Mechanisms for the Formation of Rear Inflow and Bow Echo within the 23 April 2007 Squall Line over South China

Through radar observations and convection-permitting simulations, this study examines the formation and evolution of bowing segments within a squall line on 23 April 2007 during the rainy season in South China. The squall line results from the merger of several discrete convective clusters along a quasi-stationary front as the cold pools grow into one contiguous outflow. Eventually, a large bow echo of about 200 km in length develops in the middle of the squall line from consolidation of several smaller-scale bow-shaped segments.

Rear inflow was found to have played an essential role in the formation and evolution of the large bow echo. The rear inflow originates first from the lower branch of a buoyancy-driven rotor beneath the rearward-sloping updraft near the leading edge of the active squall-line convection then expands rearward, upward and northward. The upper portion of the rear inflow is also contributed by a pressure deficit located near the top of the cold pool at the earlier stage and later enhanced by the circulation induced by book-end vortices which is most evident on the northern portion of the large bow. The formation of the big bow was found to have formed through merging of multiple pairs of book-end vortices.

Likely due to influences from complex inhomogeneous background, this real-data case study shows that the anticyclonic center of the book-end vortices is not necessarily stronger than the cyclonic center as documented in past studies initialized with a horizontally homogeneous environment. Vorticity-budget and vortex-line analyses show that the book-end vortices could be generated with the anticyclonic and cyclonic vortex centers produced by tilting of not only the same vortex line but also separate vortex lines associated with horizontal vorticity forced by either convection or the ambient shear flow in front of the squall line. Front-to-rear storm-relative flow may strengthen the anticyclonic vorticity center more than the cyclonic vorticity center of the book-end vortices.