High wind events in the lee of the Sierra Nevada—are they downslope windstorms?

Abstract

High winds events occur frequently in the Owens Valley east of the Sierra Nevada. These events are generally thought to be associated with mountain waves and downslope wind storms. Using long-term surface and upper air observations, we studied the climatology of high wind events and the synoptic conditions leading to their development. The surface observations are from nine stations, of which seven are located along the axis of the Owens Valley in the lee of the southern Sierra and two are located farther north in Mono County. The mean winds in the Owens Valley are bi-directional throughout the year and the climatology is dominated by a thermally-driven diurnal wind system, especially in the summer, when winds blow up the valley during daytime and down the valley at night. The high wind events, with speed above 10 m s-1, are most frequent from March through May and during afternoon hours. Instead of westerly winds at the surface, as one would expect from downslope wind storms associated with mountain wave events, theses high winds generally blow along the valley axis either from the NNW or SSE. Westerly downslope windstorms are quite rare in the climatology. The relationship between surface winds and geostrophic winds at the ridge top are examined, where ridgetop winds are estimated from interpolation from surrounding rawinsondes or from the NCEP North American Regional Reanalysis. The results show that several mechanisms are responsible for the development of the high wind events, including forced channeling of strong winds aloft along the valley axis, or pressure driven channeling in which the winds in the valley are driven by the along-valley component of the geostrophic pressure gradient, and to a lesser degree, downward transport of horizontal momentum,.