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 these models to the greenhouse gas‐induced climate change projections on regional scales. A series of long time‐slice control experiments have been performed recently with the ECHAM3 atmospheric general circulation model at T21, T42 and T106 horizontal resolutions (30 years each with T21 and T42 and 5 years with T106). All these model experiments use a common sea‐surface temperature climatology in their control experiment inferred from a coupled ocean–atmosphere climate model (ECHAM1+LSG) experiment. In this paper, we examine the ability of the ECHAM3 model 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 scales. We focus on the model's simulation of selected variables representative of the thermal, dynamic and hydrological components in zonal mean cross‐ sections and area‐averaged monthly as well as seasonal regional 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, however, the model does not have a high degree of correspondence with observations as regards the spatial distribution of mean sea‐level pressure, surface air temperature, 850 hPa winds and precipitation. On regional scales, T106 resolution best captures both the spatial and temporal characteristics of the Indian climatology. Both the diurnal and seasonal cycles of area‐averaged surface air temperature over the region simulated by the model at T106 resolution are within 1 to 2°C as 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 with finer resolution. Although the modelled and observed mean summer precipitation is similar in overall structure at T106 resolution, underestimation of the total seasonal rainfall in the model even at high resolution is a reflection of the sensitivity of simulated precipitation to local climate forcings, e.g. tropical convergence zone, and deficiencies of parameterization schemes for convection and land surface processes. © 1997 by the Royal Meteorological Society. Int. J. Climato ., 17: 847–858 (1977) (No. of Figures: 8. No. of Tables: 1. No. of References: 15.)