The Structure and Evolution of a Continental Winter Cyclone. Part I: Frontal Structure and the Occlusion Process

Abstract

The production of a narrow, heavy, occasionally convective snowband that fell within a modest surface cyclone on 19 January 1995 is examined using gridded model output from a successful numerical simulation performed using the University of Wisconsin-Nonhydrostatic Modeling System. It is found that the snowband was produced by a thermally direct vertical circulation forced by significant lower-tropospheric warm frontogenesis in the presence of across-front effective static stability differences as measured in terms of the equivalent potential vorticity (PV e). The sometimes convective nature of the snowband resulted from the development of freely convective motions forced by frontal lifting of the environmental stratification. Model trajectories demonstrate that a stream of warm, moist air ascended through the trowal portion of the warm-occluded structure that developed during the cyclone life cycle. The lifting of air in the trowal was, in this case, forced by lower-tropospheric frontogenesis occurring in the warm-frontal portion of the warm occlusion. This trowal airstream accounts for the production of the so-called wrap-around precipitation often associated with occluded cyclones and, in this case, accounted for the northern third of the heavy snowband.

Using Frontogenesis to Identify Sting Jets in Extratropical Cyclones

“…Petterssen frontogenesis has been used previously to diagnose fronts in the central United States (e.g., Koch 1984;Keshishian et al 1994;Martin 1998a;Schultz 2004), the western United States (Steenburgh and Mass 1994;Schultz and Knox 2007;Steenburgh et al 2009;Schumacher et al 2010;West and Steenburgh 2010), and idealized baroclinic waves (Sch€ ar and Wernli 1993;Schultz and Zhang 2007); to calculate climatologies of frontogenesis (Satyamurty and De Mattos 1989); to determine regions of ascent associated with precipitation bands within heavy rainstorms (Sanders 2000) and within snowstorms (e.g., Bosart and Lin 1984;Keshishian and Bosart 1987;Roebber et al 1994;Martin 1998b;Trapp et al 2001;Novak et al 2004Novak et al , 2006Novak et al , 2008Novak et al , 2009Novak et al , 2010; to determine the time of extratropical transition of a tropical cyclone (Harr and Elsberry 2000); and to indicate regions of predecessor precipitation ahead of tropical cyclones (e.g., Galarneau et al 2010;Moore et al 2013). Lackmann (2011, sections 6.2 and 6.3) provides a moredetailed description of Petterssen frontogenesis.…”

Reexamination of the 9–10 November 1975 “Edmund Fitzgerald” Storm Using Today's Technology

“…This lack of observed data, both in space and time, makes it difficult to assess the severity of conditions on Lake Superior throughout the storm, and how those conditions varied. Numerous studies (Uccellini et al 1987;Whitaker et al 1988;Martin 1998a;Mann et al 2002;Roebber et al 2002;Poulos et al 2002;Meyers et al 2003) have shown the utility of using numerical weather prediction models to better diagnose the specific conditions of the atmosphere throughout an event. It was therefore determined that high-resolution numerical simulations could be used to help attain a more complete picture of the wind and wave conditions during the storm.…”

“…Previous studies that included successful model simulations of events (Martin 1998b;Roebber et al 2002;Poulos et al 2002;Meyers et al 2003) utilized high-resolution output from the simulations to help diagnose conditions in greater detail than would be possible through observational data alone. Following along these lines, we believe that if the atmospheric simulation successfully resolves synoptic-scale conditions, it also offers a reasonable estimate of mesoscale details over Lake Superior throughout the storm.…”

Band on the Run - Chasing the Physical Processes Associated with Heavy Snowfall

“…These two satellite imagery perspectives illustrate the juxtaposition of two critical airstreams that contributed to the heavy snowfall bands in South Dakota. The warm, moist airstream, known as the warm conveyor belt (WCB; Carlson 1980), rises as it moves cyclonically to the northwest, forming a trough of warm air aloft (trowal; Martin 1998). For this event, evidence of the trowal is seen as an axis of high equivalent potential temperature (q e ) wrapping into western Minnesota at 650 hPa at 1500 UTC (Fig.…”