The effects of gap and along-valley flow on surface winds during the Terrain-induced Rotor Experiment

A specific objective of the Terrain-induced Rotor Experiment (T-REX) was the improvement in modeling and prediction of downslope windstorms. T-REX took place in Owens Valley, California, a narrow rift valley to the lee of the southern Sierra Nevada. The onset of westerly surface flow in Owens Valley is complicated by the presence of gaps in the upstream topography, which allows westerly winds to enter the valley earlier at favored locations, and the formation of a strong, southerly up-valley flow, which can inhibit the penetration of westerlies into the valley. This study investigates these factors in the case of T-REX IOP .

During IOP 6, the University of Wyoming King Air performed three research flights. The first two flights, on the afternoon of 24 March 2006 and the morning of 25 March 2006, used a repeated box pattern, which contained multiple tracks parallel to the Sierra ridge including across Kearsarge Pass. During the 24 March flight, a distinct westerly flow was observed downstream of Kearsarge Pass which was not present downwind of the adjoining ridges or at the surface where southerly flow was dominant. In the morning flight of 25 March, westerly flow was observed along the entire downstream ridge and intermittent bursts of westerlies were seen in the surface observing network. A numerical simulation of this event was performed with the Naval Research Laboratory's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). The model correctly produced only southerly surface flow on the 24th and isolated gap jet structures on the morning of the 25th preceding a more continuous push of westerlies.