Heat, Water, and Vorticity Balance in Frontal Zones

Abstract

The data source for this study is a collection of five years of serial upper-air soundings taken during storms at five stations in the Southern California coastal and offshore region, along with supporting aerial, radar, and surface precipitation observations. Detailed analyses of frontal systems revealed mesoscale motions and processes which are an important and integral part of the frontal structure. In particular, the flow pattern within the prefrontal precipitation region is found to be characterized by waves aloft and a matching cell structure below, with wavelength of 200 to 300 km and with crusts oriented parallel to the front. Within these cells are found small convection bands with which are associated sharp peaks in the precipitation distribution. The overall pattern slopes aloft over the front, and this slope, along with horizontal and vertical mixing, is an essential element in the dynamic balance within the frontal zone. The intensity of the mesoscale vertical motion responsible for clouds and precipitation in the frontal zone appears to have some association with the degree of convective activity. In the stronger fronts, vertical velocity peaks of 20 cm sec−1 or more are found to be the rule. The thermal balance is dominated largely by vertical differences in horizontal advection which are balanced by convective heat exchange. A strong, low-level current of warm air from the south overridden by cold air from the west determines to varying degrees the convective instability and results in a considerable amount of available potential energy being converted directly into convection-scale kinetic energy, thus by-passing its conversion to the cyclone-scale kinetic energy. In fronts possessing greater stability, vertical velocities are less and the eastward movement of the front is greater, suggesting that in these cases the broad-scale deformation of the cyclone-scale thermal pattern, which controls the deepening and the occlusion processes, is accelerated.