Some aspects of the thermodynamic structure of monsoon depressions

Abstract

Using equivalent potential temperatures the thermodynamic structure of monsoon depressions (in the north-south section) has been studied. During the mid-monsoon months of July and August 1970 and 1971, eight depressions moved inland from the Bay of Bengal, Vertical profiles of equivalent potential temperatures across the section plying the data Lhasa, Gauhati, Calcutta, Bhubaneshwar, Visakhapatnam and Madras were constructed and discussed. The variations ill convective intability in relation to the clouding and weather associated with a monsoon depression are pointed out. Vertical sections of moisture and virtual temperature distributions across a monsoon depression The study brings out the existence of (i) a region of neutral convective instability and high moisture content upto the mid-troposphere just south of the centre of the depression where heavy overcast skies, cb clouds and heavy rain occur and (ii) a layer of minimum equivalent potential temperatures and low moisture centent, centre about 3.0 km asl, 40 to 5° south of the depression suggesting a possible region of suboidence. In this region though convective instability is high in the lower troposphere, it is not realised presumbably because of the synoptic scale subs1dence. The moist ascending air close to the centre and the descending (subsiding) air further south form a circulation in the vertical plane and the distribution of virtual temperatures shows that this vertical circulation is energy-producing.