Comparison of two windstorm events during the Sierra Rotors Project and Terrain-induced Rotor Experiment

The Sierra Rotors Project (SRP) and the larger Terrain-induced Rotor Experiment (T-REX) were both conducted in Owens Valley, California in the spring of 2004 and 2006, respectively. The purpose of this two-phase field campaign was to study mountain wave and rotor activity to the lee of the southern Sierra Nevada. One of the scientific objectives of the program is the study and improvement of prediction of downslope windstorms. During the four months of field observations, their were a total of eight events characterized by wind gusts in excess of 25.9 m/s (58 mph). In this study, we examine two of these events: SRP IOP 16 and a non-IOP during T-REX.

SRP IOP 16 was the strongest wind event observed during either of the two field programs with a maximum wind gust of 36.0 m/s (80.6 mph). The large-scale flow was dominated by a rapidly deepening, positively tiled trough over the Intermountain West and a surface cold front approaching Owens Valley from the northeast. The winds along the upper Sierra slopes gradually increased in strength and shifted to westerly between 0900 LST and 1100 LST, with the peak gust recorded about 1430 LST before a sharp shift to strong northerlies at 1800 LST. The event was mostly dry, but a sole lenticular cloud did form over the valley, and wind profilers measured large vertical velocities and strong turbulence. Real-time simulations with the Naval Research Laboratory's Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPS®) predicted strong westerlies in a shallow layer at the same time as the observations as well as the sharp shift to deeper northerlies.

On 5 April 2006, during the T-REX field project, a westerly wind event with a peak wind gust of 27.5 m/s (61.6 mph) rapidly developed during the early afternoon hours. The event occurred after the passage of a fast-moving shortwave trough, and the westerly winds were short in duration and shallow in depth with the wind profilers and Doppler lidars unable to receive returns above the strong surface flow. Photographs of cloud formations from the early stages of the event also suggest a complex flow structure at mid-levels. Real-time COAMPS simulations again predicted the wind event as a shooting flow associated with a shallow mountain lee wave. Comparisons between the two events are made and additional higher-resolution COAMPS simulations are used to diagnose the important formation mechanisms and characteristics.