A study of the onset of westerly downslope winds in Owens Valley

Mechanisms by which westerly momentum associated with the cross-mountain flow over the Sierra Nevada penetrates to the Owens Valley floor are investigated using observations from the Terrain-induced Rotor Experiment (T-REX) and a combination of real-data and idealized numerical simulations. Wind profiler observations show that westerly winds in Owens Valley can be either vertically deep or shallow. Other T-REX observations employed in this analysis include surface observations from the DRI mesonetwork and the consecutive radiosonde soundings from both the upstream and valley locations. The former are used to identify the onset time of westerly winds and their penetration distance in the cross-valley direction, whereas the latter are examined to identify important changes in the vertical structure of the incoming airflow (dynamic forcing) and the effects of diurnal heating on the thermal structure of the valley atmosphere (thermal forcing). Paired upstream-downstream soundings are also used to investigate the effects of the large-scale heating contrast on the onset time of the westerlies.

Simplified T-REX soundings are used as the basis for a series of idealized, two-dimensional simulations of airflow over double bell-shaped topography with an elevated valley using the Naval Research Laboratory's Coupled Ocean-Atmopshere Mesoscale Prediction System (COAMPS). A two-layer stability profile (constant tropospheric and stratospheric values) and a three-layer wind speed profile (constant tropospheric shear, constant wind jet, and constant reverse shear) are used. In the initial set of dynamic simulations, westerly flow begins to reach the valley floor only when the wind speeds at both ridge top and in the upper-level jet maximum exceed 15 m/s and 50 m/s, respectively. Inversions near 5 km reduce the amount of penetration, while inversions near the ridge top result in a hydraulic-type subcritical to supercritical transition. When an idealized heating profile is applied to the steady state dynamic solutions, westerlies already near the valley floor can sweep across the entire valley within 3-4 hours. For more moderate upstream shears, sufficiently strong heating can result in penetration in approximately six hours, corresponding to an afternoon onset of westerlies. In weaker sheared cases, heating generates dramatic changes in the flow regime. These thresholds show good agreement with observations in many cases, but some cases do have later than predicted onsets.