A dense surface network for atmospheric observations in a mountain valley: Deployment and observations in the Sierra Rotors project

A dense network of surface stations was deployed in Owens Valley in the lee of the Sierra Nevada in eastern California during February 2004 in support of the DRI-led Sierra Rotors Project (March-April 2004). Atmospheric rotors are intense horizontal vortices that form in association with mountain waves, and whose turbulence poses a hazard to aviation. The Sierra Rotors Project (SRP) was designed in part to establish basic quantitative characteristics of rotors in Owens Valley, in preparation for a larger effort, the upcoming Terrain-induced Rotor EXperiment (T-REX).

The installed surface network consists of 16 automated weather stations with telemetry, laid in three transects perpendicular to the approximately north-south axis of the central portion of Owens Valley, south of Independence, CA. This portion of Owens Valley is bounded by the the High Sierra to the west (>4400 m), and by the Inyo Mountains to the east (~3300 m). The eastern slopes of the High Sierra contain the steepest orographic gradients (~30%) in the contiguous United States. These mountains bound a U-shaped valley with a floor at 1200 m. Two transects about 2 km apart contain six stations, whereas the third parallel transect about 5 km further south has 4 stations. The average separation between individual stations along these lines is approximately 3 km. The 10-m towers are equipped with sensors for wind speed and wind direction, temperature, relative humidity, and pressure. To capture rapid dynamic evolution of waves and rotors, sensors are sampled every 3 s, with the 30-second averaged data saved on stations' data loggers before transmitted via 900 MHz spread-spectrum radio communication to a base station in Independence. From the base station, data is sent over Internet to the central repository at DRI, allowing near real-time online access to the graphically-displayed 10-minute averaged data. In the first year of operation no data have been lost, except for one act of vandalism.

The high quality data has provided information on both violent windstorms and on the repetitive flows of the mountain-valley diurnal cycle. During the strongest event of the SRP, a ground "footprint" of the rotor was captured by the Owens Valley surface network. In addition to being a unique asset for mesoscale dynamics studies in the lee of the Sierra Nevada, the data from this network can furnish valuable insights on the representativeness of climate measurements for programs such as the Climate Reference Network. This dense network will constitute one element in a larger effort to more comprehensively observe the entire Sierra Nevada, including 10-minute data currently provided via Internet from three sites in the nearby White Mountains, at elevations of 3100, 3800, and 4300 m.