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W. James Steenburgh
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W. James Steenburgh

Abstract

Synoptic, orographic, and lake-effect precipitation processes during a major winter storm cycle over the Wasatch Mountains of northern Utah are examined using radar imagery, high-density surface data, and precipitation observations from Alta Ski Area [2600–3200 m above mean sea level (MSL)] and nearby Salt Lake City International Airport (1288 m MSL). The storm cycle, which occurred from 22 to 27 November 2001, included two distinct storm systems that produced 108 in. (274 cm) of snow at Alta Ski Area, including 100 in. (254 cm) during a 100-h period. Each storm system featured an intrusion of low equivalent potential temperature (θ e ) air aloft, well in advance of a surface-based cold front. Prefrontal precipitation became increasingly convective as low-θ e air aloft moved over northern Utah, while cold frontal passage was accompanied by a convective line and a stratiform precipitation region. Postfrontal destabilization led to orographic and lake-effect snowshowers that produced two-thirds of the observed snow water equivalent at Alta.

Storm stages were defined based on the passage of the above features and their accompanying changes in stability and precipitation processes. Contrasts between mountain and lowland precipitation varied dramatically from stage to stage and storm to storm, and frequently deviated from climatology, which features a nearly fourfold increase in precipitation between Salt Lake City and Alta. Based on the two storms, as well as other studies, a schematic diagram is presented that summarizes the evolution of Intermountain West snowstorms featuring an intrusion of low-θ e air aloft ahead of a surface cold front. Implications for short-range quantitative precipitation forecasting and seasonal-to-annual hydrometeorological prediction are discussed.

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W. James Steenburgh
and
James R. Holton

Abstract

The conceptual model for height tendency presented by Hirschberg and Fritsch directly links upper-level virtual temperature tendency with low-level height tendency, overlooking the essential dynamics of mass divergence. An analysis of the complete height tendency equation shows that upper-level virtual temperature change car only indirectly induce low-level height change by driving ageostrophic circulations. To avoid misconceptions about middle- and lower-tropospheric height tendency, the dynamics of height tendency are reviewed.

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W. James Steenburgh
and
Clifford F. Mass

Abstract

Observational analyses and numerical simulations are used to investigate the interaction of an intense extra-tropical cyclone with the coastal orography of the Pacific Northwest. Known as the “Inauguration Day cyclone,” the system made landfall upon the Washington State coast on 20 January 1993, producing one of the most damaging wind storms in Pacific Northwest history. The strongest winds accompanying the storm were associated with an intense low-level pressure gradient that was concentrated along a bent-back front. Mesoscale pressure perturbations produced by the time-dependent interaction of the cyclone and bent-back front with the coastal orography were isolated using numerical simulations. The simulations showed that during the period of peak winds over Puget Sound, there was only a minor enhancement of the local pressure gradient by troughing to the lee (east) of the Olympic Mountains. Gradual amplification of this Olympic Mountain lee trough over a period of 2–3 h extended the period of strong winds by enhancing the pressure gradient over Puget Sound as the bent-back front moved out of the region.

The influence of orographically induced coastal ridging and pressure surges was also investigated. It was found that the evolution of coastal ridging was closely connected to the progressive northward development of onshore flow behind the bent-back front. There was no evidence that a self-propagating feature, such as a Kelvin wave or gravity current, was triggered during the landfall of the cyclone and its attendant fronts. The momentum budget in the coastal zone following passage of the bent-back ftont is also discussed.

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W. James Steenburgh
and
Clifford F. Mass

Abstract

This paper describes the life cycle of a lee trough associated with the passage of a baroclinic wave over the Rocky Mountains based on two overlapping simulations by the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model. The evolution of the lee trough may be summarized as follows. A surface cyclone and lee trough developed and intensified as an upper-level short-wave trough approached and moved over the Rockies. Near the low center,.the lee trough was found to be frontogenetical, with the frontogenesis being forced by 1) confluent deformation associated with the wind fields of the developing cyclone and lee trough and 2) differential vertical motion along the eastern slopes of the Rocky Mountains and the adjacent sloping terrain of the High plains. In turn, the frontogenesis drove an ageostrophic circulation that sharpened the lee trough. The lee trough represented a break in trajectory origin, with trajectories ending ahead of the lee trough originating over the Great Plains, and trajectories ending behind the trough beginning over the high terrain of the Rocky Mountains.

The lee trough developed a warm occlusion-like structure as it moved eastward across the Great Plains and was overtaken by an upper-level short-wave trough and associated upper-level baroclinic zone. The upper-level baroclinic zone provided the “elevated cold front” of the occlusion, while at the surface, an occlusion-like thermal ridge formed as the warm tongue and baroclinity of the lee trough were overtaken by a zone of low-level cold advection. This zone of cold advection could not be traced to a preexisting surface-based frontal zone, but rather developed from the convective mixing of baroclinity front aloft to the surface.

A conceptual model is presented that describes the influence of the Rocky Mountains and sloping terrain of the High Plains upon the evolution of die cyclone and lee trough. Primarily due to the complex terrain of the region, this evolution deviates drastically from the Norwegian cyclone model. For example, near the Rockies, neither a classical warm sector in which warm air is advected from the south nor a classical surface cold front separating warm-sector air from polar air are evident. Well downstream of the mountains, however, the cyclone begins to develop a more classical appearance.

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David M. Schultz
and
W. James Steenburgh

Abstract

A mesoscale-model simulation is used to examine the evolution of the cold front and accompanying cloud bands in eastern Mexico associated with Superstorm 1993 (12–14 March). The simulated cold front differed in structure and evolution from a classical cold front, in agreement with evidence from observations and European Centre for Medium-Range Weather Forecasts analyses. The surface cold front, as defined by the leading edge of strong northerlies and cold advection, initially possessed a rearward tilt with height over southern Texas. Within 6 h, the leading edge of the front moved equatorward and developed a large-scale forward tilt of greater than 200 km in the horizontal from the surface to 700 hPa. This forward tilt occurred as a mid- to upper-tropospheric baroclinic zone arrived from over the Sierra Madre, descended into eastern Mexico, and interacted with the surface cold front. Embedded within this large-scale forward tilt was a locally enhanced horizontal potential temperature gradient that also tilted forward ∼100 km from the surface to 850 hPa. Tilting frontogenesis associated with ascent at the leading edge of the surface front was responsible for the smaller-scale forward-tilting structure. This surface-based ascent is believed to have caused the primary cloud band observed from satellite imagery that is coincident with the leading edge of the front, whereas a second region of ascent, elevated at the leading edge of the mid- to upper-tropospheric baroclinic zone, is believed to have caused the prefrontal cloud band revealed by satellite imagery. Subsidence behind the forward-tilting cold-frontal structure at and above 850 hPa (and concomitant divergence underneath) resulted in frontolysis of the surface front, and eventually the dissipation of the primary cloud band, leading to the dominance of the prefrontal cloud band. Finally, the Superstorm 1993 cold front is compared and contrasted to nonclassical cold-frontal structures found in the literature and a general context for frontal interaction is discussed.

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Jason C. Shafer
and
W. James Steenburgh

Abstract

Motivated by the intensity and severity of winds and temperature falls that frequently accompany rapidly developing cold fronts in northern Utah, this paper presents a 25-yr climatology of strong cold frontal passages over the Intermountain West and adjoining western United States. Using conventional surface observations and the North American Regional Reanalysis, strong cold frontal passages are identified based on a temperature fall of 7°C or greater in a 2–3-h period, a concurrent pressure rise of 3 hPa or greater, and the presence of a large-scale 700-hPa temperature gradient of at least 6°C (500 km)−1. The number of strong cold frontal passages exhibits a strong continental signature with very few events (<10) along the Pacific coast and more than 200 events east of the Continental Divide. The number of events increases dramatically from the Cascade Mountains and Sierra Nevada to northern Utah, indicating that the Intermountain West is a frequent cold front breeding ground.

A composite of the 25 strongest events at Salt Lake City (based on the magnitude of the temperature fall) reveals that confluent deformation acting on a broad baroclinic zone over central Nevada commonly initiates Intermountain frontogenesis. The confluent deformation develops in southwesterly large-scale flow and appears to be enhanced by flow deflection around the Sierra Nevada. Quasi-stationary development and intensification of the southwest–northeast-oriented cold front then occurs as a mobile upper-level trough approaches from the west. The front becomes mobile as cold advection and ascent associated with the upper-level trough overtake the low-level front. Cloud and precipitation observations suggest that differential diabatic heating contributes to the rapid frontal intensification in many events.

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Gregory L. West
and
W. James Steenburgh

Abstract

Recent studies indicate that strong cold fronts develop frequently downstream of the Sierra Nevada over the Intermountain West. To help ascertain why, this paper examines the influence of the Sierra Nevada on the rapidly developing Intermountain cold front of 25 March 2006. Comparison of a Weather Research and Forecasting (WRF) model control simulation with a simulation in which the height of the Sierra Nevada is restricted to 1500 m (roughly the elevation of the valleys and basins of the Intermountain West) shows that the interaction of southwesterly prefrontal flow with the formidable southern High Sierra produces a leeward orographic warm anomaly that enhances the cross-front temperature contrast. Several processes generate this orographic warm anomaly, including flow modification by the Sierra Nevada (i.e., windward blocking of low-level Pacific air, leeward subsidence, and increased southerly flow from the Mojave Desert and lower Colorado River basin into the Intermountain West), diabatic heating and water vapor loss associated with orographic precipitation, and increased sensible heating and reduced subcloud diabatic cooling in the downstream cloud and precipitation shadow. In contrast, the postfrontal air mass experiences comparatively little orographic modification as it moves across the relatively low northern Sierra Nevada. These results show that the Sierra Nevada can enhance frontal development, which may contribute to the high frequency of strong cold-frontal passages over the Intermountain West.

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Clifford F. Mass
and
W. James Steenburgh

Abstract

Observational analyses and a high-resolution simulation using The Pennsylvania State University–NCAR Mesoscale Model Version 5 (MM5) were used to describe the coastally trapped wind reversal of 19–21 July 1994. Major findings include the following.

  • The event was initiated and controlled by changes in the synoptic-scale flow, and particularly by the development of offshore flow over the coastal terrain of Oregon. Although synoptic control was dominant, blocking of the inversion-capped marine layer and mesoscale coastal pressure ridging were important components of the event.

  • The synoptic evolution associated with the trapped reversal was characterized by a shift from climatological near-zonal 500-mb flow to high-amplitude upper-level ridging over the eastern Pacific. As this upper ridge moved northeastward, the 850-mb flow over Oregon became northeasterly and then southeasterly, and 850-mb heights fell over western Oregon.

  • The combination of falling heights aloft and increasing low-level offshore flow, with associated downslope subsidence warming and offshore advection of warm continental air, resulted in sea level pressure falling along the Oregon coast and the northward extension of the California thermal trough into northern Oregon. The extension of the thermal trough caused a reversal of the alongshore pressure gradient along the Oregon coast, so that pressure increased to the south, as well as the attenuation or reversal of the normal cross-shore pressure gradient over the coastal waters.

  • As the coastal trough intensified, there was an increase in nearly geostrophic, onshore-directed coastal flow, with appreciable blocking and deflection of the inversion-capped marine layer by the coastal terrain. With an increase in the northward-directed pressure gradient force due to the troughing to the north, and a lesser contribution from damming of the marine air on the coastal terrain, the blocked low-level winds developed a coastally trapped southerly component over a considerable expanse of the southern and central Oregon coast.

  • Before wind reversal, the northerly flow over the coast was nearly geostrophic. After the coastal winds turned southerly, near-geostrophic balance in the cross-shore direction was established, with the offshore-directed pressure gradient force associated with the coastal pressure ridge balanced by the eastward-directed Coriolis force associated with the southerly flow. In contrast, the momentum balance in the alongshore direction was highly ageostrophic after wind reversal, with near-antitriptic balance close to shore between the northward-directed pressure gradient force and friction, while farther offshore the pressure gradient and Lagrangian accelerations were roughly in balance.

  • The offshore scale of the blocking within the marine layer was less than in the stable layer immediately above, a result consistent with previous theoretical and modeling studies. This difference in offshore blocking scale resulted in the offshore wind reversal occurring in the stable layer aloft before it occurred at the surface.

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W. James Steenburgh
and
Daryl J. Onton

Abstract

The large-scale and mesoscale structure of the Great Salt Lake–effect snowstorm of 7 December 1998 is examined using radar analyses, high-density surface observations, conventional meteorological data, and a simulation by the Pennsylvania State University–National Center for Atmospheric Research fifth generation Mesoscale Model (MM5). Environmental conditions during the event were characterized by a lake–700-hPa temperature difference of up to 22.5°C, a lake–land temperature difference as large as 10°C, and conditionally unstable low-level lapse rates. The primary snowband of the event formed along a land-breeze front near the west shoreline of the Great Salt Lake. The snowband then migrated eastward and merged with a weaker snowband as the land-breeze front moved eastward, offshore flow developed from the eastern shoreline, and low-level convergence developed near the midlake axis. Snowfall accumulations reached 36 cm and were heaviest in a narrow, 10-km-wide band that extended downstream from the southern shore of the Great Salt Lake. Thus, although the Great Salt Lake is relatively small in scale compared to the Great Lakes, it is capable of inducing thermally driven circulations and banded precipitation structures similar to those observed in lake-effect regions of the eastern United States and Canada.

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