Comparative study of the simulation and mechanism of two different severe thunderstorms embedded in a squall line in North China

Abstract

Based on a cloud-permitting numerical simulation carried out using the Weather Research and Forecasting Model, version 4.1.5, a squall line (SL) system accompanied by severe thunderstorms in North China in July 2019 was simulated. Both the observation and simulation showed that the convective organizational modes of the thunderstorms experienced three main stages: a quasi-linear (QL), bow echo (BE) and SL stage. Across these three stages, the thunderstorms consisted of two different convective systems. One was a thunderstorm with severe wind gusts (SWGs), which was concentrated in the BE and SL stages, and the other was a thunderstorm without gales but that featured high frequency lightning (HFL), which initiated and enhanced in the QL and SL stages, respectively. In the BE stage, SWGs began to extend backwards from the apex of the bow structure. In the SL stage later, the extent of SWGs increased significantly and dominated over a wide range behind the convective line as the convection nearly died out. Further analysis showed that the features and formation mechanisms of the SWG system were significantly different between the two stages. During the BE period, SWGs were mainly caused by the downdrafts in the convective core, which could be attributed to the formation and development of the horizontal vortex pair in the lower level. In the SL stage, however, SWGs mainly attributed to the sinking of the rear inflow, which was associated with the wide extent of negative buoyancy in the uneven convective environment, enhanced by the evaporation of the rainwater near the surface. HFL enhanced strongly on an extreme cold surface in the SL stage as the thunderstorm grew vigorously. The outflow along the edge of the cold dome converged with the front inflow and continuously triggered convection. The development and the interaction between the multiple convective cells triggered lightning, which was enhanced by the strong condensation heating in the storms.