Dual-Doppler analysis of a squall line in southern China

In this study, the kinematic and thermodynamic structure of a subtropical squall line on 24 April 2007 is investigated using the Dual-Doppler Radar data collected from the South China Heavy Rainfall Field Experiment (SCHeREX). The synoptic environment of the squall line was characterized of a moderate CAPE and strong vertical shear (0-3 km, 18 m/s). The results show that the squall line has a nearly east-west orientation, and propagates south-eastward rapidly with the peak speed of about 17 m/s. The kinematic, dynamic and thermodynamic structures of the squall line at the mature stage are similar to those of the previously observed fast moving tropical and subtropical squall lines. The flow structure is almost two-dimensional, including a deep storm relative front to rear flow on the front of the squall line, and a shallow rear to front flow on the back. From front to rear, the features in precipitation structure 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 cells. 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 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 the new convective cells successively form and move into the squall line from its west side, which is favorable to the maintenance of the squall line. The mass and momentum budget calculations in the convective region reveal that the vertical transportation of the horizontal momentum flux perpendicular to the squall line is of counter gradient while the momentum flux parallel to the line is of down gradient.