Large-scale rainfall over India during the summer monsoon and its relation to the lower and upper tropospheric vorticity

Abstract

The daily variation of the lower and upper tropospheric wind field for the monsoon seasons of 1972 and 1973 is investigated, It is shown that the changes in the large-scale rainfall are directly related to those in the large-scale vorticity above the frictional boundary layer on a day-to-day basis throughout the two seasons, large cyclonic vorticity being associated with active spells and anticyclonic vorticity with weak monsoon spells. In the upper troposphere, the regional average of the anticyclonic vorticity is generally found to increase in active periods, decrease slightly in breaks and spectacularly towards the end of the break and increase again during revival. The maximum value of the kinetic energy of the easterlies during a season is found to occur during the break or weak monsoon spell for the seasons studied. The major difference in the upper tropospheric circulation of the two seasons is in the meridional component which is southerly over large regions over India in 1972 and northerly over the entire region on most days in the season of 1973. The contribution of the meridional advection of vorticity in determining the upper level divergence is found to be significant and its variations are found to be correlated to those of the lower-level convergence.