Observations of Thermally Driven Wind Jets at the Exit of Weber Canyon, Utah

Abstract

hermally driven valley-exit jets were investigated at 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. Exit jets at Weber Canyon are most frequent in late summer or early fall. Strong low-level-wind jets formed at the canyon exit on 75 of 90 nights (83%) during the measurement campaign, with the best-developed winds forming during synoptically undisturbed, clear-sky periods. Winds inside the canyon consisted of a weak down-valley flow layer that occupied most of the 1000-m depth of the canyon. The flow was observed to descend, thin, and accelerate at the valley exit, producing winds that were typically 2.5 times as strong but much more shallow than those inside the canyon. Maximum nighttime jet-axis wind speeds of 15?20 m s?1 are typically found about 80?120 m above the ground at the canyon exit on clear undisturbed nights in the late summer and fall. The jets form 1?3 h after sunset, approach a near-steady state during the late night, and continue until 5?6 h after sunrise, although slowly losing speed after sunrise. The jet is a local modification at the canyon exit of the 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. The potential for utilizing the exit jet for wind power generation is discussed.