The Effects of Strong Ageostrophic Outflows on the Formation of Surface Mesoscale Pressure Systems in Squall Lines

Based on the previous study of the streamline field triggered by singularities in a two-dimensional potential flow, the wind field caused by vorticity lines in an incompressible flow is deduced in this paper.The result shows an elliptic cyclonic (anticyclonic) circulation in association with a positive (negative) vorticity line. By use of the shallow-water model, the flow fields are simulated in a weak wind background under the influence of mesoscale vorticity lines. In the case of two vorticity line, one positive and the other negative,a mesoscale vortex couplet forms in the flow.When three vorticity lines are considered, three mesoscale circulations develop, and a mesohigh and two mesolows similar to the thunderstorm high, wake low and pre-squall mesolow of a mature squall line are produced. Theoretical analysis and numerical simulations show that the formation of the surface mesoscale pressure systems in squall lines may be partly attributed to the dynamical effects of the ageostrophic outflows. The strong downdrafts under the thundercloud base of the squall line lead to surface ageostrophic outflows, and produce positive-negative-positive arranged vertical vorticity bands (VBs) along the direction normal to the squall line, then the mesoscale circulations develop and mesoscale pressure systems form or strengthen during the geostrophic adjustment. By use of the scale separation method, this dynamic mechanism is confirmed by a case study of a severe storm passing over eastern China on 17 June 1974.