Numerical Simulation with the NCAR Global Circulation Model of the Mean Conditions During the Asian-African Summer Monsoon

Abstract

A global circulation model (GCM) developed at the National Center for Atmospheric Research (NCAR) has been used to simulate the large-scale features of the Asian-African summer monsoon. The model has 6 vertical layers of 3-km thickness with a 2½° horizontal latitude-longitude grid. The physical processes incorporated are solar and infrared radiation, with cloudiness explicitly calculated from a model-generated relative humidity distribution. The latent heat released from precipitation is derived from stable lifting and cumulus convection. Also included in the model are subgrid-scale vertical and horizontal transports of momentum, sensible heat, and latent heat. We compare the computed sea-level pressure, wind, cloudiness, and precipitation patterns with. observed data and, in particular, concentrate on the strong low-level monsoon jet near eastern Kenya and Somalia. The model correctly simulates this jet in position; however, the wind maxima are weaker than observed. Because of the relatively coarse model resolution, we fail to obtain the important monsoon depressions which form in the Bay of Bengal or near Bombay. In nature, these depressions account for a large part of the precipitation in India and its surrounding regions. This study demonstrates that a global circulation model is capable of simulating many of the features observed in the Asian-African summer monsoon.