SIMULATION OF THE INDIAN MONSOON CLIMATOLOGY IN ECHAM3 CLIMATE MODEL - SENSITIVITY TO HORIZONTAL RESOLUTION

The ability of the current generation of climate models, in their long -term simulations, to replicate the observed atmospheric behaviour on a wide range of spatial and time scales provides support in applying t hese models to the greenhouse gas-induced climate change projections o n regional scales. A series of long time-slice control experiments hav e been performed recently with the ECHAM3 atmospheric general circulat ion model at T21, T42 and T106 horizontal resolutions (30 years each w ith T21 and T42 and 5 years with T106). All these model experiments us e a common sea-surface temperature climatology in their control experi ment inferred from a coupled ocean-atmosphere climate model (ECHAM1+LS G) experiment. In this paper, we examine the ability of the ECHAM3 mod el to simulate the Indian monsoon climatology at these three different horizontal resolutions. Because the Indian summer monsoon circulation evolves through a characteristic sequence of events, it is important that the climate models should be able to realistically portray these important features of the circulation over adequate spatial and time s cales. We focus on the model's simulation of selected variables repres entative of the thermal, dynamic and hydrological components in zonal mean cross-sections and area-averaged monthly as well as seasonal regi onal distributions. Generally, with respect to large-scale features of the circulation, the largest differences among the simulations occur at T42 relative to T21. At both T21 and T42 horizontal resolution, how ever, the model does not have a high degree of correspondence with obs ervations as regards the spatial distribution of mean sea-level pressu re, surface air temperature, 850 hPa winds and precipitation. On regio nal scales, T106 resolution best captures both the spatial and tempora l characteristics of the Indian climatology. Both the diurnal and seas onal cycles of area-averaged surface air temperature over the region s imulated by the model at T106 resolution are within 1 to 2 degrees C a s compared with observed climatology. The development and migration of the monsoon trough over Central India and the adjoining Bay of Bengal during the monsoon season is best simulated at T106 resolution. There is a distinct improvement in the spatial distribution as well as the total area-averaged summer monsoon rainfall in the model simulations w ith finer resolution. Although the modelled and observed mean summer p recipitation is similar in overall structure at T106 resolution, under estimation of the total seasonal rainfall in the model even at high re solution is a reflection of the sensitivity of simulated precipitation to local climate forcings, e.g. tropical convergence zone, and defici encies of parameterization schemes for convection and land surface pro cesses. (C) 1997 by the Royal Meteorological Society.