Large-scale influences on mesoscale valley circulations in a deep valley

This paper investigates the large-scale influences on thermally and mechanically driven mesoscale valley circulations in a deep and broad valley, California's Owens Valley. Measurements from the Terrain-Induced Rotor Experiment (T-REX), which was carried out in March and April of 2006, reveal a complex valley-wind system with often non-classical structure and evolution, such as daytime down-valley winds despite clear skies and strong radiative heating, nighttime up-valley winds, or simultaneous down- and up-valley winds at different heights within the valley. The analysis combines T-REX measurement data and the output of high-resolution large-eddy simulations using the Advanced Regional Prediction System (ARPS) in order to isolate the forcing mechanisms. In particular the relative importance of local thermal forcings in comparison to the larger-scale forcings is investigated. Using horizontal grid resolutions of 1km and 350m, the numerical model reproduces the observed event-to-event variability very well. This includes a layered structure with simultaneous down- and up-valley winds during Enhanced Observing Periods (EOPs) 1 and 2, a moderate down-valley flow during EOP 3, and strong down-valley winds during EOP 4 and 5. We show that the valley wind in Owens Valley is highly susceptable to large-scale influences and propose an explanation of why this is the case.