On the difference in the seasonal cycle of rainfall over India in the Community Climate System Model (CCSM2) and Community Atmospheric Model (CAM2)

Abstract

The Community Climate System Model (CCSM2) provides a realistic simulation of seasonal cycle of rainfall over India. The Community Atmosphere Model (CAM2), which is the atmospheric component of CCSM2, when forced with observed monthly mean SST, does not simulate accurately the seasonal cycle of rainfall over India. This unusual behaviour is investigated by using a diagnostic model. The rainfall simulated by both CCSM2 and CAM2 is much higher than observed for a value of given vertical integrated water vapor (Pwat). The main difference between the two models is the seasonal variation of Pwat. Pwat in CAM2 is larger than CCSM2 in the month of May over India. This is on account of the advection of air with high Pwat from Arabian Sea to Indian region. In CCSM2, Pwat is not high because the simulated SST in the Arabian sea in CCSM2 is 2°C lower than observed. Hence the combined effect of lower Pwat and a steeper gradient in the Pwat-rainfall relationship cause the simulation of seasonal cycle of rainfall in CCSM2 to be realistic.