ABSTRACT:The trends and epochal variability of southwest monsoon over the country as a whole and four homogeneous regions are examined using monthly rainfall data of 640 political districts of India. The district rainfall data is computed from station rainfall data. The same station data is used to analyse the trends in the frequency of rainfall events of different intensities for examining extreme rainfall events. The existence of the multidecadal epochal variability of rainfall is clearly established in the all-India monsoon rainfall as well as monsoon rainfall over the four homogenous regions. However, over different homogenous regions, the phases of multidecadal variability are found to be different. Principal component analysis brings out Northeast India (NEI) rainfall as more dominant mode for all-India rainfall. Significant decrease in southwest monsoon rainfall over NEI is observed during the post 1950 period. Decreasing trends are also observed over the monsoon core region during the post-1950 period. Over these regions, monsoon rainfall has increased significantly during the pre-1950 period. It has been shown that the decreasing trend in monsoon rainfall during the post 1950 period is the result of multidecadal epochal variability. Geographical regions that experienced significant changes in the frequency of days of rainfall with different intensities are also identified. Significant change/turning points are also detected in the southwest monsoon rainfall. Frequency of moderate rainfall events (5 mm ≤ daily rainfall < 100 mm) decreased significantly during the period 1951-2010 over the monsoon core region of India whereas no significant changes are observed in the frequencies of heavy (daily rainfall >100 mm) or very heavy rain (daily rainfall >150 mm) during the southwest monsoon season. Climatic shift or change point in monsoon rainfall in India is also detected by an established statistical test.

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Thunderstorm is a severe weather phenomenon, the impact of which is being increasingly felt by all the sectors of society. In this study attempt has been made to develop thunderstorm climatology over Indian region based on latest representative climatological data. In all, data of 450 observatories comprising of 390 IMD observatories, 50 IAF observatories, six Bangladesh observatories, two Pakistan observatories, and one each in Nepal and Sri Lanka have been analysed. Inclusion of climatological data of Indian Air Force and Bangladesh has helped in developing representative climatology over Indian region. The study has brought out higher (100-120 days) annual frequency of thunderstorm as compared to those given by earlier studies (80-100 days). The highest annual frequency (100-120 days) is observed over Assam and Sub Himalayan West Bengal in the east and Jammu region in the north. The lowest frequency (less than 5 days) is observed over Ladakh region. In the plains Gangetic West Bengal and Bangladesh record between 80 and 100 days of thunderstorm annually. Kerala records highest (80-100 days) thunderstorm frequency of thunderstorm over peninsula. Udhampur observatory (132 days) in Jammu sub-division records highest number of thunderstorms in the country followed by Kumbhigram (Silchar) observatory (129 days) in south Assam and Hasimara (123 days) in Sub Himalayan West Bengal. In the plains Saurashtra and Kutch record lowest number (less than 15 days) of thunderstorm in the country. Thunderstorms are primarily short lived mesoscale weather phenomena. Existing synoptic network of part time observatories have limitations in recording all the occurrences of thunderstorms at the observatory and adjoining areas. Inclusion of data from such observatories results in lower frequency of the event and vitiates climatology. Efforts have been made in this study to develop thunderstorm climatology by using quality data of full time current weather (Airport) observatories, class I and selected class II IMD observatories. Since number of full time observatories in most of the sub divisions is few, the study is not able to bring out finer spatial variation of thunderstorm activity with in sub-divisions. Thunderstorm is a high impact severe weather event, which affects all the sectors of the society. Therefore, both from operational and climatological point of view, there is urgent need to establish at least one full time current weather observatory in each district to ensure proper reporting of all thunderstorm occurrences and to build district level thunderstorm climatology in the country.

South Asian countries have developed a unique program for forecasting severe thunderstorms through field experiments and research on mesoscale modeling.