A summary of the Sierra Rotors wave and rotor events

The Sierra Rotors Project (SRP) was the initial phase of a coordinated effort to experimentally document terrain-induced rotors; the second phase of this effort was the recently completed Terrain-induced Rotor Experiment (T-REX). The Sierra Rotors Project took place in March and April 2004 in Owens Valley, California. Observations were collected on the lee side of the Sierra by the DRI surface network of automatic weather stations and two NCAR Integrated Sounding Systems (ISS). Special radiosonde launches were carried out on the upwind side of the Sierra in the San Joaquin Valley in support of the SRP special operations. Real-data numerical simulations at high horizontal resolution of 333 m were carried out with the NRL COAMPS model of nine mountain wave events, which occurred during the course of SRP.

In this paper, we summarize the mountain wave and rotor activity during the Sierra Rotors Project using observations from the surface mesonetwork and wind profilers in conjunction with the high-resolution numerical simulations. Wave events, which are diagnosed using all available observational data, are classified by strength into three categories: weak, moderate, and strong. The criteria used include the degree of penetration of wave-induced westerly winds into the valley and the strength of wave-induced pressure anomalies, determined from the surface network data, and the maximum strength of the updraft, determined from the wind profiler data and numerical simulations. For illustration, during weak wave events typical thermal circulations occur at the surface, whereas during moderate and strong wave events westerly winds penetrate down to the valley floor. The numerical simulation results, verified with surface observations, are also used to diagnose the presence of rotors, which appear to form only during the strongest wave events. In these extreme cases, the easterly flow associated with a rotor circulation can be observed by the surface mesonet.

Six of the examined nine wave events had the westerly momentum mix down to the surface, with one event involving the "touchdown" of a rotor circulation. In the remaining three events westerly momentum did not penetrate to the valley floor, but there was evidence of wave activity near crest level.