Kinematic and thermodynamic structure of the 23 april 2007 squall line over south China

In this study, dual-Doppler observation is used to investigate the kinematic and thermodynamic structure of a subtropical squall line system occurred over south China on 24 April 2007. The synoptic environment of the squall line was characterized by a mesoscale shear/convergence zone induced by the southwesterly flow and the northerly flow and with a moderate CAPE and strong vertical shear between the surface and 5km.

Results show that the squall line has a nearly east-west orientation, and propagates south-eastward rapidly at a speed of about 17ms-1. The mesoscale structures of the squall line include a deep-layer storm-relative front-to-rear flow on front of the squall line, and a shallow (about 3km) rear-to-front flow on the back. From front to rear, the structures include the intense convective band, the transition zone, and the trailing stratiform region. The convective region is composed of many individual cells characterized by intense updrafts and high reflectivity values, with relatively weak convective downdrafts between and behind the convective region. The convective updrafts are warmed by the release of latent heat, while cooling prevails in the convective downdraft. In the low level, the cold rear-to-front flow collides with the advancing warm environmental warm air at the leading edge to form a dynamic high pressure, which in turn forces the formation of new cells. A buoyancy-induced low pressure area lies beneath the convective updraft corresponding to the ascending warm environmental air. It is also noted that in the mesoscale shear/convergence zone the new convective cells successively form and move into the squall line from its west side. Therefore, the mesoscale shear/convergence zone plays an important role on the development and maintenance of the squall line.

The Advanced Research WRF (ARW) modeling system is used to simulate the squall line at high resolution (5-km spacing) from the mature stage to dissipated stage. Doppler velocities as a positive impact on the short-range prediction are assimilated into WRF based on WRF3DVAR. The simulation is used to investigate the role of rear-to-front flow on the back of squall line and flow from north-east to south-west in mesoscale shear/convergence zone in squall line's west side when new convective cells form. The result shows that the storm-induced features, such like east-west low-level flow, and environmental factors, such as southwesterly flow and northerly flow play important roles in determine the evolution of squall line system.