WRF simulation of a severe hailstorm over Baramati: a study into the space–time evolution

Space–time evolution of a severe hailstorm occurred over the western India as revealed by WRF-ARW simulations are presented. We simulated a specific event centered over Baramati (18.15°N, 74.58°E, 537 m AMSL) on March 9, 2014. A physical mechanism, proposed as a conceptual model, signifies the role of multiple convective cells organizing through outflows leading to a cold frontal type flow, in the presence of a low over the northern Arabian Sea, propagates from NW to SE triggering deep convection and precipitation. A ‘U’ shaped cold pool encircled by a converging boundary forms to the north of Baramati due to precipitation behind the moisture convergence line with strong updrafts (~15 ms−1) leading to convective clouds extending up to ~ 8 km in a narrow region of ~ 30 km. The outflows from the convective clouds merge with the opposing southerly or southwesterly winds from the Arabian Sea and southerly or southeasterly winds from the Bay of Bengal resulting in moisture convergence (maximum 80 × 10−3 g kg−1 s−1). The vertical profile of the area-averaged moisture convergence over the cold pool shows strong convergence above 850 hPa and divergence near the surface indicating elevated convection. Radar reflectivity (50–60 dBZ) and vertical component of vorticity maximum (~0.01–0.14 s−1) are observed along the convergence zone. Stratiform clouds ahead of the squall line and parallel wind flow at 850 hPa and nearly perpendicular flow at higher levels relative to squall line as evidenced by relatively low and wide-spread reflectivity suggests that organizational mode of squall line may be categorized as ‘Mixed Mode’ type where northern part can be a parallel stratiform while the southern part resembles with a leading stratiform. Simulated rainfall (grid scale 27 km) leads the observed rainfall by 1 h while its magnitude is ~2 times of the observed rainfall (grid scale ~100 km) derived from Kalpana-1. Thus, this study indicates that under synoptically favorable conditions, WRF-ARW could simulate thunderstorm evolution reasonably well although there is some space–time error which might, perhaps, be the reason for lower CAPE (observed by upper air sounding) on the simulation day.