A modeling study on the development of a bowing structure and associated rear inflow within a squall line over South China

Through convection-permitting simulations, this study examines a large bowing structure within a squall line that occurred during the rainy season in South China. The bowing structure is closely associated with a local enhancement of (and balance between) the cold pool and the line-normal environmental low-level vertical shear. Rear inflow plays an essential role in the formation and evolution of this large bowing structure. It is found that the low-level rear inflow is largely a natural consequence of the baroclinically generated horizontal vorticity near the surface, while the midtropospheric rear inflow is forced by several pairs of bookend vortices. Vorticity budget and vortex-line analyses show that the bookend vortices form mainly through the tilting of horizontal vorticity. Consolidation of these pairs of bookend vortices forms a broad zone of contiguous rear inflow. The environmental flow and horizontal pressure gradient force associated with the midlevel pressure deficit induced by the rearward-tilting buoyant updrafts, on the other hand, are not primarily responsible for the formation of the rear inflow.