Heavy rainfall occurrences in northeast India



In operational meteorology, forecasting heavy rainfall (HRF) events has been a long-standing challenge in India. This is especially true in certain regions where the physical geography lends itself to the creation of such HRF events. Northeast India (NEI) is one such region within the Asian monsoon zone, which receive very HRF during the pre-monsoon and summer monsoon season and the summer–autumn transition month of October. These events cause flooding, damage crops and bring life to standstill. In the present work, the characteristics of HRF events in NEI are studied. The seasonal and spatial variations of HRF occurrences are analysed using 31 year (1971–2001) of daily rainfall data from 15 rainfall stations. Using the daily data obtained from the Indian Meteorological Department, the most favorable locations were found for the stations between 27.5°N and 28.1°N. The most favorable time of occurrence of these events are between 10 June and 5 August. July records the largest number of HRF events followed by June and August. The aggregate of extreme rain events over the region has a significant decreasing trend over the region. Before the monsoon sets in, there is considerable thunderstorm (TS) activity in this region in the month of April and May that are also the cause of HRF events. While many of these HRF events occur associated with the pre-monsoon Nor'westers (tornadoes), some severe TSs may occur during the monsoon season. So, we present a climatology of severe TS days. Also we present the annual and seasonal variation of convective available potential energy (CAPE) and convective inhibition energy (CINE) at Guwahati as the index of the thermal instability. Between 1973 and 2001, CAPE shows a decreasing trend whereas CINE shows an increasing trend which seems reasonable due to the HRF events' decreasing trend. Copyright © 2012 Royal Meteorological Society