Abstract : I.5
Nocturnal flow dynamics and pressure-driven channeling in a deep valley

Juerg Schmidli, Gregory Poulos, Fotini Chow, Megan Daniels
schmidli@ucar.edu
National Center for Atmospheric Research, EOL

This paper analyzes the nighttime flow dynamics and heat budget in a deep and broad valley, California's Owens Valley, under weak to moderate synoptic forcing. Measurements from the Terrain-Induced Rotor Experiment (T-REX) reveal a pronounced valley-wind system with often non-classical structure and evolution, such as daytime down-valley winds despite clear skies and strong radiative forcing, nighttime up-valley winds, and a layering of the atmosphere. Here we focus on one particular nighttime event which was characterized by a layer of up-valley flow between low-level, katabatic down-valley flow and high-level, synoptic north-westerly (i.e. down-valley) flow. The analysis is based on the T-REX measurment 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 very well the observed layering and its temporal evolution. The ARPS output data are then used to calculate the along-valley pressure gradient and the components of the heat budget of the valley atmosphere. The analysis shows that the mid-level up-valley flow is due to pressure-driven channeling, i.e. the component of the geostrophic pressure gradient along the valley's axis, overcoming the thermal forcing. The layer of up-valley flow increases in strength and depth as time progresses and the along-valley component of the synoptic pressure gradient increases. Furthermore, analysis of the heat budget shows that potential temperature advection is a dominant cooling factor during the evening transition and the onset of the main down-valley flow, but that turbulent heat flux divergence and radiation flux divergence are of comparable magnitude later on during the night.

Further analysis emphasizes the importance of Owens Valley's unusual geometry for understanding the evolution of its complex valley-wind system.