scholarly journals Impact of non-hydrostatic effects and trapped lee waves on mountain-wave drag in directionally sheared flow

2014 ◽  
Vol 141 (690) ◽  
pp. 1572-1585 ◽  
Author(s):  
C. L. Yu ◽  
M. A. C. Teixeira
Keyword(s):  
Wave Drag ◽  
Mountain Wave ◽  
Sheared Flow ◽  
Lee Waves

An Investigation of a Commercial Aircraft Encounter with Severe Clear-Air Turbulence over Western Greenland

2012 ◽  
Vol 51 (1) ◽  
pp. 42-53 ◽  
Author(s):  
R. D. Sharman ◽  
J. D. Doyle ◽  
M. A. Shapiro
Keyword(s):  
High Resolution ◽  
Prediction Models ◽  
Weather Prediction ◽  
Simulation Models ◽  
Wave Drag ◽  
Commercial Aircraft ◽  
Mountain Wave ◽  
Air Turbulence ◽  
Lee Waves ◽  
Horizontal Grid

AbstractThis study presents digital flight data recorder (DFDR) analyses and high-resolution numerical simulations relevant to a severe clear-air turbulence (CAT) encounter over western Greenland by a Boeing 777 aircraft at 10-km elevation at 1305 UTC 25 May 2010. The environmental flow was dominated by an extratropical cyclone to the southeast of the Greenland tip, resulting in easterly flow at all levels. The results of the analyses indicate that the CAT encounter was related to mountain-wave breaking on the western lee (downslope) of the Greenland plateau. The simulations were not of especially high resolution (5-km horizontal grid spacing) by today’s standards, yet the simulation results do produce large-amplitude lee waves and overturning in good agreement with the encounter location as indicated by the DFDR. The success of this and other simulations in reproducing mountain-wave turbulence (MWT) events suggests that operational implementation of high-resolution nonhydrostatic simulation models, possibly an ensemble of models, over MWT-prone areas could produce more reliable forecasts of MWT than are currently available using gravity-wave-drag or MWT-postprocessing algorithms derived from global weather prediction models of relatively coarse scale.

scholarly journals A Gravity Wave Drag Matrix for Complex Terrain

2018 ◽  
Vol 75 (8) ◽  
pp. 2599-2613 ◽  
Author(s):  
Ronald B. Smith ◽  
Christopher G. Kruse
Keyword(s):  
New Zealand ◽  
Complex Terrain ◽  
Momentum Flux ◽  
Correlation Coefficients ◽  
Wave Drag ◽  
Positive Definite ◽  
Wind Vector ◽  
Mountain Wave ◽  
Wind Speeds ◽  
Aircraft Data

Abstract We propose a simplified scheme to predict mountain wave drag over complex terrain using only the regional-average low-level wind components U and V. The scheme is tuned and tested on data from the South Island of New Zealand, a rough and highly anisotropic terrain. The effect of terrain anisotropy is captured with a hydrostatically computed, 2 × 2 positive-definite wave drag matrix. The wave drag vector is the product of the wind vector and the drag matrix. The nonlinearity in wave generation is captured using a Gaussian terrain smoothing inversely proportional to wind speed. Wind speeds of |U| = 10, 20, and 30 m s−1 give smoothing scales of L = 54, 27, and 18 km, respectively. This smoothing treatment of nonlinearity is consistent with recent aircraft data and high-resolution numerical modeling of waves over New Zealand, indicating that the momentum flux spectra shift to shorter waves during high-drag conditions. The drag matrix model is tested against a 3-month time series of realistic full-physics wave-resolving flow calculations. Correlation coefficients approach 0.9 for both zonal and meridional drag components.

Forcing of oceanic mean flows by dissipating internal tides

2012 ◽  
Vol 708 ◽  
pp. 250-278 ◽  
Author(s):  
Nicolas Grisouard ◽  
Oliver Bühler
Keyword(s):  
Numerical Study ◽  
Three Dimensional ◽  
Wave Drag ◽  
Perturbation Series ◽  
Internal Tides ◽  
Time Averaging ◽  
Mean Flow ◽  
Wave Source ◽  
Lee Waves ◽  
Lee Wave

AbstractWe present a theoretical and numerical study of the effective mean force exerted on an oceanic mean flow due to the presence of small-amplitude internal waves that are forced by the oscillatory flow of a barotropic tide over undulating topography and are also subject to dissipation. This extends the classic lee-wave drag problem of atmospheric wave–mean interaction theory to a more complicated oceanographic setting, because now the steady lee waves are replaced by oscillatory internal tides and, most importantly, because now the three-dimensional oceanic mean flow is defined by time averaging over the fast tidal cycles rather than by the zonal averaging familiar from atmospheric theory. Although the details of our computation are quite different, we recover the main action-at-a-distance result from the atmospheric setting, namely that the effective mean force that is felt by the mean flow is located in regions of wave dissipation, and not necessarily near the topographic wave source. Specifically, we derive an explicit expression for the effective mean force at leading order using a perturbation series in small wave amplitude within the framework of generalized Lagrangian-mean theory, discuss in detail the range of situations in which a strong, secularly growing mean-flow response can be expected, and then compute the effective mean force numerically in a number of idealized examples with simple topographies.

scholarly journals Regional to Global Evolution of Impacts of Parameterized Mountain-Wave Drag in the Lower Stratosphere

2020 ◽  
Vol 33 (8) ◽  
pp. 3093-3106 ◽  
Author(s):  
Christopher G. Kruse
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Climate Model ◽  
Wave Drag ◽  
Mountain Wave ◽  
Global Features ◽  
Mountain Ranges ◽  
The North ◽  
Global Evolution ◽  
And Diffusion

AbstractMountain ranges are regional features on Earth, as are the regions of mountain-wave drag (MWD) exerted by dissipating atmospheric gravity waves generated by flow over them. However, these regional drags have significant global- or zonal-mean impacts on Earth’s atmospheric general circulation (e.g., slowing of the polar night jet). The objective of this work is to understand the regional to global evolution of these impacts. The approach is to track the evolution of MWD-generated potential vorticity (PV) over the winter using the Whole Atmosphere Community Climate Model (WACCM). Within an ensemble of winter-long runs with and without MWD, lower-stratospheric PV is generated over mountains and advected downstream, generating large-scale PV banners. These PV banners are diffused but survive this diffusion and are reinforced over downstream mountain ranges, accumulating into zonal-mean or global features within WACCM. A simple 2D model representing sources, advection, and diffusion of “passive PV” recreates the salient features in the WACCM results, suggesting the winter-long evolution of MWD-generated PV can be crudely understood in terms of horizontal advection and diffusion within a global vortex. In the Northern Hemisphere, cyclonic, equatorward PV banners accumulate zonally into a single zonally symmetric positive PV anomaly. The anticyclonic, poleward PV banners also accumulate into a zonally symmetric feature, but then diffuse over the North Pole into a negative PV polar cap. In the Southern Hemisphere, the same processes are at work, though the different geographic configuration of mountain ranges leads to different patterns of impacts.

Climatology of the Sierra Nevada Mountain-Wave Events

2008 ◽  
Vol 136 (2) ◽  
pp. 757-768 ◽  
Author(s):  
Vanda Grubišić ◽  
Brian J. Billings
Keyword(s):  
Sierra Nevada ◽  
Cloud Formation ◽  
Wave Form ◽  
Mountain Range ◽  
Mountain Wave ◽  
Single Wave ◽  
Sierra Nevada Mountain ◽  
Lee Waves ◽  
Lee Wave ◽  
Wave Trains

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.

scholarly journals Effects of Horizontal Geometrical Spreading on the Parameterization of Orographic Gravity Wave Drag. Part I: Numerical Transform Solutions

2015 ◽  
Vol 72 (6) ◽  
pp. 2330-2347 ◽  
Author(s):  
Stephen D. Eckermann ◽  
Jun Ma ◽  
Dave Broutman
Keyword(s):  
Gravity Waves ◽  
Wave Breaking ◽  
Climate Models ◽  
Functional Form ◽  
Vertical Displacement ◽  
Wave Drag ◽  
Geometrical Spreading ◽  
Vertical Coordinate ◽  
Sheared Flow ◽  
Weather And Climate

Abstract Numerical transform solutions for hydrostatic gravity waves generated by both uniform and sheared flow over elliptical obstacles are used to quantify effects of horizontal geometrical spreading on amplitude evolution with height. Both vertical displacement and steepness amplitudes are considered because of their close connections to drag parameterizations in weather and climate models. Novel diagnostics quantify the location and value of the largest wavefield amplitudes most likely to break at each altitude. These horizontal locations do not stray far from the obstacle peak even at high altitudes. Resulting vertical profiles of wave amplitude are normalized to remove density and refraction effects, thereby quantifying the horizontal geometrical spreading contribution, currently absent from parameterizations. Horizontal geometrical spreading produces monotonic amplitude decreases with height through wave-action conservation as waves propagate into progressively larger horizontal areas. Accumulated amplitude reductions are appreciable for all but the most quasi-two-dimensional obstacles with long axes orthogonal to the flow, and even these are impacted appreciably if the obstacle is rotated by more than 20°–30°. Profiles are insensitive to the obstacle’s functional form but vary strongly in response to changes in its horizontal aspect ratio. Responses to background winds are captured by a vertical coordinate transformation that remaps profiles to a universal form for a given obstacle. These results show that horizontal geometrical spreading has comparable or larger effects on wave amplitudes as the refraction of vertical wavenumbers and thus is important for accurate parameterizations of wave breaking and drag.

Lee waves in a stratified flow Part 1. Thin barrier

1968 ◽  
Vol 32 (3) ◽  
pp. 549-567 ◽  
Author(s):  
John W. Miles
Keyword(s):  
Integral Equation ◽  
Half Space ◽  
Dynamic Pressure ◽  
Separation Of Variables ◽  
Stratified Flow ◽  
Wave Drag ◽  
Variational Approximations ◽  
Lee Waves ◽  
Lee Wave ◽  
Thin Barrier

The lee-wave amplitudes and wave drag for a thin barrier in a two-dimensional stratified flow in which the upstream dynamic pressure and density gradient are constant (Long's model) are determined as functions of barrier height and Froude number for a channel of finite height and for a half-space. Variational approximations to these quantities are obtained and validated by comparison with the earlier results of Drazin & Moore (1967) for the channel and with the results of an exact solution for the half-space, as obtained through separation of variables. An approximate solution of the integral equation for the channel also is obtained through a reduction to a singular integral equation of potential theory. The wave drag tends to increase with decreasing wind speed, but it seems likely that the flow is unstable in the region of high drag. The maximum attainable drag coefficient consistent with stable lee-wave formation appears to be roughly two and almost certainly less than three.

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