Onset of strong downslope winds during the Sierra Rotors Project

During the field phase of the Sierra Rotors Project (SRP), which took place during the spring of 2004, several strong downslope wind events were documented by the SRP observing network. The observational assets during SRP included a network of 16 surface stations, two wind profilers, and upstream and downstream radiosondes that augmented the larger-scale NWS radiosonde network. Most events show significant time lag between the strong westerlies at the mountain top (see question below) level documented by radiosondes and forecasted by large-scale models and the onset of westerlies in the Owens Valley documented by the surface network and wind profilers. For example, from 12 to 17 of April 2004, while westerly prevailed at the mountain top level, the surface network indicated that the onset of the westerly in the valley occurred around late afternoons. Navy's COAMPS® has been used to simulate some of the documented westerly events in a multiple-nesting mode. Reasonable agreement between the COAMPS simulations and the observations is found. The delay of the onset of the westerly inside the Owens valley has been investigated based on diagnoses of the COAMPS real-data simulations and additional sensitivity tests. Preliminary results point to the multiscale interactions among four elements, namely, synoptic-scale balanced flow (horizontal scale L~500 km or larger), terrain induced gravity waves (L~10-30 km), the mesoscale response to the differential heating of the Sierra-Rocky plateau (L~ 500 km or larger) scale and thermally-driven valley winds (L~10 km). A theoretical model will be presented that will further elucidate the relevant dynamics associated with the onset of the westerly in the Owens Valley.