The central parts of India receive 80-90 percent of rainfall during the monsoon season (June-September). A major portion of rainfall occurring over these parts is associated with monsoon depressions moving across the area. During the monsoon season, generally 6-7 monsoon depressions form over north Bay of Bengal and move along the monsoon trough. These systems have the potential to produce widespread distribution, with very heavy to exceptionally heavy rainfall. These synoptic systems attracted the attention for serious research by many researchers. One of the important aspects of monsoon depression is the asymmetric distribution of rainfall around its centre. Chowdhury and Gaikwad, Venkataraman et al., and Rajamani and Raoa have shown that a primary zone of the heaviest rainfall occurs in the south-west sector about 200 to 400 km from the centre, while a secondary zone of comparatively less rainfall is located about 800 km west of the depression centre. The area around the first 100 km of the depression centre is generally free from intense rainfall activities. Rajamani and Rao suggested that this observed feature of maximum zone of rainfall over the south-west sector can be explained by the omega (to) equation for the vertical velocity. Due to the maximum pressure gradient south of the centre, the maximum vorticity at the surface would be in that region and minimum at west to north-west. Rao speculated that at the initial stage as a depression over the sea, it might have a uniform pattern of rainfall. Due to paucity of observations over the north Bay of Bengal, this aspect was however not confirmed. The distribution of water vapour, cloud amount, and cloud liquid water content around the depression over the sea are also not examined. With the availability of geophysical products derived from microwave satellite data, it is now possible to examine these aspects more in detail. Two main objectives of this scientific communication are: (i) To demonstrate the potential of use of satellite microwave radiometry, for monitoring the formation and development of synoptic systems like monsoon depression over the sea. (ii) To confirm the asymmetric thermodynamic structure of monsoon depression during the initial stage of depression over the sea, where it is stationary.
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