High-resolution modeling of the nighttime boundary layer evolution in the Owens Valley: Comparison to observations

This paper focuses on the evolution of the nighttime boundary layer in a deep valley during a period a fairly clear skies and weak to moderate synoptic forcing. In our approach, we combine the evaluation of observations from the T-REX field campaign, carried out in March/April 2006 in the Owens Valley in eastern California, with the analysis of high-resolution large-eddy simulations undertaken with the Advanced Regional Prediction System (ARPS). The observations include data from 1.5 hourly radiosonde ascents, from aircraft measurements, wind profilers and LIDARs, three 30m flux towers and an array of smaller towers and surface stations. The simulations are run with a grid resolution of 100m on the finest grid.

The analysis period is characterized by the passage of a weak high-pressure ridge with some high cirrus clouds during the night. The above-ridge-level winds remain fairly constant at about 10m/s while turning from NW to W during the night. Maximum flow velocities within the valley atmosphere were typically around 5m/s during the night with even weaker flow near the surface. Highly variable flow evolution was evident at valley bottom tall flux towers and also within shallow slope flows on the 3 degree alluvial slope on the western valley sidewall. Inversion strength, as a consequence of occasional mixing from overlying flow, varied from near 0K/km to 120K/km in the lowest 30m above ground. A upper level northerly jet of 8m/s developed in the valley center before abating by 10 UTC. This study will present initial analyses of this complex valley atmosphere evolution and discuss the steps required to achieve accurate simulations of the observed complex flow evolution.