Abstract

This study is based on all the monsoon depressions that moved westward across India between Calcutta, Allahabad, and Delhi on the right and Gopalpur, Nagpur, and Ahmadabad on the left during July and August for the period 1891–1960. Statistical distribution, of 24-hr motion and of the intensity of the depression, the relation between 24-hr motion and concurrent 24-hr rainfall, and the relation between the intensity of the depression and subsequent 24-hr rainfall are examined. In addition, the average rainfall per depression day and its standard deviation, the contribution of depression rainfall to the total rainfall, and the efficiency of the depression as a rain giver are computed. Mean patterns of 24-hr rainfall within 500 km of the center of the depression along longitudes 87°E, 80°E, and 75°E are obtained, and the main points of difference between them are discussed.

In the quadrants to the right of the depression track, the rainfall field is flat. In the quadrants to the left, however, large gradients of rainfall exists, particularly along and west of 80°E; the maximum rainfall is located in the left front quadrant, about 150 km from the center and 50–150 km from the depression track. Heavy rainfall extends about 250 km from the depression track in the left sector. Reasons for maximum rainfall in the left front quadrant were sought. We examined statistical distributions 24-hr rainfall in the four quadrants of depression along the three longitudes and found that the gamma probability model gives a good fit to 24-hr rainfall in each of the four quadrants of depression. Using the gamma model, we computed probabilities of rainfall of different intensities in the four quadrants. In this study, orography within the sector is kept constant by considering the rainfall from the same set of rain gage stations along each of the three longitudes.