Abstract
Abstract A cold-frontal passage through northern Utah was studied using observations collected during intensive observing period 4 of the Intermountain Precipitation Experiment (IPEX) on 14–15 February 2000. To illustrate some of its nonclassic characteristics, its origins are considered. The front developed following the landfall of two surface features on the Pacific coast (hereafter, the cold-frontal system). The first feature was a surface pressure trough and wind shift associated with a band of precipitation and rope cloud with little, if any, surface baroclinicity. The second, which made landfall 4 h later, was a wind shift associated with weaker precipitation that possessed a weak temperature drop at landfall (1°C in 9 h), but developed a stronger temperature drop as it moved inland over central California (4°–6°C in 9 h). As the first feature moved into the Great Basin, surface temperatures ahead of the trough increased due to downslope flow and daytime heating, whereas temperatures behind the trough decreased as precipitation cooled the near-surface air. Coupled with confluence in the lee of the Sierra Nevada, this trough developed into the principal baroclinic zone of the cold-frontal system (8°C in less than an hour), whereas the temperature drop with the second feature weakened further. The motion of the surface pressure trough was faster than the posttrough surface winds and was tied to the motion of the short-wave trough aloft. This case, along with previously published cases in the Intermountain West, challenges the traditional conceptual model of cold-frontal terminology, structure, and evolution.
Highlights
The conventional explanation for the movement of cold fronts is that they move by the advection of postfrontal cold air (e.g., Bjerknes 1919; Sanders 1955; Saucier 1955, p. 270; Wallace and Hobbs 1977, 116–117; Bluestein 1993, p. 259)
Consider a cold front traveling over the Pacific Ocean and making landfall in the western United States
Cold front subsequently pass through the western United States? Is it realistic to expect cold postfrontal air masses to be advected from the Pacific Ocean, across mountain ranges of 2000–3000-m elevation, and through the Intermountain West? Does this postfrontal air mass retain its properties of temperature and moisture throughout its passage across this complex terrain? If the advection of the postfrontal airmass does not control the speed of motion of cold fronts, the question of what controls frontal movement across the western United States—as well as other locations where complex terrain disrupts the lower-tropospheric frontal structure—becomes a relevant question for synoptic meteorology
Summary
The conventional explanation for the movement of cold fronts is that they move by the advection of postfrontal cold air (e.g., Bjerknes 1919; Sanders 1955; Saucier 1955, p. 270; Wallace and Hobbs 1977, 116–117; Bluestein 1993, p. 259). The goal of this article is to elucidate and explain these nonclassic characteristics and to synthesize across several previously published cases the kinds of processes that affect frontal structure and intensity in the western United States. This event occurred during the field phase of the Intermountain Precipitation Experiment (IPEX), a research program designed to improve the quantitative prediction of precipitation over the Intermountain West of the United States through better understanding of the relevant physical processes (Schultz et al 2002). We investigate the earlier structure and evolution of the cold-frontal system during IPEX IOP 4 from its arrival on the west coast of North America, its eastward movement across the western United States, and through to its arrival in northern Utah.
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