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Dale R. Durran - One of the best experts on this subject based on the ideXlab platform.
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the dissipation of trapped Lee Waves part i leakage of inviscid Waves into the stratosphere
Journal of the Atmospheric Sciences, 2015Co-Authors: Dale R. Durran, Matthew O G Hills, Peter N BlosseyAbstract:AbstractLeaky 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-tropo...
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Mountain Meteorology: Lee Waves and Mountain Waves
Encyclopedia of Atmospheric Sciences: Second Edition, 2014Co-Authors: Dale R. DurranAbstract:The basic properties of mountain Waves are discussed using linear theory for small-amplitude disturbances. Important nonlinear processes governing Waves launched by larger mountains are then considered. The final topic is mountain-wave momentum flux and the interaction of these Waves with the larger-scale flow.
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nonstationary trapped Lee Waves generated by the passage of an isolated jet
Journal of the Atmospheric Sciences, 2012Co-Authors: Matthew O G Hills, Dale R. DurranAbstract:AbstractThe 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 gradien...
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A Modeling Study of Nonstationary Trapped Mountain Lee Waves. Part II: Nonlinearity
Journal of the Atmospheric Sciences, 1998Co-Authors: Louisa B. Nance, Dale R. DurranAbstract:The generation of nonstationary trapped mountain Lee Waves through nonlinear wave dynamics without any concomitant change in the background flow is investigated by conducting two-dimensional mountain wave simulations. These simulations demonstrate that finite-amplitude Lee-wave patterns can exhibit temporal variations in local wavelength and amplitude, even when the background flow is perfectly steady. For moderate amplitudes, a nonlinear wave interaction involving the stationary trapped wave and a pair of nonstationary Waves appears to be responsible for the development of nonstationary perturbations on the stationary trapped wave. This pair of nonstationary Waves consists of a trapped wave and a vertically propagating wave, both having horizontal wavelengths approximately twice that of the stationary trapped wave. As the flow becomes more nonlinear, the nonstationary perturbations involve a wider spectrum of horizontal wavelengths and may dominate the overall wave pattern at wave amplitudes significantly below the threshold required to produce wave breaking. Sensitivity tests in which the wave propagation characteristics of the basic state are modified without changing the horizontal wavelength of the stationary trapped wave indicate these nonstationary perturbations are absent when the background flow does not support nonstationary trapped Waves with horizontal wavelengths approximately twice that of the stationary trapped mode. These sensitivity tests also show that a second nonstationary trapped wave can assume the role of the nonstationary vertically propagating wave when the Scorer parameter in the upper layer is reduced below the threshold that will support the vertically propagating wave. In this case, a resonant triad composed of three trapped Waves appears to be responsible for the development of nonstationary perturbations. The simulations suggest that strongly nonlinear wave dynamics can generate a wider range of nonstationary trapped modes than that produced by temporal variations in the background flow. It is suggested that the irregular variations in Lee-wave wavelength and amplitude observed in real atmospheric flows and the complex fluctuations above a fixed point that are occasionally found in wind profiler observations of trapped Lee Waves are more likely to be generated by nonlinear wave dynamics than changes in the background flow.
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A Modeling Study of Nonstationary Trapped Mountain Lee Waves. Part I: Mean-Flow Variability
Journal of the Atmospheric Sciences, 1997Co-Authors: Louisa Nance, Dale R. DurranAbstract:Abstract The impact of mean-flow variability on finite-amplitude trapped mountain Lee Waves is investigated by conducting two-dimensional mountain wave simulations for a set of idealized, time-dependent background flows. The Lee-wave patterns generated by these time-dependent flows depend on two factors: 1) the degree to which the transition in the background flow changes the amplitude of the stationary trapped Lee wave and 2) the difference between the group velocities of the trapped Waves generated before and after the transition. When the transition in the background flow significantly reduces the amplitude of the stationary Lee wave, the Lee-wave pattern generated prior to the transition gradually drifts downstream away from the mountain or back over the mountain, depending on the sign of this wave packet’s group velocity after the transition. When the transition in the background flow changes the resonant wavelength while leaving the Lee-wave amplitude relatively unchanged, the Lee-wave train develop...
Tomohiro Nakamura - One of the best experts on this subject based on the ideXlab platform.
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Processes of breaking of large‐amplitude unsteady Lee Waves leading to turbulence
Journal of Geophysical Research: Oceans, 2013Co-Authors: S. Abe, Tomohiro NakamuraAbstract:[1] The transition to turbulence after excitation of large-amplitude (~200 m) unsteady Lee Waves in Amchitka Pass, Alaska, is investigated using a nonhydrostatic vertically two-dimensional model with realistic topography. The model resolves motions two orders smaller than a large-amplitude unsteady Lee wave, which is excited in the Lee of the ridge, and shows that transition processes near the ridge top and downstream of the first trough of the unsteady Lee wave are different. Near the ridge top, three stages of transition are identified. In the first stage, convection begins on the upstream sides (forward wave breaking) and downstream sides (backward wave breaking) of the crests of the unsteady Lee wave. In the next stage, Kelvin-Helmholtz (KH) Waves develop in regions of enhanced shear between statically unstable regions and downslope flow on the bottom. In the last stage, Tollmien-Schlichting (TS) Waves develop on the bottom, under the KH Waves, and form vortices, which finally break down. To the best of the authors’ knowledge, this is the first paper to report on the occurrence of backward wave breaking and the possibility of TS wave excitation in the ocean. Downstream of the first trough of the unsteady Lee wave, flow is separated from the bottom by an adverse pressure gradient attributed to the unsteady Lee wave. The separated flow forms vortices, which are shed quasi-periodically. Diapycnal mixing is enhanced by the development of KH and TS Waves and flow separation, as well as by convection due to overturning isopycnals induced by the unsteady Lee wave.
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processes of breaking of large amplitude unsteady Lee Waves leading to turbulence
Journal of Geophysical Research, 2013Co-Authors: S. Abe, Tomohiro NakamuraAbstract:[1] The transition to turbulence after excitation of large-amplitude (~200 m) unsteady Lee Waves in Amchitka Pass, Alaska, is investigated using a nonhydrostatic vertically two-dimensional model with realistic topography. The model resolves motions two orders smaller than a large-amplitude unsteady Lee wave, which is excited in the Lee of the ridge, and shows that transition processes near the ridge top and downstream of the first trough of the unsteady Lee wave are different. Near the ridge top, three stages of transition are identified. In the first stage, convection begins on the upstream sides (forward wave breaking) and downstream sides (backward wave breaking) of the crests of the unsteady Lee wave. In the next stage, Kelvin-Helmholtz (KH) Waves develop in regions of enhanced shear between statically unstable regions and downslope flow on the bottom. In the last stage, Tollmien-Schlichting (TS) Waves develop on the bottom, under the KH Waves, and form vortices, which finally break down. To the best of the authors’ knowledge, this is the first paper to report on the occurrence of backward wave breaking and the possibility of TS wave excitation in the ocean. Downstream of the first trough of the unsteady Lee wave, flow is separated from the bottom by an adverse pressure gradient attributed to the unsteady Lee wave. The separated flow forms vortices, which are shed quasi-periodically. Diapycnal mixing is enhanced by the development of KH and TS Waves and flow separation, as well as by convection due to overturning isopycnals induced by the unsteady Lee wave.
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breaking of unsteady Lee Waves generated by diurnal tides
Geophysical Research Letters, 2010Co-Authors: Tomohiro Nakamura, Yutaka Isoda, Humio Mitsudera, Shohgo Takagi, Maki NagasawaAbstract:[1] Diapycnal mixing caused through breaking of large-amplitude internal Lee Waves generated by sub-inertial diurnal tides, which are modulated with a 18.6-year period, is hypothesized to be fundamental to both the intermediate-layer ventilation and the bi-decadal oscillation around the North Pacific Ocean. The first observational evidence of such wave breaking is presented here. The breaking wave observed had ∼200 m height and ∼1 km width, and its associated diapycnal mixing was estimated to be ∼1.5 m2 s−1, with a temporal average ∼104 times larger than typical values in the open oceans. Our estimate suggests that a similar mixing process occurs globally, particularly around the Pacific and Antarctic Oceans.
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the generation of large amplitude unsteady Lee Waves by subinertial k1 tidal flow a possible vertical mixing mechanism in the kuril straits
Journal of Physical Oceanography, 2000Co-Authors: Tomohiro Nakamura, Toshiyuki Awaji, Takaki Hatayama, Kazunori Akitomo, Takatoshi Takizawa, Tokihiro Kono, Yasuhiro Kawasaki, Masao FukasawaAbstract:Numerical experiments with a two-dimensional nonhydrostatic model are performed to investigate tidally generated internal Waves in the Kuril Straits and their effect on vertical mixing. The results show that sill-scale internal Waves at the K1 tidal frequency are confined to the sill slopes because the K1 tide is subinertial in the Kuril Straits. In contrast to previous theories, the authors show that intense short internal Waves generated at the sill breaks by the subinertial K1 tidal current can propagate upstream as the tidal current slackens. Theoretical considerations identify these short Waves as unsteady Lee Waves, which tend to be trapped at the generation region and grow into large-amplitude Waves, eventually inducing vigorous mixing along their ray paths. In particular, superposition of a propagating unsteady Lee wave and a newly generated Lee wave over a sill causes significant wave breaking leading to a maximum vertical diffusivity of ;103 cm2 s21. This quite intense mixing reaches down to the density layer of the North Pacific Intermediate Water (NPIW). In contrast, the M2 tidal current does not cause such strong vertical mixing, because most of generated internal Waves propagate away as first-mode internal tides and because the barotropic flow amplitude is small. The authors therefore suggest the possibility that generation of Lee Waves through interactions between the K1 current and the bottom topography of the Kuril Straits contributes to the observed modification of the Okhotsk Sea water required in the formation of the NPIW.
Bruno Voisin - One of the best experts on this subject based on the ideXlab platform.
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Lee Waves from a sphere in a stratified flow
Journal of Fluid Mechanics, 2007Co-Authors: Bruno VoisinAbstract:Two asymptotic analyses of the generation of Lee Waves by horizontal flow at velocity $U$ of a stratified fluid of buoyancy frequency $N$ past a sphere of radius $a$ are presented, for either weak or strong stratification, corresponding to either large or small internal Froude number $\mathit{F} = U/(Na)$, respectively. For $\mathit{F} \gg 1$, the fluid separates into two regions radially: an inner region of scale $a$ with three-dimensional irrotational flow unaffected by the stratification, and an outer region of scale $U/N$ with small-amplitude Lee Waves generated by the $O(1)$ vertical motion in the inner region. For $\mathit{F} \ll 1$, the fluid separates into five layers vertically: from the lower dividing streamsurface situated at a distance $U/N$ above the bottom of the sphere to the upper dividing streamsurface situated at a distance $U/N$ below the top, a middle layer with two-dimensional horizontal irrotational flow; from the upper dividing streamsurface to the top of the sphere, and from the lower dividing streamsurface to the bottom, top and bottom transition layers, respectively, with three-dimensional flow; above the top and below the bottom, upper and lower layers, respectively, with small-amplitude Lee Waves generated by the $O(\mathit{F})$ vertical motion in the transition layers. The Waves are calculated where they have small amplitudes. The forcing is represented by a source of mass: for $\mathit{F} \gg 1$, the surface distribution of singularities equivalent to the sphere in three-dimensional irrotational flow; for $\mathit{F} \ll 1$, the horizontal distribution of singularities equivalent, in the upper (resp. lower) layer, to the flat cut-off obstacle made of the top (resp. bottom) portion of the sphere protruding above (resp. below) the upper (resp. lower) dividing streamsurface. The analysis is validated by comparison of the theoretical wave drag with its existing experimental determinations. For $\mathit{F} \gg 1$, the drag coefficient decreases as $(\ln\mathit{F}+7/4-\gamma)/(4\mathit{F}^2)$, with $\gamma$ the Euler constant; for $\mathit{F} \ll 1$, it increases as $(32\surd2)/(15\pi)\mathit{F}^{3/2}$. The Waves have the crescent shape of the three-dimensional Lee Waves from a dipole, modulated by interferences associated with the finite size of the forcing. For strong stratification, the hydrostatic approximation is seen to produce correct leading-order drag, but incorrect Waves.
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Lee Waves from a sphere in a stratified flow
Journal of Fluid Mechanics, 2007Co-Authors: Bruno VoisinAbstract:Two asymptotic analyses of the generation of Lee Waves by horizontal flow at velocity U of a stratified fluid of buoyancy frequency N past a sphere of radius a are presented, for either weak or strong stratification, corresponding to either large or small internal Froude number F = U /( Na ), respectively. For F ⋙1, the fluid separates into two regions radially: an inner region of scale a with three-dimensional irrotational flow unaffected by the stratification, and an outer region of scale U / N with small-amplitude Lee Waves generated by the O (1) vertical motion in the inner region. For F ⋘1, the fluid separates into five layers vertically: from the lower dividing streamsurface situated at a distance U / N above the bottom of the sphere to the upper dividing streamsurface situated at a distance U / N below the top, there is a middle layer with two-dimensional horizontal irrotational flow; from the upper dividing streamsurface to the top of the sphere, and from the lower dividing streamsurface to the bottom, there are top and bottom transition layers, respectively, with three-dimensional flow; above the top and below the bottom, there are upper and lower layers, respectively, with small-amplitude Lee Waves generated by the O ( F ) vertical motion in the transition layers. The Waves are calculated where they have small amplitudes. The forcing is represented by a source of mass: for F ⋙1, the surface distribution of singularities equivalent to the sphere in three-dimensional irrotational flow; for F ⋘1, the horizontal distribution of singularities equivalent, in the upper (resp. lower) layer, to the flat cut-off obstacle made up of the top (resp. bottom) portion of the sphere protruding above (resp. below) the upper (resp. lower) dividing streamsurface. The analysis is validated by comparison of the theoretical wave drag with existing experimental determinations. For F ⋙1, the drag coefficient decreases as (ln F +7/4-γ)/(4 F 4 ), with γ the Euler constant; for F ⋘1, it increases as . The Waves have the crescent shape of the three-dimensional Lee Waves from a dipole, modulated by interferences associated with the finite size of the forcing. For strong stratification, the hydrostatic approximation is seen to produce correct leading-order drag, but incorrect Waves.
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The trapping of Lee Waves
1999Co-Authors: Bruno Voisin, Gaël HuerreAbstract:Lee Waves are that particular kind of internal gravity Waves that are produced by stratified flow over an obstacle. In many situations of geophysical interest, the scales over which Lee Waves propagate are such that the vertical variations of the stratification come into play, causing vertical trapping. This communication addresses the question of the effect of trapping on Lee Waves, using the Green's function formalism of Voisin (1994). For each given internal wave mode, at each point of reception, the Waves are expressed in terms of the retarded time of their emission. The retarded time follows from the resolution of a system of implicit equations expressing propagation at the group velocity and Doppler shift; it depends only on the vertical variations of the stratification and on the temporal variations of the wind in speed and direction. For constant wind and non-anomalous stratification, in each horizontal plane, the Waves are contained within a wedge and composed of divergent and (possibly) transverse Waves, in a manner reminiscent of the Kelvin wave wake of surface ships. General conclusions about the geometry and structure of the wave field are obtained, and applied to two particular stratifications: a fluid layer of finite depth and linear density profile, and a thermocline corresponding to a hyperbolic tangent density profile. Comparison with original experiments is also performed.
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The trapping of Lee Waves
1997Co-Authors: Gaël Huerre, Bruno VoisinAbstract:In most natural systems internal gravity Waves do not propagate freely but are trapped, inside regions of strong density gradients called thermoclines. Trapping takes place as a consequence of refraction by the buoyancy frequency profile, and reflection on fluid boundaries. In particular, in the oceans and in the atmosphere, Lee Waves generated by flows over obstacles change structure as trapping occurs: free Lee Waves have hyperbolic crests and throughs, while trapped Lee Waves have divergent and transverse crests and throughs located within a wedge similar to the Kelvin wake of a ship. This paper adapts the approach elaborated by Voisin (J. Fluid Mech. 1994) for the generation of internal Waves by bodies in arbitrary motion (or, equivalently, by flows with arbitrary time dependence) to situations where trapping is significant. First the Green's function of trapped internal Waves is calculated, and from it the wave field generated by a moving source is deduced, generalizing Keller & Munk (Phys. Fluids 1970) and Sturova (Fluid Dyn. 1985). Waves are expressed in terms of the retarded time of their emission; the dispersion relationship being known, at each point and time the retarded time and the wavenumber satisfy a system of two equations stating propagation at the group velocity and stationarity of the wavecrests with respect to the source. Lee Waves, corresponding to uniform horizontal translation, are then considered, for two stratifications: a fluid of limited depth and constant buoyancy frequency, and a thermocline associated with a hyperbolic tangent density profile. For translation at velocity $U$ of a body of radius $a$ in a fluid of maximum buoyancy frequency $N_0$ where internal Waves are trapped in a layer of thickness $\epsilon$, two internal Froude numbers can be defined: $U/N_0a$, which measures the effect of the dimension of the source, and $U/N_0\epsilon$, which measures the intensity of trapping. Detailed study of their influence is performed. Mechanisms leading to vertical mode selection are discussed, and theoretical results are compared with experiments.
Miguel A. C. Teixeira - One of the best experts on this subject based on the ideXlab platform.
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Drag associated with 3D trapped Lee Waves over an axisymmetric obstacle in two-layer atmospheres
Quarterly Journal of the Royal Meteorological Society, 2017Co-Authors: Miguel A. C. Teixeira, Pedro M. A. MirandaAbstract:Mountain wave drag is evaluated explicitly using linear theory and verified against numerical simulations for the flow of idealized two-layer atmospheres with piecewise-constant stratification over an axisymmetric mountain. Static stability is either higher in the bottom layer and lower in the top layer (Scorer's atmosphere), or neutral in the bottom layer and positive in the top layer, separated by a sharp temperature inversion (Vosper's atmosphere). The drag receives contributions from long mountain Waves propagating vertically in the upper layer and from short trapped Lee Waves propagating downstream either in the lower layer, or at the inversion. This trapped Lee wave drag, which is typically not represented in parametrizations, acts on the atmosphere at low levels. As in flow over a 2D ridge, this drag has several maxima as a function of the height of the interface between the two layers for Scorer's atmosphere, and is maximized by a marked Scorer parameter contrast between those layers. In Vosper's atmosphere, there is a single trapped Lee wave drag maximum for Froude numbers near one, when the wind speed matches the phase speed of the dominant interfacial Waves, and this drag is maximized for relatively low interface elevations, for which Waves at the inversion have higher amplitude. The 3D flow geometry allows resonant wave modes to have various horizontal orientations and a continuous spectrum, forming a dispersive ‘Kelvin ship wave' pattern, and expanding the regions in parameter space where the drag is non-zero relative to 2D flow, but it also dispersively decreases the drag magnitude. Nevertheless, the trapped Lee wave drag on an axisymmetric obstacle can still equal or exceed the drag associated with vertically propagating Waves and the reference hydrostatic drag valid for a uniformly stratified atmosphere.
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impact of non hydrostatic effects and trapped Lee Waves on mountain wave drag in directionally sheared flow
Quarterly Journal of the Royal Meteorological Society, 2015Co-Authors: Miguel A. C. TeixeiraAbstract:The orographic gravity-wave drag produced in flow over an axisymmetric mountain when both vertical wind shear and non-hydrostatic effects are important was calculated using a semi-analytical two-layer linear model, including unidirectional or directional constant wind shear in a layer near the surface, above which the wind is constant. The drag behaviour is determined by partial wave reflection at the shear discontinuity, wave absorption at critical levels (both of which exist in hydrostatic flow) and total wave reflection at levels where the Waves become evanescent (an intrinsically non-hydrostatic effect), which produces resonant trapped Lee-wave modes. As a result of constructive or destructive wave interference, the drag oscillates with the thickness of the constant-shear layer and the Richardson number within it (Ri), generally decreasing at low Ri and when the flow is strongly non-hydrostatic. Critical-level absorption, which increases with the angle spanned by the wind velocity in the constant-shear layer, shields the surface from reflected Waves, keeping the drag closer to its hydrostatic limit. Although, for the parameter range considered here, the drag seldom exceeds this limit, a substantial drag fraction may be produced by trapped Lee Waves, particularly when the flow is strongly non-hydrostatic, the lower layer is thick and Ri is relatively high. In directionally sheared flows with Ri=O(1), the drag may be misaligned with the surface wind in a direction opposite to the shear, a behaviour that is due totally to non-trapped Waves. The trapped Lee-wave drag, the reaction force of which is felt on the atmosphere at low levels, may therefore have a distinctly different direction from the drag associated with vertically propagating Waves, which acts on the atmosphere at higher levels.
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Drag produced by trapped Lee Waves and propagating mountain Waves in a two-layer atmosphere
Quarterly Journal of the Royal Meteorological Society, 2012Co-Authors: Miguel A. C. Teixeira, J. L. Argain, Pedro M. A. MirandaAbstract:The surface drag force produced by trapped Lee Waves and upward propagating Waves in non-hydrostatic stratified flow over a mountain ridge is explicitly calculated using linear theory for a two-layer atmosphere with piecewise-constant static stability and wind speed profiles. The behaviour of the drag normalized by its hydrostatic single-layer reference value is investigated as a function of the ratio of the Scorer parameters in the two layers l2/l1 and of the corresponding dimensionless interface height l1H, for selected values of the dimensionless ridge width l1a and ratio of wind speeds in the two layers. When l2/l1 → 1, the propagating wave drag approaches 1 in approximately hydrostatic conditions, and the trapped Lee wave drag vanishes. As l2/l1 decreases, the propagating wave drag progressively displays an oscillatory behaviour with l1H, with maxima of increasing magnitude due to constructive interference of reflected Waves in the lower layer. The trapped Lee wave drag shows localized maxima associated with each resonant trapped Lee wave mode, occurring for small l2/l1 and slightly higher values of l1H than the propagating wave drag maxima. As l1a decreases, i.e. the flow becomes more non-hydrostatic, the propagating wave drag decreases and the regions of non-zero trapped Lee wave drag extend to higher l2/l1. These results are confirmed by numerical simulations for l2/l1 = 0.2. In parameter ranges of meteorological relevance, the trapped Lee wave drag may have a magnitude comparable to that of propagating wave drag, and be larger than the reference single-layer drag. This may have implications for drag parametrization in global climate and weather-prediction models. Copyright © 2012 Royal Meteorological Society
Raymond W. Schmitt - One of the best experts on this subject based on the ideXlab platform.
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Mapping turbulent diffusivity associated with oceanic internal Lee Waves offshore Costa Rica
Ocean Science, 2016Co-Authors: Will Fortin, W. Steven Holbrook, Raymond W. SchmittAbstract:Abstract. Breaking internal Waves play a primary role in maintaining the meridional overturning circulation. Oceanic Lee Waves are known to be a significant contributor to diapycnal mixing associated with internal wave dissipation, but direct measurement is difficult with standard oceanographic sampling methods due to the limited spatial extent of standing Lee Waves. Here, we present an analysis of oceanic internal Lee Waves observed offshore eastern Costa Rica using seismic imaging and estimate the turbulent diffusivity via a new seismic slope spectrum method that extracts diffusivities directly from seismic images, using tracked reflections only to scale diffusivity values. The result provides estimates of turbulent diffusivities throughout the water column at scales of a few hundred meters laterally and 10 m vertically. Synthetic tests demonstrate the method's ability to resolve turbulent structures and reproduce accurate diffusivities. A turbulence map of our seismic section in the western Caribbean shows elevated turbulent diffusivities near rough seafloor topography as well as in the mid-water column where observed Lee wave propagation terminates. Mid-water column hotspots of turbulent diffusivity show levels 5 times higher than surrounding waters and 50 times greater than typical open-ocean diffusivities. This site has steady currents that make it an exceptionally accessible laboratory for the study of Lee-wave generation, propagation, and decay.
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Mapping turbulent diffusivity associated with oceanic internal Lee Waves offshore Costa Rica
Ocean Science Discussions, 2015Co-Authors: W. F. J. Fortin, W. S. Holbrook, Raymond W. SchmittAbstract:Abstract. Breaking internal Waves play a primary role in maintaining the meridional overturning circulation. Oceanic Lee Waves are known to be a significant contributor to diapycnal mixing associated with internal wave dissipation, but direct measurement is difficult with standard oceanographic sampling methods due to the limited spatial extent of standing Lee Waves. Here, we present an analysis of oceanic internal Lee Waves observed offshore eastern Costa Rica using seismic imaging and estimate the turbulent diffusivity via a new seismic slope spectrum method that extracts diffusivities directly from seismic images, using tracked reflections only to scale diffusivity values. The result provides estimates of turbulent diffusivities throughout the water column at scales of a few hundred meters laterally and 10 m vertically. Synthetic tests demonstrate the method's ability to resolve turbulent structures and reproduce accurate diffusivities. A turbulence map of our seismic section in the western Caribbean shows elevated turbulent diffusivities near rough seafloor topography as well as in the mid-water column where observed Lee wave propagation terminates. Mid-water column hotspots of turbulent diffusivity show levels five times higher than surrounding waters and fifty times greater than typical open-ocean diffusivities. This site has steady currents that make it an exceptionally accessible laboratory for the study of Lee-wave generation, propagation and decay.