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The spatial distribution of southwest monsoon rainfall is studied over Maharashtra State (India), which includes part of the well‐known Western Ghats mountain range, near its western boundary, running almost from north to south, perpendicular to the summer monsoon current in the lower troposphere. Meso‐scale analysis of daily rainfall is performed for Maharashtra State, including the Western Ghats, for the two mid‐monsoon months of July and August, during the 10‐year period of 1971–1980. Strong and weak monsoon days were identified for the 5‐year period of 1976–1980. The meso‐scale pattern of average daily rainfall is obtained separately for strong and for weak monsoon conditions. All these average patterns show the following features: (i) the rainfall increases rapidly from the Arabian Sea coast close to the line of maximum height of the Western Ghats; (ii) there are two rainfall maxima corresponding to the two mountain peaks parallel to the coast line; (iii) between the two mountain peaks, there is a valley which is narrow at the western end (upwind end), broadening towards the east (on the downwind side). Ground contour height of the valley rises eastwards and ends as a part of the Deccan Plateau east of the Ghats. Here the valley opens out like a funnel with higher mountains flanking its two sides. In the valley, the rainfall increases from the coast up to the line of maximum height of the Ghats, and then decreases eastwards towards the plateau. The rainfall isopleths also take a funnel‐shaped configuration. An interesting feature is that near the wider section of the valley funnel, there is a rainfall minimum and then the rainfall increases further eastwards on the downwind side. This feature of rainfall minimum is somewhat similar to the rainfall minimum reported by Asnani and Kinuthia (personal communication); Asnani (Asnani GC. 1993. Tropical Meteorology, Vol. I. Prof. G.C. Asnani: Pune, India; 603) attributed the rainfall minimum to the Bernoulli effect. A somewhat similar phenomenon is assumed in the present study area. Copyright © 2000 Royal Meteorological Society

The structure of the monsoon depression and the observed flow features prior to and at the time of monsoon depression formation (compositeof 15depressions) are examined. The composite monsoon depression (transient eddy) has a scale of 25 0 longitude and extends up to 300 mb and has the greatest intensity at 700 mb. It shows north-northeast to south-southwest tilt in the lower levels indicating that it may draw upon zonal kinetic energy for its growth. The disturbance has lower temperatures to its west and tilts westwards with height indicating that eddy available potential energy is not converted from zonal available potential energy by large scale advection. There appears to be a reduction of vertical shear at the time of formation of monsoon depressions and this possibly aids cumulus convection. The profiles of potential vorticity indicate extremes (i) in the upper troposphere and (ii) at several midtropospheric levels in the region of the monsoon trough indicating the possibility of combined barotropic-baroclinic instability. Using multi-level quasi-geostrophic model and employing the eigen-valuetechnique it is shown that the monsoon zonal current is not baroclinically unstable. A barotropic stability analysis is also done for monsoon zonal current in the lower and middle tropospheres. It yields very slowly growing unstable modes at lower tropospheric levels with wave lengths of 2500 km and 5000 km.

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