3.2 Nighttime Cold-Air Intrusions and Transient Warming in a Steep Valley: A Nested Large-eddy Simulation Study

Monday, 20 August 2012: 1:45 PM
Priest Creek C (The Steamboat Grand)
Bowen Zhou, Univ. of California, Berkeley, CA; and F. K. Chow

This numerical study investigates the nighttime flow dynamics in Owens Valley, California on Apr. 17th 2006, during the Terrain-Induced Rotor Experiment (T-REX). Owens Valley is located between the Sierra-Nevada range on the west and the Inyo-White mountains on the east. It is approximately 150 km long, 15-30 km wide and 2-3 km deep. Simulations are initialized from North American Mesoscale Analysis (NAM-ANL) and one-way nested to a 240 m horizontally-spaced grid that covers the entire valley. A finer nested domain with 50 m horizontal and 20 m vertical spacing resolves the local flows within the valley. Both mesoscale and microscale (LES) simulations are performed with the Advanced Regional Prediction System (ARPS). Results are verified against tower and surface based observations.

During the evening transition period, pronounced nocturnal cold-air intrusions on the eastern sidewall of the valley are observed. A regional-scale cold-air flow from the north of the valley is channeled through the “gap” between the Inyo and the White mountains. The flow discharges down the steep eastern sidewall as a strong katabatic current, overshooting the altitude corresponding to its neutral buoyancy. Gravity waves are generated across the valley as the intruding cold-air restores to its equilibrium position in the middle altitudes of the valley. The resulting cross-valley flow creates strong directional shear against the down-valley flow in the lower layers of the valley atmosphere. Shear instability leads to breaking Kelvin-Helmholtz waves at the two-layer interface, resulting in vertical turbulent mixing across the depth of the valley. Later in the night, a nighttime warming episode observed with field measurements at the valley floor is reproduced with good magnitude and temporal precision. This warming as a result of flow transition from down-slope (from the western sidewall) to down-valley flow is well-resolved on the fine LES grid.

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