Abstract : 2D.5
T-REX EOPs III: Mesoscale valley circulations and sensitivity to synoptic conditions

Juerg Schmidli, Gregory Poulos, Stephen Mobbs
schmidli@ucar.edu
National Center for Atmospheric Research, EOL-ISF

This paper compares the mesoscale valley circulations in a deep and broad valley, California's Owens Valley, for varing synoptic conditions. Measurements from the Terrain-Induced Rotor Experiment (T-REX), carried out in March/April 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 is based on the T-REX measurement data and the output of high-resolution large-eddy simulations using the Advanced Regional Prediction System (ARPS). Using horizontal grid resolutions of 1km and 350m, the 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. The analysis shows that the resulting valley wind structure and evolution is due to a subtle interaction between thermally-induced flows, pressure-driven channeling (i.e. forcing by the along-valley component of the synoptic pressure gradient), and downward transport of horizontal momentum from above the valley.