Revealing Key Dynamical Mechanisms of a Severe Supercell Within a QLCS Using Rapid Update 4DVar Assimilation of C-Band Phased Array Weather Radar Data

We investigate the evolution of a supercell storm within a quasi-linear convective system (QLCS) that occurred in the Beijing area on 12 June 2022. Using high spatiotemporal resolution observations from a C-band phased array radar (PAR), assimilated into a four-dimensional variational data assimilation system, we primarily analyze dynamical processes contributing to the development of the supercell storm and its associated mesocyclone. Our study shows that just before the convective cell is triggered, a significant convergence zone develops to the west of the terrain, forming several meso-γ vortices near the surface. During the merger of the convective cell and the QLCS from upper to lower levels, a strong downdraft generated by the QLCS enhances low-level horizontal convergence, further producing a stretching effect on the vortices within the storm and significantly increasing vertical vorticity. With the formation of the mesocyclone in the mature stage of supercell storm, the height of the rotational center rises to 4.5 km, and the maximum rotational velocity reaches 20 m/s. Our results indicate that the surface convergence lines and the meso-γ vortices along them strengthen low-level convergence and generate strong updrafts, triggering the initial storm. These intense updrafts transform horizontal vorticity into vertical vorticity and transport it upward. Additionally, the process of convective merging leads to strengthen low-level horizontal convergence, which forcibly stretches the mesovortex, enhancing vertical vorticity and allowing the convective storm to develop in a strong, organized manner and form the supercell storm.