Life Cycle and Mesoscale Frontal Structure of an Intermountain Cyclone

Abstract

High-resolution analyses and MesoWest surface observations are used to examine the life cycle and mesoscale frontal structure of the ''Tax Day Storm,'' an intermountain cyclone that produced the second lowest sea level pressure observed in Utah during the instrumented period and the strongest cold frontal passage at the Salt Lake City International Airport in the past 25 years. A key mesoscale surface feature contributing to the cyclone's evolution is a confluence zone that extends downstream from the Sierra Nevada across the Great Basin. Strong contraction (i.e., deformation and convergence) within this Great Basin confluence zone (GBCZ) forms an airstream boundary that is initially nonfrontal but becomes the locus for surface frontogenesis as it collects and concentrates baroclinicity from the northern Great Basin, including that accompanying an approaching baroclinic trough. Evaporative and sublimational cooling from postfrontal precipitation, as well as cross-front contrasts in surface sensible heating, also play an important role, accounting for up to 40% of cross-front baroclinicity. As an upper-level cyclonic potential vorticity anomaly and quasigeostrophic forcing for ascent move over the Great Basin, cyclone development occurs along the GBCZ and developing cold front rather than within the Sierra Nevada lee trough, as might be inferred from classic models of lee cyclogenesis. Front-mountain interactions ultimately produce a very complex frontal evolution over the basinand-range topography of northern Utah.The analysis further establishes the role of the GBCZ in intermountain frontogenesis and cyclone evolution. Recognition of this role is essential for improving the analysis and prediction of sensible weather changes produced by cold fronts and cyclones over the Intermountain West.

Multi-Reanalysis Climatology of Intermountain Cyclones

“…This evolution is remarkably similar to that observed by West and Steenburgh (2010) during the 2002 Tax Day Storm and suggests that the orographic modification of IC genesis is more complex than may be inferred from studies of Rocky Mountain lee cyclogenesis (e.g., Palmén and Newton 1969;Bannon 1992;Steenburgh and Mass 1994;Davis 1997;Schultz and Doswell 2000;Thomas and Martin 2007). In particular, preexisting low-level baroclinicity over the Intermountain West, which may be concentrated in some cases along the GBCZ (West and Steenburgh 2010), appears to act as a surface thermal anomaly in the classical PV view of cyclone development (Hoskins et al 1985). This shifts the locus for cyclone development from the Sierra Nevada to farther downstream along the developing GBCZ and frontal boundary.…”

“…15c). Downstream of the high Sierra, the Great Basin confluence zone (GBCZ), an airstream boundary that frequently contributes to cold-frontal development over the Intermountain West and separates southerly to southwesterly flow over Utah and southern Nevada from westerly flow over northern Nevada (Shafer and Steenburgh 2008;West and Steenburgh 2010), extends across the Intermountain West. Although one might expect a traditional barrier-parallel lee trough as observed during Rocky Mountain lee cyclogenesis (e.g., Steenburgh and Mass 1994), the most pronounced surface trough lies normal to the Sierra Nevada along the GBCZ and near the leading edge of the 700-hPa baroclinic zone.…”

“…1), has long been recognized as a region of frequent cyclone activity (e.g., Petterssen 1956;Klein 1957;Whittaker and Horn 1981;Lee 1995). Hazardous weather produced by intermountain (i.e., Nevada or Great Basin) cyclones (ICs) includes high winds, blowing dust, wildfire runs, and dramatic cold-frontal temperature falls (e.g., Shafer and Steenburgh 2008;Steenburgh et al 2009;West and Steenburgh 2010). For example, from 15 to 16 April 2002, a powerful IC produced the second lowest reduced sea level pressure (982 hPa) ever recorded in Utah (West and Steenburgh 2010).…”

“…Hazardous weather produced by intermountain (i.e., Nevada or Great Basin) cyclones (ICs) includes high winds, blowing dust, wildfire runs, and dramatic cold-frontal temperature falls (e.g., Shafer and Steenburgh 2008;Steenburgh et al 2009;West and Steenburgh 2010). For example, from 15 to 16 April 2002, a powerful IC produced the second lowest reduced sea level pressure (982 hPa) ever recorded in Utah (West and Steenburgh 2010). Winds accompanying the storm reached 35 m s 21 in Salt Lake City, Utah, and 42 m s 21 in the nearby Wasatch Mountains.…”

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.…”