THE LOW-LEVEL STRUCTURE AND EVOLUTION OF A DRY ARCTIC FRONT OVER THE CENTRAL UNITED-STATES .2. COMPARISON WITH THEORY

This investigation examines the meso- and microscale aspects of the 9 March 1992 cold front that passed through Kansas during the daylight h ours. The principal feature of this front is the relatively rapid fron togenesis that occurred. The total change in the cross-frontal tempera ture is about 6 K, with most of the change occurring between about 082 0 and 1400 local Lime and over a relatively small subsection of the to tal frontal width. The surface data are able to resolve a sharp horizo ntal transition zone of 1-2 km. The principal physical processes that produce this frontogenesis are shown to be the cross-frontal different ial sensible heating, associated with differential cloud cover, and th e convergence of warm and cold air toward the front. The former proces s is responsible for an increase in the magnitude of the differential temperature change across the front; the latter process concentrates t he existing temperature differential across an ever-decreasing transit ional zone until a near discontinuity in the horizontal temperature di stribution is essentially established during the period of a few hours . Two approaches are taken to demonstrate that these processes control the observed frontogenesis. First, surface data from an enhanced arra y, set up during the Storm-scale Operational and Research Meteorology Fronts Experiment System Test, are used to evaluate the terms that con tribute to the time rate of change of the gradient of potential temper ature, d/del theta/\ dt, following the motion of the front. Then, the processes of differential sensible heating and convergence are incorpo rated into a simple two-dimensional nonlinear model that serves to pro vide a forecast of the surface temperature and velocity fields from gi ven initial conditions that are appropriate at the onset of the surfac e heating. Verification of the model predictions by observed data conf irms that both processes contribute to the observed daytime frontogene sis on 9 March 1992. A critique of the model does, however, suggest th at the accuracy of some quantitative evaluations could be improved.