Spatial patterns of long-term trends in thunderstorms in India

Long-term trends in the frequency of thunderstorm days are investigated using fifty years of data recorded by weather observatories throughout India. An attempt has been made to identify whether the trends are spatially cohesive and collocated with regions with maximum of thunderstorm days. This analysis of trends is significant, since most of the weather over India is of convective origin and has devastating impact on life and property. There are three thunderstorm maximum regions (a) northwest Indian Himalayas and adjoining Pakistan, (b) east India and (c) extreme southern parts of peninsular India. The results of the study revealed on a monthly scale, the frequency of thunderstorm days has a consistent spatially cohesive decreasing trend over east central and northeast India, collocated with the intra-annually shifting maximum of thunderstorm activity. The trend is more diffused for the thunderstorm maxima over north and south peninsular India. Comparison with rainfall trends from previous studies indicates that this decreasing trend of thunderstorm days is in phase with decreasing trends in rainfall activity over east central and northeast Indian regions. Detailed analysis indicates that frequency of thunderstorm days over these regions decreased during the latter half of the twentieth century followed by gradually increasing trend. This corresponds to a global warming hiatus and matches similar findings around the world. It seems likely that the decreasing moisture inflow from the surrounding oceans into the Indian subcontinent throughout the year due to weakening of the land-sea thermal gradient is causing a decreasing trend in the convection during this period. Over north and south peninsular India, where circum-global planetary waves play an important role in triggering convection, the strengthening of these waves offsets the effect of the generally decreasing moisture.