Atmospheric Rotors and Severe Turbulence in a Long Deep Valley

2016 ◽  
Vol 73 (4) ◽  
pp. 1481-1506 ◽  
Author(s):  
Lukas Strauss ◽  
Stefano Serafin ◽  
Vanda Grubišić
Keyword(s):  
Sierra Nevada ◽  
Flow Separation ◽  
Owens Valley ◽  
Mountain Waves ◽  
Side Pressure ◽  
Thermally Driven ◽  
Rotor Experiment ◽  
Downslope Flow ◽  
Using Data ◽  
Turbulence Generation

Abstract The conceptual model of an atmospheric rotor is reexamined in the context of a valley, using data from the Terrain-Induced Rotor Experiment (T-REX) conducted in 2006 in the southern Sierra Nevada and Owens Valley, California. All T-REX cases with strong mountain-wave activity have been investigated, and four of them (IOPs 1, 4, 6, and 13) are presented in detail. Their analysis reveals a rich variety of rotorlike turbulent flow structures that may form in the valley during periods of strong cross-mountain winds. Typical flow scenarios in the valley include elevated turbulence zones, downslope flow separation at a valley inversion, turbulent interaction of in-valley westerlies and along-valley flows, and highly transient mountain waves and rotors. The scenarios can be related to different stages of the passage of midlatitude frontal systems across the region. The observations from Owens Valley show that the elements of the classic rotor concept are modulated and, at times, almost completely offset by dynamically and thermally driven processes in the valley. Strong lee-side pressure perturbations induced by large-amplitude waves, commonly regarded as the prerequisite for flow separation, are found to be only one of the factors controlling rotor formation and severe turbulence generation in the valley. Buoyancy perturbations in the thermally layered valley atmosphere appear to play a role in many of the observed cases. Based on observational evidence from T-REX, extensions to the classic rotor concept, appropriate for a long deep valley, are proposed.

scholarly journals Climatology of Westerly Wind Events in the Lee of the Sierra Nevada

2017 ◽  
Vol 56 (4) ◽  
pp. 1003-1023 ◽  
Author(s):  
Stefano Serafin ◽  
Lukas Strauss ◽  
Vanda Grubišić
Keyword(s):  
Sierra Nevada ◽  
Westerly Wind ◽  
Owens Valley ◽  
Mountain Waves ◽  
Wind Variability ◽  
Westerly Winds ◽  
Thermally Driven ◽  
Strong Winds ◽  
Wind Events ◽  
Westerly Wind Events

AbstractA 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.

scholarly journals High-Resolution Simulations of Lee Waves and Downslope Winds over the Sierra Nevada during T-REX IOP 6

2012 ◽  
Vol 51 (7) ◽  
pp. 1333-1352 ◽  
Author(s):  
Peter Sheridan ◽  
Simon Vosper
Keyword(s):  
High Resolution ◽  
Sierra Nevada ◽  
Cross Sections ◽  
Weather Prediction ◽  
Vertical Motion ◽  
Owens Valley ◽  
Downslope Winds ◽  
Lee Waves ◽  
Rotor Experiment ◽  
Intensive Observation

AbstractThe 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.

scholarly journals Diurnal Variation of Downslope Winds in Owens Valley during the Sierra Rotor Experiment

2008 ◽  
Vol 136 (10) ◽  
pp. 3760-3780 ◽  
Author(s):  
Qingfang Jiang ◽  
James D. Doyle
Keyword(s):  
Diurnal Variation ◽  
Numerical Simulations ◽  
Western Slope ◽  
Owens Valley ◽  
Mountain Waves ◽  
Downslope Winds ◽  
Wind Event ◽  
Downslope Wind ◽  
Rotor Experiment ◽  
The Impact

The impact of diurnal forcing on a downslope wind event that occurred in Owens Valley in California during the Sierra Rotors Project (SRP) in the spring of 2004 has been examined based on observational analysis and diagnosis of numerical simulations. The observations indicate that while the upstream flow was characterized by persistent westerlies at and above the mountaintop level the cross-valley winds in Owens Valley exhibited strong diurnal variation. The numerical simulations using the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) capture many of the observed salient features and indicate that the in-valley flow evolved among three states during a diurnal cycle. Before sunrise, moderate downslope winds were confined to the western slope of Owens Valley (shallow penetration state). Surface heating after sunrise weakened the downslope winds and mountain waves and eventually led to the decoupling of the well-mixed valley air from the westerlies aloft around local noon (decoupled state). The westerlies plunged into the valley in the afternoon and propagated across the valley floor (in-valley westerly state). After sunset, the westerlies within the valley retreated toward the western slope, where the downslope winds persisted throughout the night.

scholarly journals The Intense Lee-Wave Rotor Event of Sierra Rotors IOP 8

2007 ◽  
Vol 64 (12) ◽  
pp. 4178-4201 ◽  
Author(s):  
Vanda Grubišić ◽  
Brian J. Billings
Keyword(s):  
Sierra Nevada ◽  
Large Amplitude ◽  
Forced Flow ◽  
Mountain Range ◽  
Wave Rotor ◽  
Owens Valley ◽  
Mountain Waves ◽  
Downslope Wind ◽  
Lee Wave ◽  
Strong Control

Abstract A large-amplitude lee-wave rotor event observationally documented during Sierra Rotors Project Intensive Observing Period (IOP) 8 on 24–26 March 2004 in the lee of the southern Sierra Nevada is examined. Mountain waves and rotors occurred over Owens Valley in a pre-cold-frontal environment. In this study, the evolution and structure of the observed and numerically simulated mountain waves and rotors during the event on 25 March, in which the horizontal circulation associated with the rotor was observed as an opposing, easterly flow by the mesonetwork of surface stations in Owens Valley, are analyzed. The high-resolution numerical simulations of this case, performed with the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) run with multiple nested-grid domains, the finest grid having 333-m horizontal spacing, reproduced many of the observed features of this event. These include small-amplitude waves above the Sierra ridge decoupled from thermally forced flow within the valley, and a large-amplitude mountain wave, turbulent rotor, and strong westerlies on the Sierra Nevada lee slopes during the period of the observed surface easterly flow. The sequence of the observed and simulated events shows a pronounced diurnal variation with the maximum wave and rotor activity occurring in the early evening hours during both days of IOP 8. The lee-wave response, and thus indirectly the appearance of lee-wave rotor during the core IOP 8 period, is found to be strongly controlled by temporal changes in the upstream ambient wind and stability profiles. The downstream mountain range exerts strong control over the lee-wave horizontal wavelength during the strongest part of this event, thus exhibiting the control over the cross-valley position of the rotor and the degree of strong downslope wind penetration into the valley.

Three-Dimensional Characteristics of Stratospheric Mountain Waves during T-REX

2011 ◽  
Vol 139 (1) ◽  
pp. 3-23 ◽  
Author(s):  
James D. Doyle ◽  
Qingfang Jiang ◽  
Ronald B. Smith ◽  
Vanda Grubišić
Keyword(s):  
Sierra Nevada ◽  
National Science Foundation ◽  
Wave Amplitude ◽  
Lower Stratosphere ◽  
Three Dimensional ◽  
Real Data ◽  
Mountain Waves ◽  
National Science ◽  
Rotor Experiment ◽  
Vertically Propagating

Abstract Measurements from the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) Gulfstream V (G-V) obtained during the recent Terrain-Induced Rotor Experiment (T-REX) indicate marked differences in the character of the wave response between repeated flight tracks across the Sierra Nevada, which were separated by a distance of approximately 50 km. Observations from several of the G-V research flights indicate that the vertical velocities in the primary wave exhibited variations up to a factor of 2 between the southern and northern portions of the racetrack flight segments in the lower stratosphere, with the largest amplitude waves most often occurring over the southern flight leg, which has a terrain maximum that is 800 m lower than the northern leg. Multiple racetracks at 11.7- and 13.1-km altitudes indicate that these differences were repeatable, which is suggestive that the deviations were likely due to vertically propagating mountain waves that varied systematically in amplitude rather than associated with transients. The cross-mountain horizontal velocity perturbations are also a maximum above the southern portion of the Sierra Nevada ridge. Real data and idealized nonhydrostatic numerical model simulations are used to test the hypothesis that the observed variability in the wave amplitude and characteristics in the along-barrier direction is a consequence of blocking by the three-dimensional Sierra Nevada and the Coriolis effect. The numerical simulation results suggest that wave launching is sensitive to the overall three-dimensional characteristics of the Sierra Nevada barrier, which has an important impact on the wave amplitude and characteristics in the lower stratosphere. Real-time high-resolution Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) forecasts successfully capture the along-barrier variations in the wave amplitude (using vertical velocity as a proxy) as well as skillfully distinguishing between large- and small-amplitude stratospheric wave events during T-REX.

scholarly journals Estimating Wind Velocities in Mountain Lee Waves Using Sailplane Flight Data*

2010 ◽  
Vol 27 (1) ◽  
pp. 147-158 ◽  
Author(s):  
R. P. Millane ◽  
G. D. Stirling ◽  
R. G. Brown ◽  
N. Zhang ◽  
V. L. Lo ◽  
...  
Keyword(s):  
Wind Velocity ◽  
Sierra Nevada ◽  
General Circulation ◽  
Three Dimensional ◽  
Horizontal Wind ◽  
Atmospheric Gravity Wave ◽  
Mountain Waves ◽  
Lee Waves ◽  
Wind Velocities ◽  
Using Data

Abstract Mountain lee waves are a form of atmospheric gravity wave that is generated by flow over mountain topography. Mountain lee waves are of considerable interest, because they can produce drag that affects the general circulation, windstorms, and clear-air turbulence that can be an aviation hazard, and they can affect ozone abundance through mixing and inducing polar stratospheric clouds. There are difficulties, however, in measuring the three-dimensional wind velocities in high-altitude mountain waves. Mountain waves are routinely used by sailplane pilots to gain altitude. Methods are described for estimating three-dimensional wind velocities in mountain waves using data collected during sailplane flights. The data used are the logged sailplane position and airspeed (sailplane speed relative to the local air mass). An algorithm is described to postprocess this data to estimate the three-dimensional wind velocity along the flight path, based on an assumption of a slowly varying horizontal wind velocity. The method can be applied to data from dedicated flights or potentially to existing flight records used as sensors of opportunity. The methods described are applied to data from a sailplane flight in lee waves of the Sierra Nevada in California.

Downslope Flows on a Low-Angle Slope and Their Interactions with Valley Inversions. Part I: Observations

2008 ◽  
Vol 47 (7) ◽  
pp. 2023-2038 ◽  
Author(s):  
C. David Whiteman ◽  
Shiyuan Zhong
Keyword(s):  
Salt Lake ◽  
Temperature Inversion ◽  
Maximum Speed ◽  
Salt Lake Valley ◽  
Typical Situation ◽  
Thermally Driven ◽  
Downslope Flows ◽  
Downslope Flow ◽  
Deep Temperature ◽  
Using Data

Abstract Thermally driven downslope flows were investigated on a low-angle (1.6°) slope on the west side of the floor of Utah’s Salt Lake Valley below the Oquirrh Mountains using data from a line of four tethered balloons running down the topographic gradient and separated by about 1 km. The study focused on the evolution of the temperature and wind structure within and above the slope flow layer and its variation with downslope distance. In a typical situation, on clear, undisturbed October nights a 25-m-deep temperature deficit of 7°C and a 100–150-m-deep downslope flow with a jet maximum speed of 5–6 m s−1 at 10–15 m AGL developed over the slope during the first 2 h following sunset. The jet maximum speed and the downslope volume flux increased with downslope distance. The downslope flows weakened in the late evening as the stronger down-valley flows expanded to take up more of the valley atmosphere and as ambient stability increased in the lower valley with the buildup of a nocturnal temperature inversion. Downslope flows over this low-angle slope were deeper and stronger than has been reported previously by other investigators, who generally investigated steeper slopes and, in many cases, slopes on the sidewalls of isolated mountains where the downslope flows are not subject to the influence of nighttime buildup of ambient stability within valleys.

Mountain Waves Entering the Stratosphere

2008 ◽  
Vol 65 (8) ◽  
pp. 2543-2562 ◽  
Author(s):  
Ronald B. Smith ◽  
Bryan K. Woods ◽  
Jorgen Jensen ◽  
William A. Cooper ◽  
James D. Doyle ◽  
...  
Keyword(s):  
Sierra Nevada ◽  
Energy Flux ◽  
Cross Flow ◽  
Flow Speed ◽  
Mountain Waves ◽  
Bernoulli Function ◽  
Rotor Experiment ◽  
Research Aircraft ◽  
First Time ◽  
The Waves

Abstract Using the National Science Foundation (NSF)–NCAR Gulfstream V and the NSF–Wyoming King Air research aircraft during the Terrain-Induced Rotor Experiment (T-REX) in March–April 2006, six cases of Sierra Nevada mountain waves were surveyed with 126 cross-mountain legs. The goal was to identify the influence of the tropopause on waves entering the stratosphere. During each flight leg, part of the variation in observed parameters was due to parameter layering, heaving up and down in the waves. Diagnosis of the combined wave-layering signal was aided with innovative use of new GPS altitude measurements. The ozone and water vapor layering correlated with layered Bernoulli function and cross-flow speed. GPS-corrected static pressure was used to compute the vertical energy flux, confirming, for the first time, the Eliassen–Palm relation between momentum and energy flux (EF = −U · MF). Kinetic (KE) and potential (PE) wave energy densities were also computed. The equipartition ratio (EQR = PE/KE) changed abruptly across the tropopause, indicating partial wave reflection. In one case (16 April 2006) systematically reversed momentum and energy fluxes were found in the stratosphere above 12 km. On a “wave property diagram,” three families of waves were identified: up- and downgoing long waves (30 km) and shorter (14 km) trapped waves. For the latter two types, an explanation is proposed related to secondary generation near the tropopause and reflection or secondary generation in the lower stratosphere.

The Impact of Moisture on Mountain Waves during T-REX

2009 ◽  
Vol 137 (11) ◽  
pp. 3888-3906 ◽  
Author(s):  
Qingfang Jiang ◽  
James D. Doyle
Keyword(s):  
Sierra Nevada ◽  
Mesoscale Model ◽  
Buoyancy Frequency ◽  
Mountain Range ◽  
Linear Wave ◽  
Owens Valley ◽  
Mountain Waves ◽  
Low Level ◽  
The Impact ◽  
Moist Layer

Abstract The impact of moist processes on mountain waves over Sierra Nevada Mountain Range is investigated in this study. Aircraft measurements over Owens Valley obtained during the Terrain-induced Rotor Experiment (T-REX) indicate that mountain waves were generally weaker when the relative humidity maximum near the mountaintop level was above 70%. Four moist cases with a RH maximum near the mountaintop level greater than 90% have been further examined using a mesoscale model and a linear wave model. Two competing mechanisms governing the influence of moisture on mountain waves have been identified. The first mechanism involves low-level moisture that enhances flow–terrain interaction by reducing windward flow blocking. In the second mechanism, the moist airflow tends to damp mountain waves through destratifying the airflow and reducing the buoyancy frequency. The second mechanism dominates in the presence of a deep moist layer in the lower to middle troposphere, and the wave amplitude is significantly reduced associated with a smaller moist buoyancy frequency. With a shallow moist layer and strong low-level flow, the two mechanisms can become comparable in magnitude and largely offset each other.

Isobaric Height Perturbations Associated with Mountain Waves Measured by Aircraft during the Terrain-Induced Rotor Experiment

2012 ◽  
Vol 29 (12) ◽  
pp. 1825-1834 ◽  
Author(s):  
Thomas R. Parish ◽  
Larry D. Oolman
Keyword(s):  
Pressure Gradient ◽  
Sierra Nevada ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Differential Gps ◽  
Mountain Waves ◽  
Horizontal Pressure Gradient ◽  
Rotor Experiment ◽  
Horizontal Pressure ◽  
Forcing Mechanisms

Abstract It has only been in the last few years that accurate measurement of the horizontal pressure gradient has been possible over complex terrain using an airborne platform. To infer forcing mechanisms for the wind, an independent measure of the height of an isobaric surface is required. Differential GPS analyses have enabled determination of the aircraft height with sufficient accuracy to infer isobaric heights. When coupled with an accurate measurement of static pressure, the horizontal pressure field can be determined. To demonstrate this measurement technique, research flight legs by the University of Wyoming King Air (UWKA) conducted in support of the Terrain-Induced Rotor Experiment (T-REX) in March and April 2006 are examined. UWKA flights conducted on 14 and 25 March and 16 April 2006 encountered strong mountain waves in response to winds directed primarily normal to the Sierra Nevada ridgeline. Winds at flight level showed pronounced variation that suggested topographic influence. The magnitude of isobaric height perturbations along UWKA flight tracks obtained using differential GPS during case study days of 14 and 25 March and 16 April are shown to exceed 70 m, corresponding to horizontal pressure perturbations greater than 4 hPa. Measurements suggest that changes in wind speed are linked primarily to the perturbation height field and that the flow can be classified as Eulerian, implying that Coriolis accelerations are negligible and flows respond to the horizontal pressure gradient force.

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