Upper air contour patterns and associated heavy rainfall during the southwest monsoon

Abstract

The necessity of dealing with the lower troposphere over India as consisting of two separate strata having opposite values of horizontal wind divergence is briefly explained. It may not be sufficient to look for areas of convergence at the lower levels areas of marked divergence at the upper levels of 500 mb and aloft may be equally, if not more, important for the understanding and forecasting of very heavy rains. The principles of deducing areas of marked horizontal divergence on qualitative reasoning based on considerations of non-geostrophic motion as well as of air flow down the gradients of vorticity and then outlined. A few cases of non-orographic phenomenal rainfall of 10-20 in 24 hours which fell over certain parts of India during the southwest monsoon of 1954 are shown to have fallen in areas of marked upper air divergence, deducible from such qualitative reasoning applied to flow patterns at 500 mb and a aloft.