Journal articles on the topic 'Mountain lee waves'
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Makarenko, N. I., and J. L. Maltseva. "Interference of lee waves over mountain ranges." Natural Hazards and Earth System Sciences 11, no. 1 (January 4, 2011): 27–32. http://dx.doi.org/10.5194/nhess-11-27-2011.
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Abstract. Internal waves in the atmosphere and ocean are generated frequently from the interaction of mean flow with bottom obstacles such as mountains and submarine ridges. Analysis of these environmental phenomena involves theoretical models of non-homogeneous fluid affected by the gravity. In this paper, a semi-analytical model of stratified flow over the mountain range is considered under the assumption of small amplitude of the topography. Attention is focused on stationary wave patterns forced above the rough terrain. Adapted to account for such terrain, model equations involves exact topographic condition settled on the uneven ground surface. Wave solutions corresponding to sinusoidal topography with a finite number of peaks are calculated and examined.
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Vosper, S. B. "Inversion effects on mountain lee waves." Quarterly Journal of the Royal Meteorological Society 130, no. 600 (July 1, 2004): 1723–48. http://dx.doi.org/10.1256/qj.03.63.
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Rhines, P. B. "Jets and Orography: Idealized Experiments with Tip Jets and Lighthill Blocking." Journal of the Atmospheric Sciences 64, no. 10 (October 1, 2007): 3627–39. http://dx.doi.org/10.1175/jas4008.1.
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Abstract This paper describes qualitative features of the generation of jetlike concentrated circulations, wakes, and blocks by simple mountainlike orography, both from idealized laboratory experiments and shallow-water numerical simulations on a sphere. The experiments are unstratified with barotropic lee Rossby waves, and jets induced by mountain orography. A persistent pattern of lee jet formation and lee cyclogenesis owes its origins to arrested topographic Rossby waves above the mountain and potential vorticity (PV) advection through them. The wake jet occurs on the equatorward, eastern flank of the topography. A strong upstream blocking of the westerly flow occurs in a Lighthill mode of long Rossby wave propagation, which depends on βa2/U, the ratio of Rossby wave speed based on the scale of the mountain, to zonal advection speed, U (β is the meridional potential vorticity gradient, f is the Coriolis frequency, and a is the diameter of the mountain). Mountains wider (north–south) than the east–west length scale of stationary Rossby waves will tend to block the oncoming westerly flow. These blocks are essentially β plumes, which are illustrated by their linear Green function. For large βa2/U, upwind blocking is strong; the mountain wake can be unstable, filling the fluid with transient Rossby waves as in the numerical simulations of Polvani et al. For small values, βa2/U ≪ 1 classic lee Rossby waves with large wavelength compared to the mountain diameter are the dominant process. The mountain height, δh, relative to the mean fluid depth, H, affects these transitions as well. Simple lee Rossby waves occur only for such small heights, δh/h ≪ aβ/f, that the f/h contours are not greatly distorted by the mountain. Nongeostrophic dynamics are seen in inertial waves generated by geostrophic shear, and ducted by it, and also in a texture of finescale, inadvertent convection. Weakly damped circulations induced in a shallow-water numerical model on a sphere by a lone mountain in an initially simple westerly wind are also described. Here, with βa2/U ∼1, potential vorticity stirring and transient Rossby waves dominate, and drive zonal flow acceleration. Low-latitude critical layers, when present, exert strong control on the high-latitude waves, and with no restorative damping of the mean zonal flow, they migrate poleward toward the source of waves. While these experiments with homogeneous fluid are very simplified, the baroclinic atmosphere and ocean have many tall or equivalent barotropic eddy structures owing to the barotropization process of geostrophic turbulence.
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Millane, R. P., G. D. Stirling, R. G. Brown, N. Zhang, V. L. Lo, E. Enevoldson, and J. E. Murray. "Estimating Wind Velocities in Mountain Lee Waves Using Sailplane Flight Data*." Journal of Atmospheric and Oceanic Technology 27, no. 1 (January 1, 2010): 147–58. http://dx.doi.org/10.1175/2009jtecha1274.1.
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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.
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Draxl, Caroline, Rochelle P. Worsnop, Geng Xia, Yelena Pichugina, Duli Chand, Julie K. Lundquist, Justin Sharp, Garrett Wedam, James M. Wilczak, and Larry K. Berg. "Mountain waves can impact wind power generation." Wind Energy Science 6, no. 1 (January 7, 2021): 45–60. http://dx.doi.org/10.5194/wes-6-45-2021.
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Abstract. Mountains can modify the weather downstream of the terrain. In particular, when stably stratified air ascends a mountain barrier, buoyancy perturbations develop. These perturbations can trigger mountain waves downstream of the mountains that can reach deep into the atmospheric boundary layer where wind turbines operate. Several such cases of mountain waves occurred during the Second Wind Forecast Improvement Project (WFIP2) in the Columbia River basin in the lee of the Cascade Range bounding the states of Washington and Oregon in the Pacific Northwest of the United States. Signals from the mountain waves appear in boundary layer sodar and lidar observations as well as in nacelle wind speeds and power observations from wind plants. Weather Research and Forecasting (WRF) model simulations also produce mountain waves and are compared to satellite, lidar, and sodar observations. Simulated mountain wave wavelengths and wave propagation speeds (group velocities) are analyzed using the fast Fourier transform. We found that not all mountain waves exhibit the same speed and conclude that the speed of propagation, magnitudes of wind speeds, or wavelengths are important parameters for forecasters to recognize the risk for mountain waves and associated large drops or surges in power. When analyzing wind farm power output and nacelle wind speeds, we found that even small oscillations in wind speed caused by mountain waves can induce oscillations between full-rated power of a wind farm and half of the power output, depending on the position of the mountain wave's crests and troughs. For the wind plant analyzed in this paper, mountain-wave-induced fluctuations translate to approximately 11 % of the total wind farm output being influenced by mountain waves. Oscillations in measured wind speeds agree well with WRF simulations in timing and magnitude. We conclude that mountain waves can impact wind turbine and wind farm power output and, therefore, should be considered in complex terrain when designing, building, and forecasting for wind farms.
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TYAGI, AJIT, and OP MADAN. "Mountain waves over Himalayas." MAUSAM 40, no. 2 (April 28, 2022): 79–84. http://dx.doi.org/10.54302/mausam.v40i2.2051.
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This paper documents the observational evidence of mountain waves over western and central Himalayas. The observed wavelength of the cases presented varies from 13 km. The most marked and well-organized mountain waves are observed to the lee of western and central Himalayas over Tibet plateau, which is also the region of highest frequency of wave occurrence. Mean stability profile associated with mountain waves has been developed and it shows high stability in the layer 5-9 km with less stability aloft.
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KUMAR, NARESH, NASEEM AHMAD, S. K. ROY BHOWMIK, and H. R. HATWAR. "Wave drag by two-dimensional mountain lee waves." MAUSAM 57, no. 4 (November 26, 2021): 591–96. http://dx.doi.org/10.54302/mausam.v57i4.498.
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A linear hydrostatic model of a stably stratified air-stream flow over a two-dimensional orographic barrier is considered assuming wind increases linearly with height and stability is constant. Analytical expressions for mountain drags and momentum fluxes are obtained for Assam-Burma hills as well as Western Ghats of India. The general expression for mountain drag also obtained for both the ridges of Assam-Burma hills.
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Röttger, J. "ST radar observations of atmospheric waves over mountainous areas: a review." Annales Geophysicae 18, no. 7 (July 31, 2000): 750–65. http://dx.doi.org/10.1007/s00585-000-0750-2.
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Abstract. Lee and mountain waves are dominant dynamic processes in the atmosphere above mountain areas. ST VHF radars had been intensively used to investigate these wave processes. These studies are summarized in this work. After discussing features of long-period quasi-stationary lee waves, attention is drawn to the frequent occurrence of freely propagating waves of shorter periods, which seem to be more common and characteristic for wave processes generated over mountainous areas. Characteristics of these waves such as their relation to the topography and background winds, the possibility of trapping by and breaking in the tropopause region and their propagation into the stratosphere is investigated. These orographically produced waves transport energy and momentum into the troposphere and stratosphere, which is considered an important contribution to the kinetic energy of the lower atmosphere. The occurrence of inertia-gravity waves in the stratosphere had been confused with lee waves, which is discussed in conclusion. Finally further questions on mountain and lee waves are drawn up, which remain to be solved and where investigations with ST radars could play a fundamental role.Key words: Meteorology and atmospheric dynamics (Middle atmosphere dynamics; Waves and tides; Instruments and techniques)
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Voelger, Peter, and Peter Dalin. "Statistical analysis of observations of polar stratospheric clouds with a lidar in Kiruna, northern Sweden." Atmospheric Chemistry and Physics 23, no. 9 (May 17, 2023): 5551–65. http://dx.doi.org/10.5194/acp-23-5551-2023.
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Abstract. In the present paper, we analyse 11 years of lidar measurements to derive general characteristics of polar stratospheric clouds (PSCs) and to examine how mountain lee waves influence PSC properties. Measurements of PSCs were made with a backscatter lidar located in Kiruna, northern Sweden, in the lee of the Scandinavian mountain range. The statistical analysis demonstrates that nearly half of all observed PSCs consisted of nitric acid trihydrate (NAT) particles, while ice clouds accounted for only a small fraction, and the remainder consisted of supercooled ternary solution (STS) and mixtures of different compositions. Most PSCs were observed around 22 km altitude. Mountain lee waves provide a distinct influence on PSC chemical composition and cloud height distribution. Ice PSCs were about 5 times as frequent, and NAT clouds were about half as frequent under wave conditions. PSCs were on average at 2 km higher altitudes when under the influence of mountain lee waves.
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Broutman, Dave, Jun Ma, Stephen D. Eckermann, and John Lindeman. "Fourier-Ray Modeling of Transient Trapped Lee Waves." Monthly Weather Review 134, no. 10 (October 1, 2006): 2849–56. http://dx.doi.org/10.1175/mwr3232.1.
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Abstract The Fourier-ray method involves ray tracing in a Fourier-transform domain. The ray solutions are then Fourier synthesized to produce a spatial solution. Here previous steady-state developments of the Fourier-ray method are extended to include a transient source of mountain waves. The method is illustrated with an initial value problem in which the background flow is started abruptly from rest and then maintained at steady velocity. The resulting wave transience is modeled in a simple way. All rays that radiate from the mountain, including the initial rays, are assigned the full amplitude of the longtime steady-state solution. Time dependence comes in through the changing position of the initial rays. This is sufficient to account for wave transience in a test case, as demonstrated by comparison with simulations from a mesoscale numerical model.
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Xue, Haile, Jian Li, Tingting Qian, and Hongping Gu. "A 100-m-Scale Modeling Study of a Gale Event on the Lee Side of a Long Narrow Mountain." Journal of Applied Meteorology and Climatology 59, no. 1 (January 2020): 23–45. http://dx.doi.org/10.1175/jamc-d-19-0066.1.
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AbstractIn this study, a gale event that occurred on the lee side of a long narrow mountain was investigated, together with the associated mountain flows, using a realistic-case large-eddy simulation (LES) that is based on the Weather Research and Forecasting Model. The mountain is located on the southeastern Tibetan Plateau, where approximately 58 gales occur annually, mostly in the afternoons during the winter season. Benefitting from realistic topography and high horizontal resolution as fine as 111 m, the LES can replicate features similar to the wind fields observed during the gale period. Investigation of the early morning wind structure over the mountain revealed that weak inflows were blocked, reversed, and divided in the upstream area and that some weak lee waves, rotors, and two clear lee vortices were evident downstream. As the upstream wind accelerated and the boundary layer developed during the daytime, the lee waves became amplified with severe downslope wind and rotors. The interaction and coherent structure of the downslope wind, rotor, and vortices were investigated to show the severe wind distribution. The mountain drags associated with blocking and amplified lee waves are displayed to show the potential impact on the large-scale model. The linear lee-wave theory was adopted to explain the wave evolution during this event together with a discussion of the uncertainty around low-level nonlinear processes.
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Dean-Day, J., K. R. Chan, S. W. Bowen, T. P. Bui, B. L. Gary, and M. J. Mahoney. "Dynamics of Rocky Mountain lee waves observed during SUCCESS." Geophysical Research Letters 25, no. 9 (May 1, 1998): 1351–54. http://dx.doi.org/10.1029/98gl01004.
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Grubišić, Vanda, Stefano Serafin, Lukas Strauss, Samuel J. Haimov, Jeffrey R. French, and Larry D. Oolman. "Wave-Induced Boundary Layer Separation in the Lee of the Medicine Bow Mountains. Part II: Numerical Modeling." Journal of the Atmospheric Sciences 72, no. 12 (November 30, 2015): 4865–84. http://dx.doi.org/10.1175/jas-d-14-0381.1.
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Abstract Mountain waves and rotors in the lee of the Medicine Bow Mountains in southeastern Wyoming are investigated in a two-part paper. Part I by French et al. delivers a detailed observational account of two rotor events: one displays characteristics of a hydraulic jump and the other displays characteristics of a classic lee-wave rotor. In Part II, presented here, results of high-resolution numerical simulations are conveyed and physical processes involved in the formation and dynamical evolution of these two rotor events are examined. The simulation results reveal that the origin of the observed rotors lies in boundary layer separation, induced by wave perturbations whose amplitudes reach maxima at or near the mountain top. An undular hydraulic jump that gave rise to a rotor in one of these events was found to be triggered by midtropospheric wave breaking and an ensuing strong downslope windstorm. Lee waves spawning rotors developed under conditions favoring wave energy trapping at low levels in different phases of these two events. The upstream shift of the boundary layer separation zone, documented to occur over a relatively short period of time in both events, is shown to be the manifestation of a transition in flow regimes, from downslope windstorms to trapped lee waves, in response to a rapid change in the upstream environment, related to the passage of a short-wave synoptic disturbance aloft. The model results also suggest that the secondary obstacles surrounding the Medicine Bow Mountains play a role in the dynamics of wave and rotor events by promoting lee-wave resonance in the complex terrain of southeastern Wyoming.
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Grubišić, Vanda, and Brian J. Billings. "Climatology of the Sierra Nevada Mountain-Wave Events." Monthly Weather Review 136, no. 2 (February 1, 2008): 757–68. http://dx.doi.org/10.1175/2007mwr1902.1.
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Abstract This note presents a satellite-based climatology of the Sierra Nevada mountain-wave events. The data presented were obtained by detailed visual inspection of visible satellite imagery to detect mountain lee-wave clouds based on their location, shape, and texture. Consequently, this climatology includes only mountain-wave events during which sufficient moisture was present in the incoming airstream and whose amplitude was large enough to lead to cloud formation atop mountain-wave crests. The climatology is based on data from two mountain-wave seasons in the 1999–2001 period. Mountain-wave events are classified in two types according to cloud type as lee-wave trains and single wave clouds. The frequency of occurrence of these two wave types is examined as a function of the month of occurrence (October–May) and region of formation (north, middle, south, or the entire Sierra Nevada range). Results indicate that the maximum number of mountain-wave events in the lee of the Sierra Nevada occurs in the month of April. For several months, including January and May, frequency of wave events displays substantial interannual variability. Overall, trapped lee waves appear to be more common, in particular in the lee of the northern sierra. A single wave cloud on the lee side of the mountain range was found to be a more common wave form in the southern Sierra Nevada. The average wavelength of the Sierra Nevada lee waves was found to lie between 10 and 15 km, with a minimum at 4 km and a maximum at 32 km.
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Jiang, Qingfang, James D. Doyle, Shouping Wang, and Ronald B. Smith. "On Boundary Layer Separation in the Lee of Mesoscale Topography." Journal of the Atmospheric Sciences 64, no. 2 (February 1, 2007): 401–20. http://dx.doi.org/10.1175/jas3848.1.
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Abstract The onset of boundary layer separation (BLS) forced by gravity waves in the lee of mesoscale topography is investigated based on a series of numerical simulations and analytical formulations. It is demonstrated that BLS forced by trapped waves is governed by a normalized ratio of the vertical velocity maximum to the surface wind speed; other factors such as the mountain height, mountain slope, or the leeside speedup factor are less relevant. The onset of BLS is sensitive to the surface sensible heat flux—a positive heat flux tends to increase the surface wind speed through enhancing the vertical momentum mixing and accordingly inhibits the occurrence of BLS, and a negative heat flux does the opposite. The wave forcing required to cause BLS decreases with an increase of the aerodynamical roughness zo; a larger zo generates larger surface stress and weaker surface winds and therefore promotes BLS. In addition, BLS shows some sensitivity to the terrain geometry, which modulates the wave characteristics. For a wider ridge, a higher mountain is required to generate trapped waves with a wave amplitude comparable to that generated by a lower but narrower ridge. The stronger hydrostatic waves associated with the wider and higher ridge play only a minor role in the onset of BLS. It has been demonstrated that although hydrostatic waves generally do not directly induce BLS, undular bores may form associated with wave breaking in the lower troposphere, which in turn induce BLS. In addition, BLS could occur underneath undular jump heads or associate with trapped waves downstream of a jump head in the presence of a low-level inversion.
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Menchaca, Maximo Q., and Dale R. Durran. "Mountain Waves, Downslope Winds, and Low-Level Blocking Forced by a Midlatitude Cyclone Encountering an Isolated Ridge." Journal of the Atmospheric Sciences 74, no. 2 (February 1, 2017): 617–39. http://dx.doi.org/10.1175/jas-d-16-0092.1.
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Abstract The interaction of a midlatitude cyclone with an isolated north–south mountain barrier is examined using numerical simulation. A prototypical cyclone develops from an isolated disturbance in a baroclinically unstable shear flow upstream of the ridge, producing a cold front that interacts strongly with the topography. The structure and evolution of the lee waves launched by the topography are analyzed, including their temporal and their north–south variation along the ridge. Typical mountain wave patterns are generated by a 500-m-high mountain, but these waves often exhibit significant differences from the waves produced in 2D or 3D simulations with steady large-scale-flow structures corresponding to the instantaneous conditions over the mountain in the evolving flow. When the mountain height is 2 km, substantial wave breaking occurs, both at low levels in the lee and in the lower stratosphere. Despite the north–south uniformity of the terrain profile, large north–south variations are apparent in wave structure and downslope winds. In particular, for a 24-h period beginning after the cold front passes the upstream side of the ridge toward the south, strong downslope winds occur only in the northern half of the lee of the ridge. Just prior to this period, the movement of the cold front across the northern lee slopes is complex and accompanied by a burst of strong downslope winds and intense vertical velocities.
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Colfescu, Ioana, Joseph B. Klemp, Massimo A. Bollasina, Stephen D. Mobbs, and Ralph R. Burton. "The Dynamics of Observed Lee Waves over the Snæfellsnes Peninsula in Iceland." Monthly Weather Review 149, no. 5 (May 2021): 1559–75. http://dx.doi.org/10.1175/mwr-d-20-0288.1.
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AbstractOn 20 October 2016, aircraft observations documented a significant train of lee waves above and downstream of the Snæfellsnes Peninsula on the west coast of Iceland. Simulations of this event with the Weather Research and Forecasting (WRF) Model provide an excellent representation of the observed structure of these mountain waves. The orographic features producing these waves are characterized by the isolated Snæfellsjökull volcano near the tip of the peninsula and a fairly uniform ridge along its spine. Sensitivity simulations with the WRF Model document that the observed wave train consists of a superposition of the waves produced individually by these two dominant orographic features. This behavior is consistent with idealized simulations of a flow over an isolated 3D mountain and over a 2D ridge, which reproduce the essential behavior of the observed waves and those captured in the WRF simulations. Linear analytic analysis confirms the importance of a strong inversion at the top on the boundary layer in promoting significant wave activity extending far downstream of the terrain. However, analysis of the forced and resonant modes for a two-layer atmosphere with a capping inversion suggest that this wave train may not be produced by resonant modes whose energy is trapped beneath the inversion. Rather, these appear to be vertically propagating modes with very small vertical group velocity that can persist far downstream of the mountain. These vertically propagating waves potentially provide a mechanism for producing near-resonant waves farther aloft due to interactions with a stable layer in the midtroposphere.
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Grubišić, Vanda, and Brian J. Billings. "The Intense Lee-Wave Rotor Event of Sierra Rotors IOP 8." Journal of the Atmospheric Sciences 64, no. 12 (December 1, 2007): 4178–201. http://dx.doi.org/10.1175/2006jas2008.1.
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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.
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Réchou, A., V. Barabash, P. Chilson, S. Kirkwood, T. Savitskaya, and K. Stebel. "Mountain wave motions determined by the Esrange MST radar." Annales Geophysicae 17, no. 7 (July 31, 1999): 957–70. http://dx.doi.org/10.1007/s00585-999-0957-9.
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Abstract. A European campaign of ground-based radar, lidar and optical measurements was carried out during the winter of 1996/1997 (28 December-2 February) to study lee waves in the northern part of Scandinavia. The participants operated ozone lidars, backscatter lidars and MST radars at ALOMAR/Andoya and Esrange/Kiruna, and an ALIS imaging system in Kiruna. The Andoya site was generally windward of the Scandinavian mountains, the Kiruna site on the leeward side. The goal of the experiment was to examine the influence of lee waves on the formation of Polar Stratospheric Clouds (PSCs). This paper studies the radar data from MST-radar ESRAD located at Esrange [68.°N, 21.°E], i.e. in the lee of the mountains. We present three cases where strong lee waves were observed: in one case they propagated upwards to the lower stratosphere and in the other two cases they were trapped or absorbed in the troposphere. We examine the local waves and the direction and strength of the local wind using the radar, the synoptic meteorological situation using weather maps (European Meteorological Bulletin) and the synoptic stratospheric temperatures using ECMWF data. We observed that waves propagate up to the stratosphere during frontal passages. When anticyclonic ridges are present, the propagation to the stratosphere is very weak. This is due to trapping of the waves at or below the tropopause. We also show that the radar data alone can be used to characterise the different weather conditions for the three cases studied (through the variation of the height of the tropopause). The synoptic stratospheric temperatures in the three cases were similar, and were above the expected threshold for PSC formation. Lidar and visual observation of PSCs and nacreous clouds, respectively, showed that these were present only in the case when the lee waves propagated up to the lower stratosphere.Key words. Atmospheric composition and structure (aerosols and particles) · Electromagnetic (wave propa- gation) · Meteorology and atmospheric dynamics (mesoscale meteorology)
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Grubišić, Vanda, and Ivana Stiperski. "Lee-Wave Resonances over Double Bell-Shaped Obstacles." Journal of the Atmospheric Sciences 66, no. 5 (May 1, 2009): 1205–28. http://dx.doi.org/10.1175/2008jas2885.1.
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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.
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Lott, François. "A New Theory for Downslope Windstorms and Trapped Mountain Waves." Journal of the Atmospheric Sciences 73, no. 9 (August 22, 2016): 3585–97. http://dx.doi.org/10.1175/jas-d-15-0342.1.
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Abstract Linear mountain gravity waves forced by a nonlinear surface boundary condition are derived for a background wind that is null at the surface and increases smoothly to reach a constant value aloft and for a constant buoyancy frequency. In this configuration, the mountain waves have a critical level just below the surface that is dynamically controlled by the surface and minimum Richardson number J. When the flow is very stable , and when the depth over which dissipations act is smaller than the mountain height, this critical-level dynamics easily produces large downslope winds and foehns. The downslope winds are more intense when the stability increases and much less pronounced when it decreases (when J goes below 1). In contrast, the trapped lee waves are very small when the flow is very stable, start to appear when , and can become pure trapped waves (e.g., not decaying downstream) when the flow is unstable (for ). For the trapped waves, these results are explained by the fact that the critical level absorbs the gravity waves downstream of the ridge when , while absorption decreases when J approaches 0.25. Pure trapped lee waves follow that when the absorption can become null in the nondissipative limit. In the background-flow profiles analyzed, the pure trapped lee waves also correspond to neutral modes of Kelvin–Helmholtz instability. The validity of the linear approximation used is tested a posteriori by evaluating the relative amplitude of the neglected nonlinear terms.
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Ralph, F. M., M. Crochet, and S. V. Venkateswaran. "A Study of Mountain Lee Waves Using Clear-Air Radar." Quarterly Journal of the Royal Meteorological Society 118, no. 506 (July 1992): 597–627. http://dx.doi.org/10.1002/qj.49711850602.
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Li, Liye, and Yi-Leng Chen. "Numerical Simulations of Two Trapped Mountain Lee Waves Downstream of Oahu." Journal of Applied Meteorology and Climatology 56, no. 5 (May 2017): 1305–24. http://dx.doi.org/10.1175/jamc-d-15-0341.1.
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AbstractTwo trapped lee-wave events dominated by the transverse mode downstream of the island of Oahu in Hawaii—27 January 2010 and 24 January 2003—are simulated using the Weather Research Forecasting (WRF) Model with a horizontal grid size of 1 km in conjunction with the analyses of soundings, weather maps, and satellite images. The common factors for the occurrences of these transverse trapped mountain-wave events are 1) Froude number [Fr = U/(Nh)] > 1, where U is the upstream speed of the cross-barrier flow, N is stability, and h is the mountain height; 2) insufficient convective available potential energy for the air parcel to become positively buoyant after being lifted to the top of the stable trade wind inversion layer; and 3) increasing cross-barrier wind speed with respect to height through the stable inversion layer, satisfying Scorer’s criteria between the inversion layer and the layer aloft. Within the inversion layer, where the Scorer parameter has a maximum, the wave amplitudes are the greatest. The two trapped mountain waves in winter occurred under strong prefrontal stably stratified southwesterly flow. On the other islands in Hawaii, where the mountaintops are below the base of the inversion, transverse trapped lee waves can occur under similar large-scale settings if the mountain height is lower than U/N. The high-spatial-and-temporal-resolution WRF Model successfully simulates the onset, development, and dissipation of these two events. Sensitivity tests for the 27 January 2010 case are performed with reduced relative humidity (RH). With a lower RH and less-significant latent heating, trapped lee waves have smaller amplitudes and shorter wavelengths.
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Teixeira, Miguel A. C., José Luis Argaín, and Pedro M. A. Miranda. "Orographic Drag Associated with Lee Waves Trapped at an Inversion." Journal of the Atmospheric Sciences 70, no. 9 (September 1, 2013): 2930–47. http://dx.doi.org/10.1175/jas-d-12-0350.1.
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Abstract The drag produced by 2D orographic gravity waves trapped at a temperature inversion and waves propagating in the stably stratified layer existing above are explicitly calculated using linear theory, for a two-layer atmosphere with neutral static stability near the surface, mimicking a well-mixed boundary layer. For realistic values of the flow parameters, trapped-lee-wave drag, which is given by a closed analytical expression, is comparable to propagating-wave drag, especially in moderately to strongly nonhydrostatic conditions. In resonant flow, both drag components substantially exceed the single-layer hydrostatic drag estimate used in most parameterization schemes. Both drag components are optimally amplified for a relatively low-level inversion and Froude numbers Fr ≈ 1. While propagating-wave drag is maximized for approximately hydrostatic flow, trapped-lee-wave drag is maximized for l2a = O(1) (where l2 is the Scorer parameter in the stable layer and a is the mountain width). This roughly happens when the horizontal scale of trapped lee waves matches that of the mountain slope. The drag behavior as a function of Fr for l2H = 0.5 (where H is the inversion height) and different values of l2a shows good agreement with numerical simulations. Regions of parameter space with high trapped-lee-wave drag correlate reasonably well with those where lee-wave rotors were found to occur in previous nonlinear numerical simulations including frictional effects. This suggests that trapped-lee-wave drag, besides giving a relevant contribution to low-level drag exerted on the atmosphere, may also be useful to diagnose lee-rotor formation.
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Metz, Johnathan J., Dale R. Durran, and Peter N. Blossey. "Unusual Trapped Mountain Lee Waves with Deep Vertical Penetration and Significant Stratospheric Amplitude." Journal of the Atmospheric Sciences 77, no. 2 (January 30, 2020): 633–46. http://dx.doi.org/10.1175/jas-d-19-0093.1.
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Abstract Simulations of the weather over the South Island of New Zealand on 28 July 2014 reveal unusual wave activity in the stratosphere. A series of short-wavelength perturbations resembling trapped lee waves were located downstream of the topography, but these waves were in the stratosphere, and their crests were oriented north–south, in contrast to both the northeast–southwest orientation of the spine of the Southern Alps and the crests of trapped waves present in the lower troposphere. Vertical cross sections through these waves show a nodal structure consistent with that of a higher-order trapped-wave mode. Eigenmode solutions to the vertical structure equation for two-dimensional, linear, Boussinesq waves were obtained for a horizontally homogeneous sounding representative of the 28 July case. These solutions include higher-order modes having large amplitude in the stratosphere that are supported by just the zonal wind component. Two of these higher-order modes correspond to trapped waves that develop in an idealized numerical simulation of the 28 July 2014 case. These higher-order modes are trapped by very strong westerly winds in the midstratosphere and are triggered by north–south-oriented features in the subrange-scale topography. In contrast, the stratospheric cross-mountain wind component is too weak to trap similar high-order modes with crest-parallel orientation.
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Wang, Longlong, Marija Bervida, Samo Stanič, Klemen Bergant, William Eichinger, and Benedikt Strajnar. "Lidar Observations of Mountain Waves During Bora Episodes." EPJ Web of Conferences 237 (2020): 06007. http://dx.doi.org/10.1051/epjconf/202023706007.
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Airflows over mountain barriers in the Alpine region may give rise to strong, gusty downslope winds, called Bora. Oscillations, caused by the flow over an orographic barrier, lead to formation of mountain waves. These waves can only rarely be observed visually and can, in general, not be reliably reproduced by numerical models. Using aerosols as tracers for airmass motion, mountain waves were experimentally observed during Bora outbreak in the Vipava valley, Slovenia, on 24-25 January 2019 by two lidar systems: a vertical scanning lidar positioned just below the peak of the lee side of the mountain range and a fixed direction lidar at valley floor, which were set up to retrieve two-dimensional structure of the airflow over the orographic barrier into the valley. Based on the lidar data, we determined the thickness of airmass layer exhibiting downslope motion, observed hydraulic jump phenomena that gave rise to mountain waves and characterized their properties.
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Udina, Mireia, Maria Rosa Soler, and Ona Sol. "A Modeling Study of a Trapped Lee-Wave Event over the Pyrénées." Monthly Weather Review 145, no. 1 (December 16, 2016): 75–96. http://dx.doi.org/10.1175/mwr-d-16-0031.1.
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Abstract A trapped lee-wave mountain event in the southern part of the Pyrénées area is analyzed using the Weather Research and Forecasting (WRF) Model. Model experiments are designed to address the WRF predictability of such an event and to explore the influence of the model parameters in resolving the mountain waves. The results show that the model is able to capture a trapped lee-wave event using the 1-km horizontal grid model outputs. Different initial conditions, the vertical grid resolution, and the resolved topography lead to changes in the wave field distribution and the wave amplitude meaning that an ensemble of different model settings may be able to quantify the uncertainty of the numerical solutions. However, the model experiments do not significantly change the wavelength of the generated mountain waves, which is shorter in the three-dimensional real simulations than the one derived from satellite imagery. Comparison with observational data from the surface stations and a wind profiler upstream of the mountain range shows that the model underestimates the horizontal wind speed and this can be the reason for the underestimation of the wavelength. In addition, the valley circulations and the formation of a rotor near the surface are explored. The formation of a low-level rotor in the model is intermittent and brief, and it interacts with other flows coming from multiple directions. The first strong wave updraft is located over the valley aligned with the highest mountain peaks and strong vorticity is captured from the surface up to the first wave crest.
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Lyapustin, A., M. J. Alexander, L. Ott, A. Molod, B. Holben, J. Susskind, and Y. Wang. "Observation of mountain lee waves with MODIS NIR column water vapor." Geophysical Research Letters 41, no. 2 (January 28, 2014): 710–16. http://dx.doi.org/10.1002/2013gl058770.
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Heller, Romy, Christiane Voigt, Stuart Beaton, Andreas Dörnbrack, Andreas Giez, Stefan Kaufmann, Christian Mallaun, et al. "Mountain waves modulate the water vapor distribution in the UTLS." Atmospheric Chemistry and Physics 17, no. 24 (December 14, 2017): 14853–69. http://dx.doi.org/10.5194/acp-17-14853-2017.
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Abstract. The water vapor distribution in the upper troposphere–lower stratosphere (UTLS) region has a strong impact on the atmospheric radiation budget. Transport and mixing processes on different scales mainly determine the water vapor concentration in the UTLS. Here, we investigate the effect of mountain waves on the vertical transport and mixing of water vapor. For this purpose we analyze measurements of water vapor and meteorological parameters recorded by the DLR Falcon and NSF/NCAR Gulfstream V research aircraft taken during the Deep Propagating Gravity Wave Experiment (DEEPWAVE) in New Zealand. By combining different methods, we develop a new approach to quantify location, direction and irreversibility of the water vapor transport during a strong mountain wave event on 4 July 2014. A large positive vertical water vapor flux is detected above the Southern Alps extending from the troposphere to the stratosphere in the altitude range between 7.7 and 13.0 km. Wavelet analysis for the 8.9 km altitude level shows that the enhanced upward water vapor transport above the mountains is caused by mountain waves with horizontal wavelengths between 22 and 60 km. A downward transport of water vapor with 22 km wavelength is observed in the lee-side of the mountain ridge. While it is a priori not clear whether the observed fluxes are irreversible, low Richardson numbers derived from dropsonde data indicate enhanced turbulence in the tropopause region related to the mountain wave event. Together with the analysis of the water vapor to ozone correlation, we find indications for vertical transport followed by irreversible mixing of water vapor. For our case study, we further estimate greater than 1 W m−2 radiative forcing by the increased water vapor concentrations in the UTLS above the Southern Alps of New Zealand, resulting from mountain waves relative to unperturbed conditions. Hence, mountain waves have a great potential to affect the water vapor distribution in the UTLS. Our regional study may motivate further investigations of the global effects of mountain waves on the UTLS water vapor distributions and its radiative effects.
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Durran, Dale R., Matthew O. G. Hills, and Peter N. Blossey. "The Dissipation of Trapped Lee Waves. Part I: Leakage of Inviscid Waves into the Stratosphere." Journal of the Atmospheric Sciences 72, no. 4 (March 31, 2015): 1569–84. http://dx.doi.org/10.1175/jas-d-14-0238.1.
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Abstract Leaky trapped mountain lee waves are investigated by examining the structure of individual linear modes in multilayer atmospheres. When the static stability and cross-mountain wind speed are constant in the topmost unbounded layer, modes that decay exponentially downstream also grow exponentially with height. This growth with height occurs because packets containing relatively large-amplitude waves follow ray paths through the stratosphere, placing them above packets entering the stratosphere farther downstream that contain relatively low-amplitude waves. Nevertheless, if the trapped wave train is generated by a compact source, all waves disappear above some line parallel to the group velocity that passes just above the source region. The rate of downstream decay due to leakage into the stratosphere is strongly dependent on the atmospheric structure. Downstream dissipation is often significant under realistic atmospheric conditions, which typically include elevated inversions and strong upper-tropospheric winds. On the other hand, idealized profiles with constant Scorer parameters throughout each of two tropospheric layers can exhibit a wide range of behaviors when capped by a third stratospheric layer with typical real-world static stability. Assuming the Scorer parameter in the stratosphere is a little larger than the minimum value necessary to allow a particular mode to propagate vertically, the rate of downstream decay is more sensitive to changes in the height of the tropopause than to further increases in the stability of the stratosphere. Downstream decay is minimized when the tropopause is high and the horizontal wavelength is short.
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Teixeira, M. A. C., J. L. Argaín, and P. M. A. Miranda. "Drag produced by trapped lee waves and propagating mountain waves in a two-layer atmosphere." Quarterly Journal of the Royal Meteorological Society 139, no. 673 (September 11, 2012): 964–81. http://dx.doi.org/10.1002/qj.2008.
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Kelley, Jeffrey D., David M. Schultz, Russ S. Schumacher, and Dale R. Durran. "Can Mountain Waves Contribute to Damaging Winds Far Away from the Lee Slope?" Weather and Forecasting 34, no. 6 (December 1, 2019): 2045–65. http://dx.doi.org/10.1175/waf-d-18-0207.1.
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Abstract On 25 December 2016, a 984-hPa cyclone departed Colorado and moved onto the northern plains, drawing a nearby Arctic front into the circulation and wrapping it cyclonically around the equatorward side of the cyclone. A 130-km-wide and 850-km-long swath of surface winds exceeding 25 m s−1 originated underneath the comma head of the lee cyclone and followed the track of the Arctic front from Colorado to Minnesota. These strong winds formed in association with a downslope windstorm and mountain wave over Colorado and Wyoming, producing an elevated jet of strong winds. Central to the distribution of winds in this case is the Arctic air mass, which both shielded the elevated winds from surface friction behind the front and facilitated the mixing of the elevated jet down to the surface just behind the Arctic front, due to steep lapse rates associated with cold-air advection. The intense circulation south of the cyclone center transported the Arctic front and the elevated jet away from the mountains and out across Great Plains. This case is compared to an otherwise similar cyclone that occurred on 28–29 February 2012 in which a downslope windstorm occurred, but no surface mesoscale wind maximum formed due to the absence of a well-defined Arctic front and postfrontal stable layer. Despite the superficial similarities of this surface wind maximum to a sting jet (e.g., origin in the midtroposphere within the comma head of the cyclone, descent evaporating the comma head, acceleration to the top of the boundary layer, and an existence separate from the cold conveyor belt), this swath of winds was not caused by a sting jet.
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Díaz-Fernández, Javier, Carlos Calvo-Sancho, Pedro Bolgiani, Juan Jesús González-Alemán, José Ignacio Farrán, Mariano Sastre, and María Luisa Martín. "On the Precursor Environments to Mountain Lee Wave Clouds in Central Iberia under CMIP6 Projections." Atmosphere 15, no. 1 (January 20, 2024): 128. http://dx.doi.org/10.3390/atmos15010128.
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Mountain lee waves present significant hazards to aviation, often inducing turbulence and aircraft icing. The current study focuses on understanding the potential impact of global climate change on the precursor environments to mountain lee wave cloud episodes over central Iberia. We examine the suitability of several Global Climate Models (GCMs) from CMIP6 in predicting these environments using the ERA5 reanalysis as a benchmark for performance. The dataset is divided into two periods: historical data (2001–2014) and projections for the SSP5–8.5 future climate scenario (2015–2100). The variations and trends in precursor environments between historical data and future climate scenarios are exposed, with a particular focus on the expansion of the Azores High towards the Iberian Peninsula, resulting in increased zonal winds throughout the Iberian Peninsula in the future. However, the increase in zonal wind is insufficient to modify the wind pattern, so future mountain lee wave cloud events will not vary significantly. The relative humidity trends reveal no significant changes. Moreover, the risk of icing precursor environments connected with mountain lee wave clouds is expected to decrease in the future, due to rising temperatures. Our results highlight that the EC-EARTH3 GCM reveals the closest alignment with ERA5 data, and statistically significant differences between the historical and future climate scenario periods are presented, making EC-EARTH3 a robust candidate for conducting future studies on the precursor environments to mountain lee wave cloud events.
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Nance, Louisa B., and Dale R. Durran. "A Modeling Study of Nonstationary Trapped Mountain Lee Waves. Part II: Nonlinearity." Journal of the Atmospheric Sciences 55, no. 8 (April 1998): 1429–45. http://dx.doi.org/10.1175/1520-0469(1998)055<1429:amsont>2.0.co;2.
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Hertenstein, Rolf F. "The Influence of Inversions on Rotors." Monthly Weather Review 137, no. 1 (January 1, 2009): 433–46. http://dx.doi.org/10.1175/2008mwr2482.1.
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Abstract Rotors are small-scale circulations about a horizontal or quasi-horizontal axis that usually form in conjunction with high-amplitude mountain waves. The moderate to severe turbulence often found within rotors is a hazard to aviation. Observations and numerical model studies have revealed two types of mountain-wave–rotor systems. The first type is associated with trapped waves, whereas the second, less common, type resembles a hydraulic jump. It has long been known that an upstream, near-mountaintop inversion plays an important role in mountain-wave/rotor formation. In this study, the role of the upstream inversion strength and height in an environment with sheared flow and over a barrier with a steep lee slope is investigated. It is found that the second mountain-wave/rotor type is more likely to form when a strong near-mountaintop inversion is present. Baroclinic generation of horizontal vorticity within the inversion along the lee slope leads to overturning in an upstream direction and spreading of inversion isentropes. The sign and magnitude of vertical shear within the upstream inversion has a modifying influence, with positive shear favoring the formation of trapped-wave systems. Stronger inversions lead to higher-reaching, more turbulent mountain-wave/rotor systems, regardless of type.
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Garvert, Matthew F., Bradley Smull, and Cliff Mass. "Multiscale Mountain Waves Influencing a Major Orographic Precipitation Event." Journal of the Atmospheric Sciences 64, no. 3 (March 1, 2007): 711–37. http://dx.doi.org/10.1175/jas3876.1.
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Abstract This study combines high-resolution mesoscale model simulations and comprehensive airborne Doppler radar observations to identify kinematic structures influencing the production and mesoscale distribution of precipitation and microphysical processes during a period of heavy prefrontal orographic rainfall over the Cascade Mountains of Oregon on 13–14 December 2001 during the second phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field program. Airborne-based radar detection of precipitation from well upstream of the Cascades to the lee allows a depiction of terrain-induced wave motions in unprecedented detail. Two distinct scales of mesoscale wave–like air motions are identified: 1) a vertically propagating mountain wave anchored to the Cascade crest associated with strong midlevel zonal (i.e., cross barrier) flow, and 2) smaller-scale (<20-km horizontal wavelength) undulations over the windward foothills triggered by interaction of the low-level along-barrier flow with multiple ridge–valley corrugations oriented perpendicular to the Cascade crest. These undulations modulate cloud liquid water (CLW) and snow mixing ratios in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5), with modeled structures comparing favorably to radar-documented zones of enhanced reflectivity and CLW measured by the NOAA P3 aircraft. Errors in the model representation of a low-level shear layer and the vertically propagating mountain waves are analyzed through a variety of sensitivity tests, which indicated that the mountain wave’s amplitude and placement are extremely sensitive to the planetary boundary layer (PBL) parameterization being employed. The effects of 1) using unsmoothed versus smoothed terrain and 2) the removal of upstream coastal terrain on the flow and precipitation over the Cascades are evaluated through a series of sensitivity experiments. Inclusion of unsmoothed terrain resulted in net surface precipitation increases of ∼4%–14% over the windward slopes relative to the smoothed-terrain simulation. Small-scale waves (<20-km horizontal wavelength) over the windward slopes significantly impact the horizontal pattern of precipitation and hence quantitative precipitation forecast (QPF) accuracy.
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Woods, Bryan K., and Ronald B. Smith. "Short-Wave Signatures of Stratospheric Mountain Wave Breaking." Journal of the Atmospheric Sciences 68, no. 3 (March 1, 2011): 635–56. http://dx.doi.org/10.1175/2010jas3634.1.
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Abstract Recent stratospheric mountain wave measurements over the Sierra Nevada indicate that downgoing secondary waves may be common or even ubiquitous in large wave events. Because of their short wavelengths, they may dominate the vertical velocity field near the tropopause, and they give a remote indicator of wave breaking farther aloft. Using a 2D numerical model, the authors have simulated the secondary wave generation process with qualitative agreement in the wave location, phase speed, wavelength (i.e., 10–20 km), and amplitude. A key to the analysis was the use of Morlet wavelet cross spectra on both the observational and simulated fields. Several characteristics of the simulated secondary waves were unexpected. First, the secondary waves are generated with good efficiency, approaching 20% of the primary upgoing wave momentum flux. Second, whereas most of the secondary waves are downward, the shorter components reflect upward from the tropopause, giving a kind of lee wave trapping in the lower stratosphere. Long waves are also observed propagating upward and downward away from the wave breaking region. Third, the phase speed of the secondary waves is nearly zero so the Eliassen–Palm relationship between momentum and energy flux is satisfied. While the 2D results are robust to grid size and subgrid parameterization, an extension of the modeling to three dimensions is disappointing. The secondary waves’ amplitudes in the 3D runs are much smaller than observed.
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Hills, Matthew O. G., and Dale R. Durran. "Nonstationary Trapped Lee Waves Generated by the Passage of an Isolated Jet." Journal of the Atmospheric Sciences 69, no. 10 (May 22, 2012): 3040–59. http://dx.doi.org/10.1175/jas-d-12-047.1.
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Abstract The behavior of nonstationary trapped lee waves in a nonsteady background flow is studied using idealized three-dimensional (3D) numerical simulations. Trapped waves are forced by the passage of an isolated, synoptic-scale barotropic jet over a mountain ridge of finite length. Trapped waves generated within this environment differ significantly in their behavior compared with waves in the more commonly studied two-dimensional (2D) steady flow. After the peak zonal flow has crossed the terrain, two disparate regions form within the mature wave train: 1) upwind of the jet maximum, trapped waves increase their wavelength and tend to untrap and decay, whereas 2) downwind of the jet maximum, wavelengths shorten and waves remain trapped. Waves start to untrap approximately 100 km downwind of the ridge top, and the region of untrapping expands downwind with time as the jet progresses, while waves downstream of the jet maximum persist. Wentzel–Kramers–Brillouin (WKB) ray tracing shows that spatial gradients in the mean flow are the key factor responsible for these behaviors. An example of real-world waves evolving similarly to the modeled waves is presented. As expected, trapped waves forced by steady 2D and horizontally uniform unsteady 3D flows decay downstream because of leakage of wave energy into the stratosphere. Surprisingly, the downstream decay of lee waves is inhibited by the presence of a stratosphere in the isolated-jet simulations. Also unexpected is that the initial trapped wavelength increases quasi-linearly throughout the event, despite the large-scale forcing at the ridge crest being symmetric in time about the midpoint of the isolated-jet simulation.
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Keller, Teddie L., Stanley B. Trier, William D. Hall, Robert D. Sharman, Mei Xu, and Yubao Liu. "Lee Waves Associated with a Commercial Jetliner Accident at Denver International Airport." Journal of Applied Meteorology and Climatology 54, no. 7 (July 2015): 1373–92. http://dx.doi.org/10.1175/jamc-d-14-0270.1.
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AbstractAt 1818 mountain standard time 20 December 2008, a Boeing 737 jetliner encountered significant crosswinds while accelerating for takeoff at the Denver International Airport (DIA), ran off the side of the runway, and burst into flames. Passengers and crew were able to evacuate quickly, and, although there were injuries, there were no fatalities. Winds around the time of the accident were predominantly from the west, with substantial spatial and temporal speed variability across the airport property. Embedded in this mostly westerly flow were intermittent gusts that created strong crosswinds for the north–south runways. According to the report from the National Transportation Safety Board, it was one of these strong gusts that initiated the events that led to the runway excursion and subsequent crash of the aircraft. Numerous aircraft reported significant mountain-wave activity and turbulence over Colorado on the day of the accident. To determine whether wave activity may have contributed to the strong, intermittent gustiness at DIA, a high-resolution multinested numerical simulation was performed using the Clark–Hall model, with a horizontal grid spacing of 250 m in the inner domain. Results from this simulation suggest that the surface gustiness at DIA was associated with undulations in a train of lee waves in a midtropospheric stable layer above the airport, creating regions of higher-velocity air descending toward the surface. In contrast, a simulation with horizontal grid spacing that was similar to that of a state-of-the-art operational forecast model (3 km) did not predict strong winds at DIA.
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Tuttle, John D., and Chris A. Davis. "Modulation of the Diurnal Cycle of Warm-Season Precipitation by Short-Wave Troughs." Journal of the Atmospheric Sciences 70, no. 6 (May 29, 2013): 1710–26. http://dx.doi.org/10.1175/jas-d-12-0181.1.
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Abstract Traveling deep tropospheric disturbances of wavelengths ~1500 km (short waves) have long been known to play an important role in the initiation and maintenance of warm-season convection. To date, relatively few studies have formally documented the climatology of short waves and their relationship to the diurnal heating cycle, the topography, and the diurnal cycle of precipitation. Those that did had to rely on low-resolution global analyses and often could not track short waves across mountain barriers. In this study, 10 yr of the (32 km) NCEP North American Regional Reanalysis (NARR) are used to objectively identify and track short waves in the North American domain. Statistics of short-wave span, duration, phase speed, latitudinal extent, and locations of preferred intensification/decay are presented. Some of the key findings from the climatology include that the lee (windward) of mountain barriers are preferred regions of intensification (decay) and short waves show little evidence of a diurnal cycle and can pass a given point at any time of the day. The second part of the study focuses on the role that short waves play in modulating the diurnal cycle of propagating convection east of the Rocky Mountains. Depending on the timing of short-wave passage, short waves may either significantly enhance the precipitation above the mean or completely disrupt the normal diurnal cycle, causing precipitation to develop at times and locations where it normally does not. While short waves play an important role in modulating the mean precipitation patterns their role is considered to be secondary in nature. The diurnal precipitation signature is prominent even when short waves are not present.
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DAS, PRASANTA, SOMENATH DUTTA, and SHYAMAL KUMAR MONDAL. "A mathematical model for the 3-D dynamics of lee wave across a meso-scale mountain corner." MAUSAM 68, no. 2 (November 30, 2021): 195–204. http://dx.doi.org/10.54302/mausam.v68i2.603.
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A mathematical model for studying the 3-D dynamical structure of lee wave across a meso-scale mountain corner has been proposed for a mean flow with realistic vertical variation of wind and temperature. The basic flow consists of both zonal wind component (U) and meridional component (V), which are assumed to be dependent of height. The Brunt-Vaisala frequency (N) is also assumed to be dependent of height. This model has been applied to the mountain corner, in the North East India, formed by broadly North-South oriented Assam Burma Hills (ABH) and broadly East-West oriented Khasi Jayantia hills (KJH). The model has been solved following the quasi-numerical approach. The perturbation vertical velocity is expressed as a double integral. Three cases have been studied and in all cases the relation between the possible transverse and divergent lee wave numbers (k, l) and also the updraft/downdraft regions associated with lee waves at different heights has been mapped and discussed.
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Reinecke, Patrick A., and Dale Durran. "The Overamplification of Gravity Waves in Numerical Solutions to Flow over Topography." Monthly Weather Review 137, no. 5 (May 1, 2009): 1533–49. http://dx.doi.org/10.1175/2008mwr2630.1.
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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.
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Ehard, Benedikt, Peggy Achtert, Andreas Dörnbrack, Sonja Gisinger, Jörg Gumbel, Mikhail Khaplanov, Markus Rapp, and Johannes Wagner. "Combination of Lidar and Model Data for Studying Deep Gravity Wave Propagation." Monthly Weather Review 144, no. 1 (December 22, 2015): 77–98. http://dx.doi.org/10.1175/mwr-d-14-00405.1.
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Abstract The paper presents a feasible method to complement ground-based middle atmospheric Rayleigh lidar temperature observations with numerical simulations in the lower stratosphere and troposphere to study gravity waves. Validated mesoscale numerical simulations are utilized to complement the temperature below 30-km altitude. For this purpose, high-temporal-resolution output of the numerical results was interpolated on the position of the lidar in the lee of the Scandinavian mountain range. Two wintertime cases of orographically induced gravity waves are analyzed. Wave parameters are derived using a wavelet analysis of the combined dataset throughout the entire altitude range from the troposphere to the mesosphere. Although similar in the tropospheric forcings, both cases differ in vertical propagation. The combined dataset reveals stratospheric wave breaking for one case, whereas the mountain waves in the other case could propagate up to about 40-km altitude. The lidar observations reveal an interaction of the vertically propagating gravity waves with the stratopause, leading to a stratopause descent in both cases.
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Feltz, W. F., K. M. Bedka, J. A. Otkin, T. Greenwald, and S. A. Ackerman. "Understanding Satellite-Observed Mountain-Wave Signatures Using High-Resolution Numerical Model Data." Weather and Forecasting 24, no. 1 (February 1, 2009): 76–86. http://dx.doi.org/10.1175/2008waf2222127.1.
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Abstract Prior work has shown that pilot reports of severe turbulence over Colorado often occur when complex interference or crossing wave patterns are present in satellite water vapor imagery downstream of the Rocky Mountains. To improve the understanding of these patterns, a high-resolution (1-km) Weather Research and Forecasting (WRF) model simulation was performed for an intense mountain-wave event that occurred on 6 March 2004. Synthetic satellite imagery was subsequently generated by passing the model-simulated data through a forward radiative transfer model. Comparison with concurrent Moderate Resolution Imaging Spectroradiometer (MODIS) water vapor imagery demonstrates that the synthetic satellite data realistically captured many of the observed mesoscale features, including a mountain-wave train extending far downstream of the Colorado Front Range, the deformation of this wave train by an approaching cold front, and the substantially warmer brightness temperatures in the lee of the major mountain ranges composing the Colorado Rockies. Inspection of the model data revealed that the mountain waves redistributed the water vapor within the lower and middle troposphere, with the maximum column-integrated water vapor content occurring one-quarter wavelength downstream of the maximum ascent within each mountain wave. Due to this phase shift, the strongest vertical motions occur halfway between the locally warm and cool brightness temperature couplets in the water vapor imagery. Interference patterns seen in the water vapor imagery appear to be associated with mesoscale variability in the ambient wind field at or near mountaintop due to flow interaction with the complex topography. It is also demonstrated that the synergistic use of multiple water vapor channels provides a more thorough depiction of the vertical extent of the mountain waves since the weighting function for each channel peaks at a different height in the atmosphere.
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Karim, S. M. Shajedul, Yuh-Lang Lin, and Michael L. Kaplan. "Formation Mechanisms of the Mesoscale Environment Conducive to a Downslope Windstorm over the Cuyamaca Mountains Associated with Santa Ana Wind during the Cedar Fire (2003)." Journal of Applied Meteorology and Climatology 61, no. 11 (November 2022): 1797–818. http://dx.doi.org/10.1175/jamc-d-22-0025.1.
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Abstract Numerical simulations were conducted to investigate the upstream environment’s impacts on the airflow over the lee slope of the Cuyamaca Mountains (CM) near San Diego, California, during the Cedar Fire that occurred from 25 to 29 October 2003. The upstream environment was largely controlled by a southwest–northeast-oriented upper-tropospheric jet streak that rotated around a positively tilted ridge within the polar jet stream. Three sequential dynamical processes were found to be responsible for modifying the mesoscale environment conducive to low-level momentum and dry air that sustained the Cedar Fire. First, the sinking motion associated with the indirect circulation of the jet streak’s exit region strengthened the midtropospheric flow over the southern Rockies and the lee slope of the Sawatch and San Juan Ranges, thus modestly affecting the airflow by enhancing the downslope wind over the CM. Second, consistent with the coupling process between the upper-level sinking motion, downward momentum transfer, and developing lower-layer mountain waves, a wave-induced critical level over the mountain produced wave breaking, which was characterized by a strong turbulent mixed region with a wind reversal on top of it. This critical level helped to produce severe downslope winds leading to the third stage: a hydraulic jump that subsequently enhanced the downstream extent of the strong winds conducive to the favorable lower-tropospheric environment for rapid fire spread. Consistent with these findings was the deep-layer resonance between the mountain surface and tropopause, which had a strong impact on strengthening the severe downslope winds over the lee slope of the CM accompanying the elevated strong easterly jet at low levels.
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Worthington, R. M. "Alignment of mountain lee waves viewed using NOAA AVHRR imagery, MST radar, and SAR." International Journal of Remote Sensing 22, no. 7 (January 2001): 1361–74. http://dx.doi.org/10.1080/01431160151144396.
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Nance, Louisa B., and Dale R. Durran. "A Modeling Study of Nonstationary Trapped Mountain Lee Waves. Part I: Mean-Flow Variability." Journal of the Atmospheric Sciences 54, no. 18 (September 1997): 2275–91. http://dx.doi.org/10.1175/1520-0469(1997)054<2275:amsont>2.0.co;2.
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Martin, Armel, and François Lott. "Synoptic Responses to Mountain Gravity Waves Encountering Directional Critical Levels." Journal of the Atmospheric Sciences 64, no. 3 (March 1, 2007): 828–48. http://dx.doi.org/10.1175/jas3873.1.
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Abstract A heuristic model is used to study the synoptic response to mountain gravity waves (GWs) absorbed at directional critical levels. The model is a semigeostrophic version of the Eady model for baroclinic instability adapted by Smith to study lee cyclogenesis. The GWs exert a force on the large-scale flow where they encounter directional critical levels. This force is taken into account in the model herein and produces potential vorticity (PV) anomalies in the midtroposphere. First, the authors consider the case of an idealized mountain range such that the orographic variance is well separated between small- and large-scale contributions. In the absence of tropopause, the PV produced by the GW force has a surface impact that is significant compared to the surface response due to the large scales. For a cold front, the GW force produces a trough over the mountain and a larger-amplitude ridge immediately downstream. It opposes somehow to the response due to the large scales of the mountain range, which is anticyclonic aloft and cyclonic downstream. For a warm front, the GW force produces a ridge over the mountain and a trough downstream; hence it reinforces the response due to the large scales. Second, the robustness of the previous results is verified by a series of sensitivity tests. The authors change the specifications of the mountain range and of the background flow. They also repeat some experiments by including baroclinic instabilities, or by using the quasigeostrophic approximation. Finally, they consider the case of a small-scale orographic spectrum representative of the Alps. The significance of the results is discussed in the context of GW parameterization in the general circulation models. The results may also help to interpret the complex PV structures occurring when mountain gravity waves break in a baroclinic environment.
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Eckermann, Stephen D., Dave Broutman, Jun Ma, and John Lindeman. "Fourier-Ray Modeling of Short-Wavelength Trapped Lee Waves Observed in Infrared Satellite Imagery near Jan Mayen." Monthly Weather Review 134, no. 10 (October 1, 2006): 2830–48. http://dx.doi.org/10.1175/mwr3218.1.
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Abstract A time-dependent generalization of a Fourier-ray method is presented and tested for fast numerical computation of high-resolution nonhydrostatic mountain-wave fields. The method is used to model mountain waves from Jan Mayen on 25 January 2000, a period when wavelike cloud banding was observed long distances downstream of the island by the Advanced Very High Resolution Radiometer Version 3 (AVHRR-3). Surface weather patterns show intensifying surface geostrophic winds over the island at 1200 UTC caused by rapid eastward passage of a compact low pressure system. The 1200 UTC wind profiles over the island increase with height to a jet maximum of ∼60–70 m s−1, yielding Scorer parameters that indicate vertical trapping of any short wavelength mountain waves. Separate Fourier-ray solutions were computed using high-resolution Jan Mayen orography and 1200 UTC vertical profiles of winds and temperatures over the island from a radiosonde sounding and an analysis system. The radiosonde-based simulations produce a purely diverging trapped wave solution that reproduces the salient features in the AVHRR-3 imagery. Differences in simulated wave patterns governed by the radiosonde and analysis profiles are explained in terms of resonant modes and are corroborated by spatial ray-group trajectories computed for wavenumbers along the resonant mode curves. Output from a nonlinear Lipps–Hemler orographic flow model also compares well with the Fourier-ray solution horizontally. Differences in vertical cross sections are ascribed to the Fourier-ray model’s current omission of tunneling of trapped wave energy through evanescent layers.
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Lund, Thomas S., David C. Fritts, Kam Wan, Brian Laughman, and Han-Li Liu. "Numerical Simulation of Mountain Waves over the Southern Andes. Part I: Mountain Wave and Secondary Wave Character, Evolutions, and Breaking." Journal of the Atmospheric Sciences 77, no. 12 (December 2020): 4337–56. http://dx.doi.org/10.1175/jas-d-19-0356.1.
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AbstractThis paper addresses the compressible nonlinear dynamics accompanying increasing mountain wave (MW) forcing over the southern Andes and propagation into the mesosphere and lower thermosphere (MLT) under winter conditions. A stretched grid provides very high resolution of the MW dynamics in a large computational domain. A slow increase of cross-mountain winds enables MWs to initially break in the mesosphere and extend to lower and higher altitudes thereafter. MW structure and breaking is strongly modulated by static mean and semidiurnal tide fields exhibiting a critical level at ~114 km for zonal MW propagation. Varying vertical group velocities for different zonal wavelengths λx yield initial breaking in the lee of the major Andes peaks for λx ~ 50 km, and extending significantly upstream for larger λx approaching the critical level at later times. The localized extent of the Andes terrain in latitude leads to “ship wave” responses above the individual peaks at earlier times, and a much larger ship-wave response at 100 km and above as the larger-scale MWs achieve large amplitudes. Other responses above regions of MW breaking include large-scale secondary gravity waves and acoustic waves that achieve very large amplitudes extending well into the thermosphere. MW breaking also causes momentum deposition that yields local decelerations initially, which merge and extend horizontally thereafter and persist throughout the event. Companion papers examine the associated momentum fluxes, mean-flow evolution, gravity wave–tidal interactions, and the MW instability dynamics and sources of secondary gravity waves and acoustic waves.