest domain. Results (1) a nd (2) contrast strongly with a previous study for Europe carried out with the same model, probably due to inherent differences between mid- latitude and tropical dynamics.