A Numerical Simulation of the Bowing Process of A Squall Line Associated with a Stationary Front in South China during 23–24 April 2007

Through real-data cloud-resolving simulations with the Weather Research and Forecasting (WRF) model, this study examines the formation and evolution dynamics of a bow echo in an squall-line event initiated along a quasi-stationary frontal boundary during the rainy season (April 2007) in South China. This squall line forms as several isolated convective clusters expand along the quasi-stationary front while the cold pools of these clusters merge into one coherent outflow boundary on the leading edge. A bow echo of about 200-km long develops in the middle of the squall line from consolidating of several smaller-scale bow-shaped segments. It is found that the rear inflow plays an essential role in the formation and evolution of the bow-shaped convective segment. The rear inflow starts from the lower branches of rotor associated with the convectively forced horizontal vorticity beneath the rearward-sloping updraft near the leading edge of the active convection in addition to the impact of horizontal pressure gradient associated with midlevel pressure deficit due to the vertical buoyancy gradient at the southern portion of the large bow. The rear inflow is further enhanced by both the circulation induced by book-end vortices and the mid-level pressure gradient, which is more evident on the northern portion of the large bow. The rear inflow on the southern portion of this large bow is located at a lower altitude and expands less rearward than that on the northern portion. Through vorticity budget and vortex line analysis, it is found that the book-end vortices are likely to be generated with the anticyclonic and cyclonic vortex centers individually produced by tilting of independent vortex line associated with horizontal vorticity generated barocinicly or by ambient vertical shear.