Statistical study of the low pressure systems during summer monsoon season over the Indian region

Abstract

The occurrence of a closed low pressure area due to low, depression or cyclonic storm is termed as Low Pressure System (LPS). LPS plays an important role in the distribution of rainfall during the southwest monsoon season. Lows produce widespread rainfall as compared to depressions and cyclonic storms which yield concentrated rainfall over a smaller area. The distribution of rainfall depends upon the track and life span of LPS. Most of the LPS formed over the Bay of Bengal travel in northwest direction strengthening the rainfall activities over the large parts of the country. In this study, the LPS formed during the monsoon season, June to September for the period 1891-2000 over the region covering India, Pakistan, Bangladesh, Bay of Bengal and Arabian Sea are taken into account. The duration of LPS is also studied in terms of LPS Days during the monsoon season. The statistical analysis of LPS and LPS Days  is carried out for the monsoon months and for the monsoon season as a whole, for the period 1891-2000. It is seen that the frequency of LPS during any of the monsoon months does not exceed six but three LPS in a month are more common.  Total number of LPS during the monsoon season ranges from 9 to 18. In June and July maximum total duration of LPS lies between 10-14 LPS Days while in August and September, it is between 15-19 LPS Days. August is the month having maximum number of LPS and LPS Days. The frequency analysis shows that monthly and seasonal LPS and LPS Days are normally distributed. It is also observed  that even though season's total number of LPS has no significant increase or decrease, the LPS Days have significantly increased during the decades, 1971-80 and 1981-90.