The central region of India was battered by prolonged, widespread and severe hailstorm events during the period of 15 February 2014 to 15 March 2014. The events were unprecedented not only in terms of time of occurrence, duration and frequency of hailstorms but also size of hailstones. In the present study, we have proposed a hypothesis which governs the mechanism for the development of severe hailstorms based on atmospheric instability, cloud microphysics and its feedback to large scale dynamics. The steep temperature lapse rate owing to warm (i.e. moist) south easterlies in the lower troposphere and cold (i.e. dry) westerlies in upper troposphere as a result of anomalous southward penetration of mid-latitude westerlies in the form of westerly trough fostered convective instability build up. The updraft associated with convectively unstable atmosphere invigorated growth of ice crystals by rapid collection of supercooled water droplets. This ultimately led to formation of graupel/hailstones through the microphysical mechanisms (e.g., riming process). The hypothesis is further tested with high resolution numerical model. The experiment pinpoints that mixed-phase hydrometeors were predominant in mid and upper troposphere and had bearing on the release of latent heat, further stronger the deep convective clouds (i.e., instability). Time evolution of cloud microphysical processes indicate dominant presence of graupel/hail which is consistent with formation of rain water, cloud ice and snow. The results underpin the interplay between large scale dynamical and cloud microphysical processes in the formation of anomalously widespread and persistent severe hailstorm events.
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