Inverted Troughs and Cyclogenesis over Interior North America: A Limited Regional Climatology and Case Studies

On 16-17 February 2007, snowbands formed in the lee of the Rocky Mountains in Wyoming, Colorado, and Nebraska on the anticyclonic-shear side of a midlevel jet streak. Two types of bands were prevalent: a longer, wider band associated with frontogenesis along an equatorward-moving cold front (major band) and multiple shorter, narrower bands farther poleward (minor bands). To understand how the upstream terrain affected the occurrence and intensity of the bands, multiple mesoscale model simulations were performed in which the terrain was incrementally smoothed. The evolutions of the synoptic patterns were similar in all simulations that included topography, but the synoptic pattern differed and no bands developed in a simulation with a flat land surface. These results allowed a focus on the changes to the banded precipitation due to the terrain resolution. Remarkably, although the exact location of the bands differed from run to run, the bands in all simulations with topography were in roughly the same region where they occurred on 16-17 February 2007. The major band was associated with frontogenesis along an equatorward-moving cold front that became stalled against the terrain. The minor bands formed from the release of conditional, symmetric, and inertial instabilities by ascent up the large-scale topography, rather than by ascent up specific small-scale topographic features. Because the bands were not tied to specific terrain features, these results suggest that the precise location of the minor bands is unpredictable.

Section: Introductionsupporting

confidence: 88%

Influence of Terrain Resolution on Banded Convection in the Lee of the Rocky Mountains

Schumacher

Schultz

Knox

2015

“…Overall, thermal advection regions within the CO cyclones followed paths predicted by quasigeostrophic theory and the associated precipitation patterns. Moreover, it is in agreement with a study by Keshishian et al (1994) that followed the frontal and thermal life cycle of a small composite of the CO cyclones.…”

Section: A Temperature Advection Fieldssupporting

confidence: 91%

“…A common manner in which storms have been investigated by synopticians is through the use of case studies. Such research has been performed on synoptic events to diagnose the components of the quasigeostrophic omega equation (e.g., Gyakum 1983Gyakum , 1991Keshishian and Bosart 1987;among others). Numerous investigators have used case studies to analyze moisture and stability of individual synoptic events as well, including Marshment and Horn (1986), who examined moisture content and stability for Colorado storms and Gyakum (1983), who investigated the stability profiles of a particularly strong East Coast storm.…”

Section: Introductionmentioning

confidence: 99%

Statistical Differences of Quasigeostrophic Variables, Stability, and Moisture Profiles in North American Storm Tracks

Mercer

Richman

2007

Three common synoptic storm tracks observed throughout the United States are the Alberta Clipper, the Colorado cyclone, and the East Coast storm. Numerous studies have been performed on individual storm tracks analyzing quasigeostrophic dynamics, stability, and moisture profiles in each. This study evaluated storms in each track to help diagnose patterns and magnitudes of the aforementioned quantities, documenting how they compare from track to track. Six diagnostic variables were computed to facilitate the comparison of the storm tracks: differential geostrophic absolute vorticity advection, temperature advection, Q-vector divergence, mean layer specific humidity, low-level stability, and midlevel stability. A dataset was compiled, consisting of 101 Alberta Clippers, 165 Colorado cyclones, and 159 East Coast cyclones and mean fields were generated for this comparison. Maxima and minima of the 25th and 75th percentiles were generated to diagnose magnitudes and patterns of strong versus weak cyclones and measure their similarities and differences to the mean patterns. Alberta Clippers were found to show the weakest magnitude of quasigeostrophic variables, while East Coast storms had the strongest magnitudes. Alberta Clippers maintained the lowest moisture content through their life cycle as well. However, East Coast storms were the most stable of the three tracks. Typically, correlations between storm tracks were high; suggesting that storm evolution is similar between tracks, in terms of the patterns of diagnostic variables measured. However, significant magnitude differences in the quasigeostrophic variables distinguished the storms in each track.

“…Eventually, the fastermoving cold front catches up to the lee trough, becoming one feature at the surface ( Fig. 4c), as discussed by numerous authors (e.g., Shapiro 1982;Locatelli et al 1989Locatelli et al , 2002aBLU;Bluestein 1993, p. 162;Keshishian et al 1994;Colle and Mass 1995;Hutchinson and Bluestein 1998;Neiman et al 1998;Neiman and Wakimoto 1999;Parsons et al 2000;Stoelinga et al 2000;Rose et al 2002). Thus, the structures and evolution of the conceptual model in Fig.…”

Section: August 2005mentioning

confidence: 62%

A Review of Cold Fronts with Prefrontal Troughs and Wind Shifts

Schultz

2005

The conceptual model of a classical surface-based cold front consists of a sharp temperature decrease coincident with a pressure trough and a distinct wind shift at the surface. Many cold fronts, however, do not conform to this model-time series at a single surface station may possess a pressure trough and wind shift in the warm air preceding the cold front (hereafter called a prefrontal trough and prefrontal wind shift, respectively). Although many authors have recognized these prefrontal features previously, a review of the responsible mechanisms has not been performed to date. This paper presents such a review. Ten disparate mechanisms with different frontal structures have been identified from the previous literature. These mechanisms include those external to the front (i.e., those not directly associated with the cold front itself): synoptic-scale forcing, interaction with lee troughs/drylines, interaction with fronts in the mid-and upper troposphere, and frontogenesis associated with inhomogeneities in the prefrontal air. Mechanisms internal to the front (i.e., those directly associated with the structure and dynamics of the front) include the following: surface friction, frontogenesis acting on alongfront temperature gradients, moist processes, descent of air, ascent of air at the front, and generation of prefrontal bores/gravity waves. Given the gaps in our knowledge of the structure, evolution, and dynamics of surface cold fronts, this paper closes with an admonition for improving the links between theory, observations, and modeling to advance understanding and develop better conceptual models of cold fronts, with the goal of improving both scientific understanding and operational forecasting.