Mechanisms Governing the Strong Surface Winds in a Bow-Shaped Squall Line Over Southern China

A high-resolution simulation of surface damaging winds within a bow-shaped squall line over Southern China was conducted using WRF v3.7. Prevailing explanations for surface damaging winds were applied to investigate the forcing mechanisms on a complicated real-world scenario. It is shown that environmental vertical wind shear was an import factor that influenced surface damaging winds. The shearing vorticity by wind shear was comparable to the baro-clinic vorticity by cold pool. The balance led to strong upward motions at the squall-line leading edge and brought plenty of rainfall. The descending rainfall cooled surrounding air and entrained the upper-level cold air downward to lower level, which strengthened the cold pool. The cold pool accelerated the squall-line propagation speed and caused extensive wind damages at the surface. Meanwhile, the horizontal vortex lines at the leading edge of cold pool were lifted by frontal updrafts and tilted to form the anti-cyclonic vortex at middle level, which maintained and intensified the rear-inflow jet behind the apex of bow echo. The rear-inflow jet was then transported downward to the surface by descending condensate, and lead to damaging winds there. The other factors such as environmental flows and midlevel pressure gradient, however, contribute less to the surface damaging winds.