Collaborative Research: An Observational, Modeling, and Climatological Study of Sierra Rotors

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This research is focused on a study of mountain-wave induced rotors in the lee of the Sierra Nevada. Rotors are intense low-level horizontal vortices that form along an axis parallel to and downstream of a mountain crest. While they are known to be very hazardous to aviation, there have been few quantitative observational and theoretical studies of this phenomenon. As a result, understanding of rotor structure and rotor dynamics is relatively incomplete, and the ability to forecast rotors shows little skill.This study will be conducted using observations and numerical simulations. The observations will be collected in and upstream of the Owens Valley, which lies directly to the east of the southern Sierra Nevada. The eastern slopes of the southern Sierra Nevada are the tallest, steepest quasi-linear topographic barrier in the contiguous United States. The westerly winds that blow across the Sierra Nevada frequently generate large-amplitude mountain waves and strong rotors over the Owens Valley.Two major objectives of this project are to: (1) establish quantitative characteristics of the rotor behavior including the location and the frequency distribution of the rotor-inducing mountain-wave events, and (2) evaluate the extent to which current operational mesoscale models can reliably forecast the occurrence of rotors. The first major objective will be accomplished with a small field experiment involving the installation of an array of automatic weather stations and wind profilers in the Owens Valley and radiosonde launches to sample the upstream flow. The numerical modeling effort in support of the second objective is designed to determine whether deficiencies in the model forecasts (as revealed by comparison with the observational data) are due to an inadequate representation of the large-scale flow striking the mountains or to difficulties in the detailed computation of the mountain waves and rotors themselves.The results of this project will serve as a basis or improving the forecasting capability of numerical models applied to steep topography. One consequence of such improvement would be to provide better forecasts of severe aviation hazards in mountainous terrain.