THE SIMULATED INDIAN MONSOON - A GCM SENSITIVITY STUDY

A series of sensitivity experiments are conducted in an attempt to und erstand and correct deficiencies in the simulation of the seasonal mea n Indian monsoon with a global atmospheric general circulation model. The seasonal mean precipitation is less than half that observed. This poor simulation in seasonal integrations is independent of the choice of initial conditions and global sea surface temperature data used. Ex periments are performed to test the sensitivity of the Indian monsoon simulation to changes in orography, vegetation, soil wetness, and clou diness. The authors find that the deficiency of the model precipitatio n simulation may be attributed to the use of an enhanced orography in the integrations. Replacement of this orography with a mean orography results in a much more realistic simulation of Indian monsoon circulat ion and rainfall. Experiments with a linear primitive equation model o n the sphere suggest that this striking improvement is due to modulati ons of the orographically forced waves in the lower troposphere. This improvement in the monsoon simulation is due to the kinematic and dyna mical effects of changing the topography, rather than the thermal effe cts, which were minimal. The magnitude of the impact on the Indian mon soon of the other sensitivity experiments varied considerably, but was consistently less than the impact of using the mean orography. Howeve r, results from the soil moisture sensitivity experiments suggest a po ssibly important role for soil moisture in simulating tropical precipi tation, including that associated with the Indian monsoon.