The Structure of a Subtropical Prefrontal Convective Rainband. Part II: Dynamic and Thermodynamic Structures and Momentum Budgets

Abstract

A thermodynamic retrieval method was employed to investigate the dynamic and thermodynamic structures of a subtropical prefrontal convective rainband associated with the Mei-Yu front on 25 June 1987 over north-western Taiwan. Three-dimensional wind fields were derived from the dual-Doppler data based on the methodology developed at Saint Louis University. Subsequently, fields of perturbation pressure and temperature were retrieved from the detailed wind field using the three momentum equations. Results show that the maintenance of this long-lived rainband at the times of dual-Doppler analysis is caused by the combined effects of a gust front arising from the convective downdrafts ahead of the front and developments of new convection along the gust front. In the lowest layer, high pressure occurs behind the cold front with low pressure to its southeast. A buoyancy-induced low pressure area lies beneath the frontal updraft corresponding to the rising warm environmental air. The precipitation core associated with the frontal updraft is elongated toward the southeast side with the environmental shear in the mid-and upper troposphere forming the convective downdraft on the warm side of the surface front. This precipitation-induced downdraft transports cooler air downward producing a high pressure area underneath the convective downdraft. This high is accompanied by a temperature deficit resulting in cold horizontal outflows in the boundary layer. Part of these cold outflows interacts with the high-?e, southwest monsoonal flow to form a gust front ahead of the surface front. New convection develops along a gust front and then merges with the old convection, thereby prolonging the lifetime of the rainband. The vertical flux convergences and divergences of horizontal momentum by organized convection are largely responsible for forming a midlevel jet and weakening a low-level jet. The momentum budget calculation shows that the horizontal and vertical flux convergences and divergences of horizontal momentum by the mean and eddy motions are the major contributor to maintain the mean momentum.