Formation of Damaging Surface Winds in a Bow-shaped Squall Line Mature Stage over South China

Observational analyses and convection-permitting simulations are conducted to study an extreme event of damaging surface winds within a bow-shaped squall line over South China. Prevailing explanations for the formation of damaging surface winds were applied to investigate the detailed processes at the mature stage of a squall line in a complicated real-world scenario. It is shown that environmental vertical wind shear was adequately strong to cause a shearing vorticity, and the shearing vorticity was comparable to the baroclinic vorticity by a cold pool. The balance led to strong upward motions at the leading edge of the squall line and brought plenty of rainfall. The descending rainfall cooled the surrounding air and entrained the upper-level cold air downward to the lower level, strengthening the cold pool by excessive evaporation and melting. The cold pool accelerated the propagation speed of the squall line and caused extensive wind damage at the surface. Meanwhile, the horizontal vortex lines at the leading edge of the cold pool were lifted by frontal updrafts and tilted to form the anti-cyclonic vortex at the middle level. This maintained and intensified the rear-inflow jet behind the apex of bow echo in association with a notable midlevel pressure gradient. The rear-inflow jet was then transported downward to the surface by descending condensate, leading to damaging winds there. Other factors, such as environmental flows, however, contributed less to the damaging surface winds.