X-band phased-array radar data assimilation for a case study on a weak tornado numerical simulation in the westerlies

This study examines a weak tornado event triggered by a westerly trough in Foshan, Guangdong, on 19 June 2022, using high-resolution numerical simulations initialized with X-band phased-array radar (X-PAR) data assimilated via the three-dimensional variational (3DVAR) method. Results show that assimilating radial velocity enhances the low-level northerly flow near the tornado location, promoting cyclonic vortex development in both the VEL(assimilating radial velocity) and XPAR(assimilating radial velocity and reflectivity) experiment. This increases the positive storm-relative helicity (SRH) and intensifies the updrafts, leading to the simulation of a bounded weak echo region and an overhang structure of the reflectivity. Analysis of the tornado track, vertical vorticity, and SRH temporal evolution indicates that before tornado decay, the hook-shaped echo corresponds to the high-vorticity area, and the boundary between the positive and negative high-value zones of SRH nearly coincides with the area of high vorticity. After the tornado enters the decay phase, the tornado track, the positive SRH and vorticity of XPAR are greater than those of VEL, indicating that assimilating reflectivity can enhance the vertical updraft of supercells and improve the tornado forecast results.