Zones of rainfall ahead of a tropical depression

Abstract

The paper presents the results of a study of the nature and distribution of precipitation in the forward quadrant of a monsoon depression on the basis of hourly rainfall data from 18 self-recording raingauge stations set up over an area of 12,600 sq, km in Gangetic West Bengal, The precipitation was mostly convectional and was often restricted at a time to very narrow columns of the atmosphere, less than 10 km in diameter, Ahead of the depression there were annular bands of wet and dry zones. The outermost limit of the wet zone was at a distance of about 300 km from the centre of the depression, Between 300 and 150 km from the centre of the depression, the wet and dry zones were at nearly steady and predictable distances from the depression. The wet zones were about 30 to 50 km wide and the dry zones about 50 to 80 km. The zones situated within a distance of about 150 km from the depression were, however, unsteady with occasional variations in the distribution of rainfall. Consequently the wet zone observed initially at and around the centre of the depression turned dry during the course of the movement of the depression.