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Stefano Serafin, Lukas Strauss, and Vanda Grubišić

Abstract

A 5-yr climatology of westerly wind events in Owens Valley, California, is derived from data measured by a mesoscale network of 16 automatic weather stations. Thermally driven up- and down-valley flows are found to account for a large part of the diurnal wind variability in this approximately north–south-oriented deep U-shaped valley. High–wind speed events at the western side of the valley deviate from this basic pattern by showing a higher percentage of westerly winds. In general, strong westerly winds in Owens Valley tend to be more persistent and to display higher sustained speeds than strong winds from other quadrants. The highest frequency of strong winds at the valley floor is found in the afternoon hours from April to September, pointing to thermal forcing as a plausible controlling mechanism. However, the most intense westerly wind events (westerly windstorms) can happen at any time of the day throughout the year. The temperature and humidity variations caused by westerly windstorms depend on the properties of the approaching air masses. In some cases, the windstorms lead to overall warming and drying of the valley atmosphere, similar to foehn or chinook intrusions. The key dynamical driver of westerly windstorms in Owens Valley is conjectured to be the downward penetration of momentum associated with mountain waves produced by the Sierra Nevada ridgeline to the west of the valley.

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Laurence Armi and Georg J. Mayr

Abstract

A combination of real and virtual topography is shown to be crucial to describe the essentials of stratified flow over mountain ranges and leeside valleys. On 14 March 2006 [Intensive Observation Period 4 of the Terrain-Induced Rotor Experiment (T-REX)], a nearly neutral cloud-filled layer, capped by a strong density step, overflowed the Sierra Nevada and separated from the lee slope upon encountering a cooler valley air mass. The flow in this lowest layer was asymmetric across and hydraulically controlled at the crest with subcritical flow upstream and supercritical flow downstream. The density step at the top of this flowing layer formed a virtual topography, which descended 1.9 km and determined the horizontal scale and shape of the flow response aloft reaching into the stratosphere. A comparison shows that the 11 January 1972 Boulder, Colorado, windstorm case was similar: hydraulically controlled at the crest with the same strength and descent of the virtual topography. In the 18 February 1970 Boulder case, however, the layer beneath the stronger virtual topography was subcritical everywhere with a symmetric dip across the Continental Divide of only 0.5 km. In all three cases, the response and strength of the flow aloft depend on the virtual topography. The layer up to the next strong density step at or near the tropopause was hydraulically supercritical for the 18 February case, subcritical for the T-REX case, and critically controlled for the 11 January case, for which a weak density step and isolating layer aloft made possible the strong response aloft for which it is famous.

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Peter Sheridan and Simon Vosper

Abstract

The downslope windstorm during intensive observation period (IOP) 6 was the most severe that was detected during the Terrain-Induced Rotor Experiment (T-REX) in Owens Valley in the Sierra Nevada of California. Cross sections of vertical motion in the form of a composite constructed from aircraft data spanning the depth of the troposphere are used to link the winds experienced at the surface to the changing structure of the mountain-wave field aloft. Detailed analysis of other observations allows the role played by a passing occluded front, associated with the rapid intensification (and subsequent cessation) of the windstorm, to be studied. High-resolution, nested modeling using the Met Office Unified Model (MetUM) is used to study qualitative aspects of the flow and the influence of the front, and this modeling suggests that accurate forecasting of the timing and position of both the front and strong mountaintop winds is crucial to capture the wave dynamics and accompanying windstorm. Meanwhile, far ahead of the front, simulated downslope winds are shallow and foehnlike, driven by the thermal contrast between the upstream and valley air mass. The study also highlights the difficulties of capturing the detailed interaction of weather systems with large and complex orography in numerical weather prediction.

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Laurence Armi and Georg J. Mayr

Abstract

Cross-barrier density differences and westerly flow established a descending stratified flow across the Sierra Nevada (United States) on 9–10 April 2006. Downslope flow and an internal hydraulic jump occurred only when the potential temperature of the westerly descending flow was at least as cold as the existing upvalley-flowing valley air mass. The onset was observed in sequences of visible satellite images and with weather stations. The University of Wyoming King Air flew through the stratified flow and imaged the structure of the internal hydraulic jump with its cloud radar. Shear-layer instabilities, which first developed near the jump face, grew and paired downstream, mixing the internal hydraulic jump layer. A single wave response to the downslope flow and internal hydraulic jump was observed aloft, but only after the downslope flow had become established.

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Željko Večenaj, Stephan F. J. De Wekker, and Vanda Grubišić

Abstract

A case study of mountain-wave-induced turbulence observed during the Terrain-Induced Rotor Experiment (T-REX) in Owens Valley, California, is presented. During this case study, large spatial and temporal variability in aerosol backscatter associated with mountain-wave activity was observed in the valley atmosphere by an aerosol lidar. The corresponding along- and cross-valley turbulence structure was investigated using data collected by three 30-m flux towers equipped with six levels of ultrasonic anemometers. Time series of turbulent kinetic energy (TKE) show higher levels of TKE on the sloping western part of the valley when compared with the valley center. The magnitude of the TKE is highly dependent on the averaging time on the western slope, however, indicating that mesoscale transport associated with mountain-wave activity is important here. Analysis of the TKE budget shows that in the central parts of the valley mechanical production of turbulence dominates and is balanced by turbulent dissipation, whereas advective effects appear to play a dominant role over the western slope. In agreement with the aerosol backscatter observations, spatial variability of a turbulent-length-scale parameter suggests the presence of larger turbulent eddies over the western slope than along the valley center. The data and findings from this case study can be used to evaluate the performance of turbulence parameterization schemes in mountainous terrain.

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Stephen A. Cohn, Vanda Grubiššićć, and William O. J. Brown

Abstract

A network of three boundary layer radar wind profilers is used to study characteristics of mountain waves and rotors and to explore the utility of such a network. The data employed were collected as part of the Terrain-Induced Rotor Experiment (T-REX), which took place in Owens Valley, California, in early 2006. The wind profilers provide a continuous time––height representation of wave and rotor structure. During intensive observing period 3 (IOP 3), the profiler network was positioned in an L-shaped configuration, capturing key features of the mountain waves and rotor, including the boundary layer vortex sheet (or shear layer), turbulence within this shear layer, the classical lower turbulence zone (LTZ), and wave motion above the LTZ. Observed features were found to be in good agreement with recent high-resolution numerical simulations. Using the wind profiler with superior time resolution (Multiple Antenna Profiler Radar), a series of updraft––downdraft couplets were observed beneath the first downwind wave crest. These are interpreted as signatures of subrotors. Such detailed observations of subrotors are rare, even though subrotors are believed to be a common feature of rotor circulations in Owens Valley. During IOP 6, the network was repositioned to form a line across the valley. A simple algorithm was used to determine the amplitude, wavelength, and phase of the primary wave over the valley and to observe their changes over time and height. In the IOP-6 case, the wavelength increased over time, the phase indicated an eastward-shifting wave crest, and the amplitude increased with height and also varied over time.

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Qingfang Jiang, Ming Liu, and James D. Doyle

Abstract

Fine dust particles emitted from Owens (dry) Lake in California documented during the Terrain-Induced Rotor Experiment (T-REX) of 2006 have been examined using surface observations and a mesoscale aerosol model. Air quality stations around Owens (dry) Lake observed dramatic temporal and spatial variations of surface winds and dust particulate concentration. The hourly particulate concentration averaged over a 2-month period exhibits a strong diurnal variation with a primary maximum in the afternoon, coincident with a wind speed maximum. The strongest dust event documented during the 2-month-long period, with maximum hourly and daily average particulate concentrations of 7000 and 1000 μg m−3, respectively, is further examined using output from a high-resolution mesoscale aerosol model simulation. In the morning, with the valley air decoupled from the prevailing westerlies (i.e., cross valley) above the mountaintop, fine particulates are blown off the dry lake bed by moderate up-valley winds and transported along the valley toward northwest. The simulated strong westerlies reach the western part of the valley in the afternoon and more fine dust is scoured off Owens (dry) Lake than in the morning. Assisted by strong turbulence and wave-induced vertical motion in the valley, the westerlies can transport a substantial fraction of the particulate mass across the Inyo Mountains into Death Valley National Park.

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Georg J. Mayr and Laurence Armi

Abstract

The potential for a stably stratified air mass upstream of the Sierra Nevada (California) to descend as foehn into the nearly 3-km-deep Owens Valley was studied for the 2 March 2006 case with observations from sondes, weather stations, and two aircraft flights. While upstream conditions remained almost unchanged throughout the day, strong diurnal heating on the downstream side warmed the valley air mass sufficiently to permit flow through the passes to descend to the valley floor only in the late afternoon. Potential temperatures of air crossing the crest were too warm to descend past a virtual floor formed by the strong potential temperature step at the top of the valley air mass, the height of which changed throughout the day primarily due to diurnal heating in the valley. The descending stably stratified flow and its rebound with vertical velocities as high as 8 m s−1 were shaped by the underlying topography and the virtual valley floor.

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Stephan F. J. De Wekker and Shane D. Mayor

Abstract

First results are presented from the deployment of the NCAR Raman-Shifted Eye-Safe Aerosol Lidar (REAL) in the Owens Valley of California during the Terrain-Induced Rotor Experiment (T-REX) in March and April 2006. REAL operated in range–height indicator (RHI) and plan position indicator (PPI) scanning modes to observe the vertical and horizontal structures of the aerosol and cloud distribution in a broad valley in the lee of a tall mountain range. The scans produce two-dimensional cross sections that when animated produce time-lapse visualizations of the microscale and mesoscale atmospheric structures and dynamics. The 2-month dataset includes a wide variety of interesting atmospheric phenomena. When the synoptic-scale flow is strong and westerly, the lidar data reveal mountain-induced waves, hydraulic jumps, and rotorlike circulations that lift aerosols to altitudes of more than 2 km above the valley. Shear instabilities occasionally leading to breaking waves were observed in cloud and aerosol layers under high wind conditions. In quiescent conditions, the data show multiple aerosol layers, upslope flows, and drainage flows interacting with valley flows. The results demonstrate that a rapidly scanning, eye-safe, ground-based aerosol lidar can be used to observe important features of clear-air atmospheric flows and can contribute to an improved understanding of mountain-induced meteorological phenomena. The research community is encouraged to use the dataset in support of their observational analysis and modeling efforts.

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C. David Whiteman, Sebastian W. Hoch, and Gregory S. Poulos

Abstract

At slope and valley floor sites in the Owens Valley of California, the late afternoon near-surface air temperature decline is often followed by a temporary temperature rise before the expected nighttime cooling resumes. The spatial and temporal patterns of this evening warming phenomenon, as seen in the March/April 2006 Terrain-Induced Rotor Experiment, are investigated using a widely distributed network of 51 surface-based temperature dataloggers. Hypotheses on the causes of the temperature rises are tested using heavily instrumented 34-m meteorological towers that were located within the datalogger array. The evening temperature rise follows the development of a shallow temperature deficit layer over the slopes and floor of the valley in which winds blow downslope. Background winds within the valley, freed from frictional deceleration from the earth’s surface by this layer, accelerate. The increased vertical wind shear across the temperature deficit layer eventually creates shear instability and mixes out the layer, creating the observed warming near the ground. As momentum is exchanged during the mixing event, the wind direction near the surface gradually turns from downslope to the background wind direction. After the short period of warming associated with the mixing, ongoing net radiative loss causes a resumption of the cooling.

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