Terrain-Induced Rotor Experiment Research Articles

Wind Profiler Observations of Mountain Waves and Rotors during T-REX

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.

Influence of Mesoscale Dynamics and Turbulence on Fine Dust Transport in Owens Valley

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.

Energy Flux and Wavelet Diagnostics of Secondary Mountain Waves

Abstract In recent years, aircraft data from mountain waves have been primarily analyzed using velocity and temperature power spectrum and momentum flux estimation. Herein it is argued that energy flux wavelets (i.e., pressure–velocity wavelet cross-spectra) provide a more powerful tool for locating and classifying different types of mountain waves. In the first part of the paper, pressure–velocity cross-spectra using various linear mountain-wave solutions are shown to be capable of disentangling collocated waves with different propagation directions and wavelength. A field of group velocity vectors can also be determined. In the second part, the energy flux wavelet technique is applied to five cases of mountain waves entering the stratosphere from the Terrain-Induced Rotor Experiment (T-REX) in 2006. Perturbation pressure along the flight track is determined using aircraft static pressure corrected hydrostatically with GPS altitude. In four of the cases, collocated long up-propagating and short down-propagating waves are seen in the stratosphere. These waves have strong, but opposite, p′w′ cospectra. In one of these cases, a patch of turbulence is collocated with the up and down waves. In two other cases, trapped waves riding on the tropopause inversion layer (TIL) are seen. These trapped waves have p′w′ quadrature spectra that reverse sign across the tropopause. These newly discovered wave types may arise from secondary wave generation (i.e., a nonlinear transfer of energy from the long vertically propagating waves to shorter modes).

Turbulence Characteristics in an Elevated Shear Layer over Owens Valley

Abstract Characteristics of turbulence in the lower and middle troposphere over Owens Valley have been examined using aircraft in situ measurements obtained from the Terrain-Induced Rotor Experiment. The two events analyzed in this study are characterized by a deep turbulent layer from the valley floor up to the midtroposphere associated with the interaction between trapped waves and an elevated shear layer. Kelvin–Helmholtz (KH) instability develops above the mountaintop level and often along the wave crests where the Richardson number is reduced. The turbulence induced by KH instability is characterized by a progressive downscale energy cascade, a well-defined inertial subrange up to 1 km, and large eddies with vertical to horizontal aspect ratios less than unity. The turbulence below the mountaintop level is largely shear induced, associated with wave steepening and breaking, and is more isotropic. Evaluation of structure functions indicates that while the turbulence energy cascade is predominately downscale, upscale energy transfer exists with horizontal scales from a few hundred meters to a few kilometers because of the transient energy dispersion of large eddies generated by KH instability and gravity wave steepening or breaking.

Large-Eddy Simulation-Based Retrieval of Dissipation from Coherent Doppler Lidar Data

Accurate estimation of dissipation rate is important in understanding and analyzing turbulent flows found in environment and engineering processes. Many previous studies have focused on measuring the local dissipation rate at a single point or averaged dissipation rate over a suitable area. Since coherent Doppler lidar is capable of providing multi-point measurements covering a large spatial extent, it is well-suited for examining the distribution of dissipation in the atmosphere. In this paper, an approach is presented that is based on retrieving the dissipation rate from coherent Doppler lidar data using large-eddy simulation. Two Coherent Doppler lidars performed range height indicator (RHI) scans of a vertical/cross-barrier plane during the Terrain-induced Rotor Experiment (T-REX). Two-dimensional velocity vectors were retrieved using a least squares method. The velocity vectors retrieved from co-planar RHI scans are used to estimate subgrid scale (SGS) quantities through a known SGS parameterization. For the T-REX datasets analyzed, the dissipation rate was found to increase in the presence of rotors, subrotors, and, as expected, in regions of high wind shear. Owing to the presence of sharper gradients in subrotors, their dissipation rate is generally larger than that of rotors.

Coplanar Doppler Lidar Retrieval of Rotors from T-REX

Abstract Dual-Doppler analysis of data from two coherent lidars during the Terrain-Induced Rotor Experiment (T-REX) allows the retrieval of flow structures, such as vortices, during mountain-wave events. The spatial and temporal resolution of this approach is sufficient to identify and track vortical motions on an elevated, cross-barrier plane in clear air. Assimilation routines or additional constraints such as two-dimensional continuity are not required. A relatively simple and quick least squares method forms the basis of the retrieval. Vortices are shown to evolve and advect in the flow field, allowing analysis of their behavior in the mountain–wave–boundary layer system. The locations, magnitudes, and evolution of the vortices can be studied through calculated fields of velocity, vorticity, streamlines, and swirl. Generally, observations suggest two classes of vortical motions: rotors and small-scale vortical structures. These two structures differ in scale and behavior. The level of coordination of the two lidars and the nature of the output (i.e., in range gates) creates inherent restrictions on the spatial and temporal resolution of retrieved fields.

Vorticity from Line-of-Sight Lidar Velocity Scans

Abstract A method is presented to compute the spanwise vorticity in polar coordinates from 2D vertical cross sections of high-resolution line-of-sight Doppler wind lidar observations. The method uses the continuity equation to derive the velocity component perpendicular to the observed line-of-sight velocity, which then yields the spanwise vorticity component. The results of the method are tested using a ground-based Doppler lidar, which was deployed during the Terrain-Induced Rotor Experiment (T-REX). The resulting fields can be used to identify and quantify the strength and size of vortices, such as those associated with atmospheric rotors. Furthermore, they may serve to investigate the dynamics and evolution of vortices and to evaluate numerical simulations. A demonstration of the method and comparison with high-resolution numerical simulations reveals that the derived vorticity can explain 66% of the mean-square vorticity fluctuations, has a reasonably skillful magnitude, exhibits no significant bias, and is in qualitative agreement with model-derived vorticity.

The Impact of Moisture on Mountain Waves during T-REX

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.

Initial-Condition Sensitivities and the Predictability of Downslope Winds

Abstract The sensitivity of downslope wind forecasts to small changes in initial conditions is explored by using 70-member ensemble simulations of two prototypical windstorms observed during the Terrain-Induced Rotor Experiment (T-REX). The 10 weakest and 10 strongest ensemble members are composited and compared for each event. In the first case, the 6-h ensemble-mean forecast shows a large-amplitude breaking mountain wave and severe downslope winds. Nevertheless, the forecasts are very sensitive to the initial conditions because the difference in the downslope wind speeds predicted by the strong- and weak-member composites grows to larger than 28 m s−1 over the 6-h forecast. The structure of the synoptic-scale flow one hour prior to the windstorm and during the windstorm is very similar in both the weak- and strong-member composites. Wave breaking is not a significant factor in the second case, in which the strong winds are generated by a layer of high static stability flowing beneath a layer of weaker mid- and upper-tropospheric stability. In this case, the sensitivity to initial conditions is weaker but still significant. The difference in downslope wind speeds between the weak- and strong-member composites grows to 22 m s−1 over 12 h. During and one hour before the windstorm, the synoptic-scale flow exhibits appreciable differences between the strong- and weak-member composites. Although this case appears to be more predictable than the wave-breaking event, neither case suggests that much confidence should be placed in the intensity of downslope winds forecast 12 or more hours in advance.

Observations of Atmospheric Structure and Dynamics in the Owens Valley of California with a Ground-Based, Eye-Safe, Scanning Aerosol Lidar*

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.

Using Surface Pressure Variations to Categorize Diurnal Valley Circulations: Experiments in Owens Valley

Abstract Harmonic analysis has been applied to data from nearly 1000 Automatic Surface Observation System (ASOS) stations over the United States to extract diurnal pressure signals. The largest diurnal pressure amplitudes (∼200 Pa) and the earliest phases (∼0600 LST for surface pressure maximum) were found for stations located within deep mountain valleys in the western United States. The origin of these unique characteristics of valley pressure signals is examined with a detailed study of Owens Valley, California. Analysis of observational data from the Terrain-Induced Rotor Experiment (T-REX) project shows that the ratio of the valley surface pressure to temperature amplitude can be used to estimate the daily maximum mixed-layer depth H. On days with strong westerly winds above the valley, the mixed layer is found to be shallower than on quiescent days because of a flushing effect in the upper parts of the valley. Idealized two-dimensional Weather Research and Forecasting Model simulations were used to explain the pressure signal. In agreement with observations, the simulations show a 3-h difference between the occurrence of a surface pressure minimum (1800 LST) and a surface temperature maximum (1500 LST). The resolved energy budget analysis reveals that this time lag is caused by the persistence of subsidence warming in the upper part of the valley after the surface begins to cool. Sensitivity tests for different valley depths and seasons show that the relative height of the mixed-layer depth with respect to the valley depth, along with the valley width-to-depth ratio, determine whether the diurnal valley circulation is a “confined” system or an “open” system. The open system has a smaller pressure amplitude and an earlier pressure phase.

Vertical Air Motion from T-REX Radiosonde and Dropsonde Data

AbstractThe primary goal of this study is to explore the potential for estimating the vertical velocity (VV) of air from the surface to the stratosphere, using widely available radiosonde and dropsonde data. The rise and fall rates of radiosondes and dropsondes, respectively, are a combination of the VV of the atmosphere and still-air rise–fall rates. The still-air rise–fall rates are calculated using basic fluid dynamics and characteristics of radiosonde and dropsonde systems. This study validates the technique to derive the VV from radiosonde and dropsonde data and demonstrates its value. This technique can be easily implemented by other users for various scientific applications.The technique has been applied to the Terrain-induced Rotor Experiment (T-REX) dropsonde and radiosonde data. Comparisons among radiosonde, dropsonde, aircraft, and profiling radar vertical velocities show that the sonde-estimated VV is able to capture and describe events with strong vertical motions (larger than ∼1 m s−1) observed during T-REX. The VV below ∼5 km above ground, however, is overestimated by the radiosonde data. The analysis of derived VVs shows interesting features of gravity waves, rotors, and turbulence. Periodic variations of vertical velocity in the stratosphere, as indicated by the radiosonde data, correspond to the horizontal wavelength of gravity waves with an averaged horizontal wavelength of ∼15 km. Two-dimensional VV structure is described in detail by successive dropsonde deployment.

Observations and Numerical Simulations of Subrotor Vortices during T-REX

Abstract High-resolution observations from scanning Doppler and aerosol lidars, wind profiler radars, as well as surface and aircraft measurements during the Terrain-induced Rotor Experiment (T-REX) provide the first comprehensive documentation of small-scale intense vortices associated with atmospheric rotors that form in the lee of mountainous terrain. Although rotors are already recognized as potential hazards for aircraft, it is proposed that these small-scale vortices, or subrotors, are the most dangerous features because of strong wind shear and the transient nature of the vortices. A life cycle of a subrotor event is captured by scanning Doppler and aerosol lidars over a 5-min period. The lidars depict an amplifying vortex, with a characteristic length scale of ∼500–1000 m, that overturns and intensifies to a maximum spanwise vorticity greater than 0.2 s−1. Radar wind profiler observations document a series of vortices, characterized by updraft/downdraft couplets and regions of enhanced reversed flow, that are generated in a layer of strong vertical wind shear and subcritical Richardson number. The observations and numerical simulations reveal that turbulent subrotors occur most frequently along the leading edge of an elevated sheet of horizontal vorticity that is a manifestation of boundary layer shear and separation along the lee slopes. As the subrotors break from the vortex sheet, intensification occurs through vortex stretching and in some cases tilting processes related to three-dimensional turbulent mixing. The subrotors and ambient vortex sheet are shown to intensify through a modest increase in the upstream inversion strength, which illustrates the predictability challenges for the turbulent characterization of rotors.

The Overamplification of Gravity Waves in Numerical Solutions to Flow over Topography

Abstract The tendency of high-resolution numerical weather prediction (NWP) models to overpredict the strength of vertically propagating mountain waves is explored. Discrete analytic mountain-wave solutions are presented for the classical problem of cross-mountain flow in an atmosphere with constant wind speed and stability. Time-dependent linear numerical solutions are also obtained for more realistic atmospheric structures. On one hand, using second-order-accurate finite differences on an Arakawa C grid to model nonhydrostatic flow over what might be supposed to be an adequately resolved 8Δx-wide mountain can lead to an overamplification of the standing mountain wave by 30%–40%. On the other hand, the same finite-difference scheme underestimates the wave amplitude in hydrostatic flow over an 8Δx-wide mountain. Increasing the accuracy of the advection scheme to the fourth order significantly reduces the numerical errors associated with both the hydrostatic and nonhydrostatic discrete solutions. The Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model is used to generate two 70-member ensemble simulations of a mountain-wave event during the Terrain-Induced Rotor Experiment. It is shown that switching from second-order advection to fourth-order advection leads to as much as a 20 m s−1 decrease in vertical velocity on the lee side of the Sierra Nevada, and that the weaker fourth-order solutions are more consistent with observations.

Lee-Wave Resonances over Double Bell-Shaped Obstacles

Abstract Lee-wave resonance over double bell-shaped obstacles is investigated through a series of idealized high-resolution numerical simulations with the nonhydrostatic Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model using a free-slip lower boundary condition. The profiles of wind speed and stability as well as terrain derive from observations of lee-wave events over the Sierra Nevada and Inyo Mountains from the recently completed Terrain-Induced Rotor Experiment (T-REX). Numerical experiments show that double bell-shaped obstacles promote trapped lee waves that are in general shorter than those excited by an isolated ridge. While the permissible trapped lee-wave modes are determined by the upstream atmospheric structure, primarily vertical wind shear, the selected lee-wave wavelengths for two obstacles that are close or equal in height are dictated by the discrete terrain spectrum and correspond to higher harmonics of the primary orographic wavelength, which is equal to the ridge separation distance. The exception is the smallest ridge separation distance examined, one that corresponds to the Owens Valley width and is closest to the wavelength determined by the given upstream atmospheric structure, for which the primary lee-wave and orographic wavelengths were found to nearly coincide. The influence two mountains exert on the overall lee-wave field is found to persist at very large ridge separation distances. For the nonlinear nonhydrostatic waves examined, the ridge separation distance is found to exert a much stronger control over the lee-wave wavelengths than the mountain half-width. Positive and negative interferences of lee waves, which can be detected through their imprint on wave drag and wave amplitudes, were found to produce appreciable differences in the flow structure mainly over the downstream peak, with negative interference characterized by a highly symmetric flow pattern leading to a low drag state.

Evening Temperature Rises on Valley Floors and Slopes: Their Causes and Their Relationship to the Thermally Driven Wind System

AbstractAt 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.

Three-Dimensional Wind Retrieval: Application of MUSCAT to Dual-Doppler Lidar

Abstract During the field campaign of the Terrain-induced Rotor Experiment (T-REX) in the spring of 2006, Doppler lidar measurements were taken in the complex terrain of the Californian Owens Valley for six weeks. While fast three-dimensional (3D) wind analysis from measured radial wind components is well established for dual weather radars, only the feasibility was shown for dual-Doppler lidars. A computationally inexpensive, variational analysis method developed for multiple-Doppler radar measurements over complex terrain was applied. The general flow pattern of the 19 derived 3D wind fields is slightly smoothed in time and space because of lidar scan duration and analysis algorithm. The comparison of extracted wind profiles to profiles from radiosondes and wind profiler reveals differences of wind speed and direction of less than 1.1 m s−1 and 3°, on average, with standard deviations not exceeding 2.7 m s−1 and 27°, respectively. Standard velocity–azimuth display (VAD) retrieval method provided higher vertical resolution than the dual-Doppler retrieval, but no horizontal structure of the flow field. The authors suggest a simple way to obtain a good first guess for a dual-lidar scan strategy geared toward 3D wind retrieval that minimizes scan duration and maximizes spatial coverage.

External Influences on Nocturnal Thermally Driven Flows in a Deep Valley

AbstractThe dynamics that govern the evolution of nighttime flows in a deep valley, California’s Owens Valley, are analyzed. Measurements from the Terrain-Induced Rotor Experiment (T-REX) reveal a pronounced valley-wind system with often nonclassical flow evolution. Two cases with a weak high pressure ridge over the study area but very different valley flow evolution are presented. The first event is characterized by the appearance of a layer of southerly flow after midnight local time, sandwiched between a thermally driven low-level downvalley (northerly) flow and a synoptic northwesterly flow aloft. The second event is characterized by an unusually strong and deep downvalley jet, exceeding 15 m s−1. The analysis is based on the T-REX measurement data and the output of high-resolution large-eddy simulations using the Advanced Regional Prediction System (ARPS). Using horizontal grid spacings of 1 km and 350 m, ARPS reproduces the observed flow features for these two cases very well. It is found that the low-level along-valley forcing of the valley wind is the result of a superposition of the local thermal forcing and a midlevel (2–2.5 km MSL) along-valley pressure forcing. The analysis shows that the large difference in valley flow evolution derives primarily from differences in the midlevel pressure forcing, and that the Owens Valley is particularly susceptible to these midlevel external influences because of its specific geometry. The results demonstrate the delicate interplay of forces that can combine to determine the valley flow structure on any given night.

Vertically nested nonhydrostatic model for multiscale resolution of flows in the upper troposphere and lower stratosphere

Vertical nesting with refined gridding in coupled mesoscale weather research and forecasting (WRF)/microscale models are presented with a particular emphasis on improved vertical resolution in the upper troposphere and lower stratosphere (UTLS). The finest mesoscale nest is coupled with a sequence of microscale nests with finer resolution in both the horizontal and the vertical. The fully three-dimensional, compressible nonhydrostatic Navier–Stokes equations are solved using a time-split method with a refined grid in the vertical, and improved resolution in the UTLS region. For nesting, both lateral and vertical boundary conditions are treated via implicit relaxation in buffer zones where all fields are relaxed to those obtained from the finest mesoscale nest. Computational results are presented demonstrating the ability of microscale nests to resolve multiscale physics of strongly nonlinear interactions and laminated structures observed in the Terrain-induced rotor experiment (T-REX) campaign of field measurements. Very high resolution real case nested simulations are conducted. The microscale nests fully resolve localized shear layers and sharp gradients of vertical velocity and potential temperature near the tropopause and in the lower stratosphere.

THE TERRAIN-INDUCED ROTOR EXPERIMENT

Abstract The Terrain-Induced Rotor Experiment (T-REX) is a coordinated international project, composed of an observational field campaign and a research program, focused on the investigation of atmospheric rotors and closely related phenomena in complex terrain. The T-REX field campaign took place during March and April 2006 in the lee of the southern Sierra Nevada in eastern California. Atmospheric rotors have been traditionally defined as quasi-two-dimensional atmospheric vortices that form parallel to and downwind of a mountain ridge under conditions conducive to the generation of large-amplitude mountain waves. Intermittency, high levels of turbulence, and complex small-scale internal structure characterize rotors, which are known hazards to general aviation. The objective of the T-REX field campaign was to provide an unprecedented comprehensive set of in situ and remotely sensed meteorological observations from the ground to UTLS altitudes for the documentation of the spatiotem-poral characteristics ...