STRUCTURE AND EVOLUTION OF WINTER CYCLONES IN THE CENTRAL UNITED-STATES AND THEIR EFFECTS ON THE DISTRIBUTION OF PRECIPITATION .1. A SYNOPTIC-SCALE RAINBAND ASSOCIATED WITH A DRYLINE AND LEE TROUGH

A convective rainband, which was approximately 1500 km in length and a ffected large areas of the central United States for about 16 h, devel oped within an evolving winter cyclone. The rainband, which will be re ferred to as the pre-drytrough rainband, formed approximately 400 km a head of a developing dryline and lee trough (drytrough, for short) tha t created an elevated, sloping layer of convective instability. The pr esence of a deep pool of high-potential-temperature air in the middle troposphere over the south-central United States, advected there from the elevated terrain to the southwest (i.e., an elevated mixed layer), produced a region of warm-air advection downstream of the high terrai n. This enhanced the lifting associated with a migrating short wave al oft and generated the pre-drytrough rainband. In previous studies the dryline, the lee trough, the elevated mixed layer, and the low-level j et in the central United States have generally been viewed as isolated features. Here the authors present a more integrated view, compelled by their common dependence on the interactions of synoptic-scale distu rbances with topography. Mesoscale structures and precipitation distri butions similar to those documented in this paper are common in winter cyclones in the central United States and they are responsible for mu ch of the severe weather associated with these systems.