12A.4 Climatology, Life Cycle, and Impacts of Intermountain Cyclones

Thursday, 27 January 2011: 9:15 AM
613/614 (Washington State Convention Center)
W. James Steenburgh, University of Utah, Salt Lake City, UT; and G. L. West, M. E. Jeglum, T. Lee, L. Bosart, J. Massey, and T. Painter

The topography in and around the Intermountain West strongly affects the genesis, migration, and lysis of extratropical cyclones and their attendent fronts. Here we examine Intermountain (a.k.a. Nevada or Great Basin) cyclone (IC) activity and evolution using climatological, observational, and numerical modeling approaches.

A multi-reanalysis climatology based on the ERA-Interim and NARR shows that Intermountain cyclone activity is greatest in two distinct regions. The first, which we call the Great Basin cyclone region, extends northeastward from the southern “high” Sierra to the Great Salt Lake Basin of northwest Utah. The second, which we call the Canyonlands cyclone region, lies over the upper Colorado River Basin of southeast Utah, a lowland region between the mountains and plateaus of central Utah and the Colorado Rockies.

An ERA-Interim-based composite of strong ICs generated in cross-Sierra (210-300°) 500-hPa flow shows that cyclogenesis is preceeded by the development of the Great Basin Confluence Zone (GBCZ), a regional airstream boundary that extends downstream from the Sierra Nevada across the Intermountain West. Cyclogenesis occurs along the GBCZ as large-scale ascent develops over the Intermountain West in advance of an approaching upper-level trough. Flow splitting around the high Sierra and the presence of low-level baroclinity along the GBCZ suggest that IC evolution may be better conceptualized from a potential vorticity perspective than from traditional quasigeostrophic models of lee cyclogenesis.

A case study of the 2002 “Tax Day Cyclone”, which produced the second lowest sea level pressure observed in Utah during the instrumented record further illustrates that the GBCZ is a key mesoscale surface feature of Intermountain cyclone evolution and contributes to frequent development of strong cold fronts over eastern Nevada and Utah. Strong contraction (i.e., deformation and convergence) along the GBCZ forms an airstream boundary that is initially non-frontal, but becomes the local for surface frontogenesis as it collects and concentrates baroclinity from the northern Great Basin. As an upper-level cyclonic potential vorticity anomaly and quasi-geostrophic 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 classical models of lee cyclogenesis.

High winds accompanying these Intermountain cyclones and cold fronts are responsible for several recent dust storms over Utah. In particular, strong prefrontal southerly winds transport dust from two major point sources in southern Utah: the Sevier dry-lake bed and Milford Flat fire scar. Deposition of this dust onto the mountain snowpack of the Wasatch Mountains results in increased absorption of solar radiation, leading to earlier snowmelt and peak stream flows during the spring.

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