7.7 Observations of Wind Jets at the Exit of Weber Canyon, Utah

Tuesday, 21 August 2012: 11:45 AM
Priest Creek C (The Steamboat Grand)
Morgan Farley-Chrust, University of Utah, Salt Lake City, UT; and C. D. Whiteman and S. W. Hoch

Thermally driven valley-exit jets occur at the exit of Utah's Weber Canyon, a main tributary of the Great Salt Lake basin. An intensive measurement campaign during July-September 2010 supplemented longer-term measurements to characterize the wind and temperature structure in the vicinity of the canyon exit.

Nighttime jet-axis wind speeds 50-150 m above the ground at the canyon exit typically reach 10-20 m s-1 on clear undisturbed nights in the late summer and fall. The jet forms 1-3 hours after sunset, approaches a near-steady state during the night and continues until 5-6 hours after sunrise, although slowly losing speed after sunrise. The jet is a local modification at the canyon exit of the normal thermally driven down-valley flow. Its continuation after sunrise is thought to be caused by the nighttime buildup and persistence of a cold-air pool in the Morgan Basin at the east end of the canyon.

Winds within the canyon and at the exit exhibit a strong seasonal and diurnal variation that is linked to the yearly course of solar insolation. Strong nocturnal low-level-wind jets formed on 75 of 90 nights (83%) during the measurement campaign. Winds inside the canyon consisted of a weak but deep down-valley flow layer that occupied most of the depth of the canyon. The flow was observed to descend, thin and compress at the exit producing winds that were typically twice as strong but much shallower than inside the canyon. Flows within the canyon and at the exit were influenced by larger-scale synoptic conditions. Clear skies and weak synoptic-scale pressure gradients were conducive to their development.

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