Hailstorms over India during the summer season

Hailstorm activity over India is seasonal and mainly occurs from March to June. The present study analyzes the diurnal spatial pattern of hail event occurrence and its intensity based on the hailstorm data of the India Meteorological Department surface observatories during March to June for the years 2014-2021. The study indicates that in all months, unlike thunderstorm events, hail events were clustered around the afternoon temperature maximum, becoming insignificant in the post evening hours. Spatially in March, hail events were mostly confined to the Himalayas. Their frequency increased in April throughout the Indian region with significant increase over the plains of India. In May, the frequency of hail events gradually decreased in the Himalayas, but persisted over the plains of India, becoming insignificant thereafter throughout the Indian region in June. Small hail events (<1 cm diameter hailstones) preferentially occurred in the Himalayas. Moderate (1-1.9 cm diameter hailstones) to severe hail events (& GE;2 cm diameter hailstones) were rarer and more uniformly distributed throughout the Himalayas and plains of north and central India. They occurred later in the day as compared to small hail events. The steep decrease in frequency of small hail events beyond April indicates that with the increase in the height of the freezing level as the months progressed, ordinary thunderstorms were less able to form significant hail particles that travelled to the ground without melting, through a progressively warm sub-cloud layer of air. On the other hand, moderate to severe hail events form in tall and deep thunderstorm cells, with significant cloud fraction above the freezing level and strong updraft velocity. Such deep thunderstorm cells occur as a result of strong large scale low to middle tropospheric thermodynamic and dynamic support, are less frequent and peak during afternoon to evening hours. As the season progresses, the warming of the subcloud layer significantly affects the size of hail particles reaching the ground. The warming atmosphere and changing character of the mesoscale convective systems in June subdued most of the hailstorm activity over the Indian region.