A Diagnostic Case Study of a Winter Low-Level Front over Southern China

Abstract

A diagnostic case study was carried out for a lower-tropospheric front over southern China in association with lee cyclogenesis over the Yun-Guei Plateau (23°?29°N, 102°?110°E) during the period of 25?27 December 1991. The front over the subtropical latitudes was found to have a structure similar to that of a typical midlatitude cold front with a maximum gradient of potential temperature and strong cyclonic vorticity concentrated within the frontal (baroclinic) zone. As the frontal zone intensified, upward motion associated with transverse circulation increased and frontal contraction occurred. It is found that the frontal zone was also the zone of maximum frontogenesis. Intensification of the frontal baroclinity was mainly due to the confluence frontogenetical effect of stretching deformation. The twisting term was observed to have a frontogenetical effect to the south of the frontal zone, though with a smaller magnitude. Results also suggest that the front in this case moves southward because of either a frontogenetical or an advective process.