Diurnally Driven Summertime Winds in the Lee of the Sierra: the Washoe Zephyr

Moderate westerly winds are a common feature of summer afternoons in western Nevada. Mark Twain was among the first to refer to these winds as the Washoe Zephyr. They are thought to occur in response to isallobaric gradients set up by faster pressure falls over the relatively warm elevated terrain of the Great Basin compared to those at similar heights in the relatively cool airmass over central California. The few studies that provide quantitative information on this phenomenon are based primarily on limited surface observations. This study extends our knowledge of the Washoe Zephyr by examining its evolution and vertical structure. The primary tool is a Doppler sodar located in a quiet agricultural area just east of the Reno-Tahoe International Airport. This instrument provides 30 m resolution profiles of horizontal and vertical wind from 60-900 m above the site elevation of ~1340 m MSL at intervals of 15-30 minutes. During July and August 1998, the Doppler sodar sampled several Washoe Zephyr events. The time of onset for these events varied considerably, ranging from 1200-1800 local time, as did their lifetimes, which lasted from 3-9 hours. Their kinematic evolution usually consisted of a downward shift of stronger westerly momentum at onset and an upward shift at decay. The vertical motion field often showed an oscillating pattern 1-2 hours prior to onset with strong downward motion immediately before onset and strong upward motion just after decay.