STRUCTURE AND EVOLUTION OF WINTER CYCLONES IN THE CENTRAL UNITED-STATES AND THEIR EFFECTS ON THE DISTRIBUTION OF PRECIPITATION .2. ARCTIC FRONTS

The structure and evolution of a shallow but intense cold front (commonly referred to as an arctic front) and its associated precipitation features that passed through the central United States from 0000 UTC 9 March to 0000 UTC 10 March 1992 are studied with the aid of observations and outputs from a numerical simulation using the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model MM4. Located above the arctic front was a region of midtropospheric, frontogenetical confluence that was attended by a thermally direct vertical circulation. A large banded precipitation feature, for the most part located behind the arctic front, was produced by ice crystals from upper-level clouds (formed by the frontogenetical confluence) falling into low-level stratocumulus associated with the arctic front. The arctic front at the surface separated a region where the precipitation reaching the ground was solid from an adjacent region where the precipitation was liquid. A westward-moving, low-level jet behind the arctic front produced upslope flow over the high terrain of the northern Great Plains, which contributed to heavy snowfalls in this region. A portion of the arctic front that moved southward, west of a low pressure center, was characterized by sharp drops in temperature and dewpoint and an increase in wind speed. However, the arctic front was not associated with either a pressure trough or much change in wind direction. The proximity of arctic fronts to such nonfrontal features as tee troughs and/or drylines often leads to the latter being misanalyzed as cold fronts.