EFFECTS OF TROPOPAUSE AND POTENTIAL VORTICITY ANOMALY ON FRONTOGENESIS

A two-dimensional,semi-geostrophic numerical model incorporating the tropopause andstratosphere is used to investigate the effects of a positive potential vorticity anomaly and latentheat release on the frontogenetic process and the structure of the resulting frontal zone.It isdemonstrated that(1)the inclusion of tropopause and stratosphere significantly changes the frontalstructure only in the upper levels;(2)a clearly defined quasi-equivalent barotropic structure and aregion of upward motion of finite width appear when a positive potential vorticity anomaly exists onthe warm side of the maximum baroclinity in the lower troposphere,especially when it is locatedon the south edge of the baroclinic zone;(3)the above mentioned structure deteriorates as thefrontogenesis proceeds in a dry atmosphere but can be maintained in a moist frontogenetic processwith condensational heating;(4)the combination of a positive potential vorticity anomaly and thelatent heat release is able to accelerate the frontogenesis significantly with the time needed to forman intense frontal zone reduced to less than 15 h.The results have significant theoreticalimportance in understanding the complex nature of frontal structure and frontogenesis,especiallyin understanding the dynamic structure of the subtropical frontal zone observed during earlysummer over East Asia.