Effect of cumulus parameterization on the Indian summer monsoon simulated by the COLA General Circulation Model

The sensitivity of the Indian summer monsoon to cumulus convection parameterization is investigated using the Center for Ocean-Land-Atmosphere studies (COLA) General Circulation Model (GCM). Two simulations for a period of nine years are performed with observed sea surface temperature, one parameterizing convection using the Kuo scheme and the other using a type of Arakawa Schubert scheme (the Relaxed Arakawa-Schubert RAS scheme). Dynamic fields such as winds, velocity potential, etc., are compared with the corresponding fields of National Center for Environmental Prediction (NCEP) reanalysis data sets. The model simulated precipitation is compared with the corresponding observed precipitation data. The seasonal climatology of nine years of model integration shows that the differences between the two cumulus convection schemes have significant impact on the Indian summer monsoon circulation and its associated precipitation. The large-scale features are better simulated with the RAS scheme than with the Kuo scheme. The RAS scheme simulates stronger low level westerly over the Arabian sea, which leads to more moisture transport from the Arabian sea to the central India, than the Kuo scheme. The convective heating over the Bay of Bengal and the Arabian sea regions is larger with the RAS scheme than with the Kuo scheme, except near the surface and in the upper troposphere. With the RAS scheme, the convective heating maximum in the upper troposphere H more intense in the Bay of Bengal region than in the Arabian sea. Compared to the RAS scheme, the Tibetan anticyclone becomes weaker and shifts eastward, accompanying weaker tropical easterly jet, with the Kuo scheme. Similarly the divergent outflow in the upper troposphere associated with Asian monsoon is reduced significantly with the Kuo scheme than with the RAS scheme. Although both schemes are unable to simulate the Indian monsoon rainfall close to observation, the RAS scheme simulates and resolves better the positions of rainfall maxima over the Bay of Bengal and west coast regions. These differences are attributed to the closures and feedbacks of the two cumulus parameterization schemes. The GCM simulated time series of rainfall with both schemes shows that the intra-seasonal variation (monsoon cycle) is phase-locked to the seasonal migration.