The climatology of westerly wind events in Owens Valley in the lee of the southern Sierra Nevada is derived from the data recorded by the mesonet of sixteen automatic weather stations located in Owens Valley near Independence, CA, which was installed by the Desert Research Institute in preparation for the Sierra Rotors Project in 2004. This long-term network, which has been collecting 30-second data since the end of February 2004, was also an integral part of the ground-based instrumentation suite in the recently completed Terrain-induced Rotor Experiment (T-REX) in spring 2006. The focus of this study is on westerly wind events associated with the Sierra Nevada mountain lee waves for which a satellite-based climatology has been recently compiled.

Preliminary results of the surface-based climatology indicate that westerly wind events occur throughout the year, most frequently in the period from March through September, predominantly in the afternoon hours. In summer months, thermally-foreced westerly wind events (Washoe Zephyr) occur exclusively in the afternoon hours. In months from September through May, in particular from March through May, a large number of lee-side westerly events are associated with westerly winds at the Sierra ridge-top in excess of 10 m/s, required for mountain wave generation by the Sierra Nevada. While strong lee-side westerlies due to wave activity also tend to occur in the afternoon hours, our analysis shows they can appear earlier in the course of the day and extend well into the night. A relationship between the climatology of westerly wind events derived from the surface wind data and of the mountain wave events based on the satellite data will be discussed.