Unraveling the dynamics of an intense pre-monsoon hailstorm and summer monsoon deep convective system over the central Himalayas using a VHF radar

Abstract

Himalayan region being influenced by extratropical and monsoon weather systems coupled with orographically forced convection, are frequently impacted by severe thunderstorms. Negligible ground based observations over this region have restricted the analysis of these storms up to coarse statistical features utilising reanalysis data and space-borne radar missions. High spatiotemporal resolution of the Very High Frequency (VHF) radars makes them well suited for the study of large convective storms. In this context, a 206.5 MHz ARIES ST Radar (ASTRAD) facility at Nainital (29.4oN, 79.5°E, 1793 m amsl) has been used to probe mesoscale dynamics of severe deep convective systems in the pre-monsoon and monsoon season for the first time over the Himalayan region. Pre-monsoon event on May 5, 2020 was observed to have characteristic features of a supercell storm resulting in a nearly two hour long hailstorm (∼1430–1630 IST). Vigorous updrafts with maxima of 15.5 ms-1 were observed extending from the mid-troposphere to up to the lower stratosphere region and downdrafts up to 15.7 ms−1 were detected in the lower troposphere within the hailstorm. Hodographs during peak convective activity revealed strongly ‘veered’ winds up to upper troposphere with Storm Relative Helicity maxima of 484 m2s−2. The strength of convection was moderate (updrafts maxima of 4.5 ms−1) in summer monsoon case on September 2, 2020. In this case, trailing stratiform region is observed with double bright band structure in the radar reflectivity factor. Weakening of the stratified stable layer structures in the UTLS region have been observed for both the case in the regions of strong updraft gradients. Spectral and hodograph analysis of the quasi sinusoidal undulations in the wind components fluctuations revealed the presence of upward propagating short-period gravity waves in the Upper Troposphere and the Lower Stratosphere (UTLS) region for both the cases. The observed period of oscillation was 35.6 min and 27 min with vertical wavelengths of ∼8.4 km and ∼ 5.7 km for the May 5 and September 2 case respectively. Momentum flux enhancement of 30–40 times was observed during convection with extremely large values during the hailstorm indicating enhanced flux exchange between troposphere and stratosphere. The findings of this study will aid the forecasting of these storms and improve their simulation in the mesoscale models, particularly over this Himalayan region having complex topography.