Sierra Rotors: A comparative study of three mountain wave and rotor events

The Sierra Rotors Project (SRP) took place in March and April of 2004 to the lee of the southern Sierra Nevada in Owens Valley, California. The primary instrumentation consisted of a long-term mesonetwork of automated weather stations, upstream rawinsonde launches, and two integrated sounding systems in the valley. In this study, we use these observations and high-resolution (333 m) numerical simulations to compare and contrast the structure and evolution of three of the strongest wave and rotor events observed during SRP: IOP 8, IOP 14, and IOP 16.

All three events were accompanied by visual evidence of wave activity. IOP 8 and IOP 14 contained visual evidence of rotor activity, while IOP 16 contained the strongest surface wind gust observed during SRP. Each event formed in a synoptic environment characterized by strong cross-barrier flow and pre-frontal stability, although in IOP 16 these ingredients formed in a unique manner. A diurnal temporal evolution of the surface wind was seen in each case, with thermal circulations in the early morning transitioning through strong westerlies in the afternoon to channeled northerlies after the event. In IOP 8, an easterly flow associated with a rotor circulation was also observed.

The high-resolution numerical simulations produce rotor circulations during each of these events. In two of the events, there is a large degree of variability in the wavelength, while for IOP 14 only short wavelengths are produced. The strong westerlies in all three events occur as either a continuous line of downslope winds which reach to the base of the mountain wave trough or as gap jets which extend further into the valley. Each of the simulations contain evidence of three-dimensional variability in the mountain wave structure. The role of the upstream stability and wind speed profile in determining both the mode of wave propagation and the downstream wavelength is also examined for each case.