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Peng-Yun Wang, Jonathan E. Martin, John D. Locatelli, and Peter V. Hobbs

Abstract

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 lee troughs and/or drylines often leads to the latter being misanalyzed as cold fronts.

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John D. Locatelli, Jonathan E. Martin, Jeffrey A. Castle, and Peter V. Hobbs

Abstract

From 8 to 9 March 1992 cold frontogenesis aloft (CFA), which was associated with the development of a vigorous baroclinic wave, triggered a series of squall lines that produced large hail and several tornadoes as they moved across the central United States. The air lifted by the CFA, which produced the squall lines, was made potentially unstable as a result of the circulation associated with a surface drytrough. This study provides further support for the view that in winter and early spring CFA plays an important role in triggering severe weather in the central United States.

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Jonathan E. Martin, John D. Locatelli, Peter V. Hobbs, Peng-Yun Wang, and Jeffrey A. Castle

Abstract

A convective rainband, which was approximately 1500 km in length and affected large areas of the central United States for about 16 h, developed within an evolving winter cyclone. The rainband, which will be referred to as the pre-drytrough rainband, formed approximately 400 km ahead of a developing dryline and lee trough (drytrough, for short) that created an elevated, sloping layer of convective instability. The presence 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 terrain. This enhanced the lifting associated with a migrating short wave aloft and generated the pre-drytrough rainband.

In previous studies the dryline, the lee trough, the elevated mixed layer, and the low-level jet 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 disturbances with topography.

Mesoscale structures and precipitation distributions similar to those documented in this paper are common in winter cyclones in the central United States and they are responsible for much of the severe weather associated with these systems.

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