Sierra Rotors: A study of the IOP 8 rotor event

The Special Observation Period of the Sierra Rotors experiment, an exploratory first phase of the multi-year effort to study rotors and related phenomena in complex terrain (Phase II of this effort is T-REX), took place in March and April 2004 in Owens Valley in the southern Sierra Nevada of California. Terrain-induced rotors are intense horizontal vortices with strong turbulence that form on the lee side of a mountain range in association with mountain waves, and are known to pose severe aeronautical hazards. The eastern slopes of the southern Sierra Nevada make up the tallest, steepest, quasi-linear topographic barrier in the contiguous United States, and are well-known for generating large-amplitude Sierra Waves and attendant strong rotors over Owens Valley.

An intense mountain wave and rotor event was documented in this experiment during Intensive Observing Period (IOP) 8 on March 24-26 2004. The event was characterized by a cold frontal passage and strong westerly flow at the mountain top level that induced mountain waves and rotors over Owens Valley. This case was simulated with the Naval Research Laboratory's (NRL) Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) run at 333 m resolution. In this study, we analyze the evolution and structure of simulated mountain waves and rotors during this event, in which horizontal circulation associated with the rotor extended to the valley floor, where it was observed as easterly flow by the DRI mesonetwork of surface stations. The model accurately reproduced the timing and spatial structure of many of the observed phenomena, including thermal circulation in the valley at the start of the event, an intense mountain wave during the period of observed easterly flow, and strong westerlies on the Sierra Nevada lee slopes.