Numerical simulation of monsoon circulations

Abstract

Short period variations in monsoon rainfall are caused by the westward passage of low pressure systems (depressions) from the northern sector of the Bay of Bengal. A primitive equation model was used to predict the movement of one such depression (20 August 1977). Four research ships from the USSR, which formed a part of the recently concluded Monsoon Experiment, provided additional meteorological data within the field of this depression. This enabled us to fix the depression's initial position with greater accuracy than would have been possible without the ships' data. The model used a co-ordinate system in which the lower boundary coincided with the earth's surface, while the upper boundary was placed at 200 mb. It resembled a three-dimensional channel with side walls at 0° and 140°E, and northern and southern boundaries at 60°N and the equator. The grid spacing was 250 km. Numerical integration was performed upto 5 days of model time. Gravity waves and other forms of 'noise' were filtered out every 24 hr by a process of adjustment referred to as 'initialisation'. Initialisation after every 24 hr was necessary because the boundary conditions in this regional model did not permit sufficiently rapid dispersal of gravity waves. In the first experiment only orographic features were included, but the second experiment considered the main features of atmospheric radiation in addition to orography. The paper presents a statement of deviations between the predicted and actual movement of the depression, and discusses reasons for such deviations.