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  • Author or Editor: Matthew E. Jeglum x
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Matthew E. Jeglum and Sebastian W. Hoch

Abstract

Climatological features of the surface wind on diurnal and seasonal time scales over a 17-yr period in an area of complex terrain at Dugway Proving Ground in northwestern Utah are analyzed, and potential synoptic-scale, mesoscale, and microscale forcings on the surface wind are identified. Analysis of the wind climatology at 26 automated weather stations revealed a bimodal wind direction distribution at times when thermally driven circulations were expected to produce a single primary direction. The two modes of this distribution are referred to as the “northerly” and “southerly” regimes. The northerly regime is most frequent in May, and the southerly regime is most frequent in August. January, May, and August constitute a “tripole seasonality” of the wind evolution. Although both regimes occur in all months, the monthly changes in regime frequency are related to changes in synoptic and mesoscale phenomena including the climatological position of the primary synoptic baroclinic zone in the western United States, interaction of the large-scale flow with the Sierra Nevada, and thermal low pressure systems that form in the Intermountain West in summer. Numerous applications require accurate forecasts of surface winds in complex terrain, yet mesoscale models perform relatively poorly in these areas, contributing to poor operational forecast skill. Knowledge of the climatologically persistent wind flows and their potential forcings will enable relevant model deficiencies to be addressed.

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Matthew E. Jeglum, Sebastian W. Hoch, Derek D. Jensen, Reneta Dimitrova, and Zachariah Silver

Abstract

Large temperature fluctuations (LTFs), defined as a drop of the near-surface temperature of at least 3°C in less than 30 min followed by a recovery of at least half of the initial drop, were frequently observed during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program. Temperature time series at over 100 surface stations were examined in an automated fashion to identify and characterize LTFs. LTFs occur almost exclusively at night and at locations elevated 50–100 m above the basin floors, such as the east slope of the isolated Granite Mountain (GM). Temperature drops associated with LTFs were as large as 13°C and were typically greatest at heights of 4–10 m AGL. Observations and numerical simulations suggest that LTFs are the result of complex flow interactions of stably stratified flow with a mountain barrier and a leeside cold-air pool (CAP). An orographic wake forms over GM when stably stratified southwesterly nocturnal flow impinges on GM and is blocked at low levels. Warm crest-level air descends in the lee of the barrier, and the generation of baroclinic vorticity leads to periodic development of a vertically oriented vortex. Changes in the strength or location of the wake and vortex cause a displacement of the horizontal temperature gradient along the slope associated with the CAP edge, resulting in LTFs. This mechanism explains the low frequency of LTFs on the west slope of GM as well as the preference for LTFs to occur at higher elevations later at night, as the CAP depth increases.

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