Simulation of monsoon disturbances in a GCM

A large part of the rainfall over India during the summer monsoon season (J une-September) is contributed by synoptic scale disturbances such as monsoo n depressions. To study the evolution of such disturbances in Atmospheric G eneral Circulation Models (AGCM), the Hadley Centre AGCM (HadAM2b) has been integrated for 15 summer monsoons (1979-1993) and the output was examined for the presence of synoptic scale disturbances such as monsoon depressions , low pressure areas, land lows and land depressions over the Indian summer monsoon region. The atmospheric initial condition For each of these integr ations was of 23rd May and observed Sea Surface Temperatures (SST) were des cribed as a boundary condition. Although the horizontal resolution of the AGCM used in this study is only 2 .5 degrees x 3.75 degrees lat. long., the model is able to simulate a few m onsoon disturbances. The important features of these simulated disturbances are presented. The features of the simulated disturbances are realistic. T he morphologies of a well simulated monsoon depression and a simulated low pressure area are presented as examples. The frequency of the simulated mon soon depressions is less than the climatological frequency of the depressio ns during all four monsoon months.