A study of the onset of westerly surface flow in Owens Valley

This study examines the mechanisms resulting in the penetration into a deep mountain valley of momentum that is associated with a cross-mountain flow and wave activity aloft. The study is based on the observational data analysis and numerical simulations. The observations used in the analysis come from two recent field campaigns, the Sierra Rotors Project (2004) and the Terrain-induced Rotor Experiment (2006). The numerical simulations have been performed using the Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPS) and involve both real case and idealized experiments. Data from a network of surface observing stations in Owens Valley was used to identify Intensive Observing Periods with westerly wind events and to detect the onset time of westerlies. Pairs of simultaneous upstream and downstream radiosonde soundings bracketing the time of onset were examined to determine whether significant changes in the upstream wind speed and stability structure or differences between upstream and valley temperature due to diurnal heating within the valley were occurring at the time of onset. The real case sensitivity experiments on a selected number of SRP and T-REX events were used to examine the effect of the downstream mountain range, radiative heating, and moisture on the onset of westerlies. The simulation results show that the downstream mountain range usually has a weakening effect on the mountain wave and surface flow, while theabsence of diurnal heating weakens or destroys the surface westerlies if strong dynamic forcing is not present. The effects of condensation processes seems to depend on the flow structure and amount of cloud in the baseline simulations. The idealized simulations with linear positive wind shear profiles show that, in the absence of diurnal heating, the flow cannot penetrate to the valley floor unless the ratio Nh/U decreases to near 2 and the Richardson number decreases to near 4 or a strong inversion is located at the mountain crest. When the effect of surface heating is included, weaker flows can penetrate the valley in approximately 6-7 hours, while surface cooling significantly weakens the wave structure