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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, 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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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.
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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:Abstract 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 s−1. This quite intense mixing reaches do...
Vanda Grubisic - One of the best experts on this subject based on the ideXlab platform.
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Wave induced boundary layer separation in the Lee of the medicine bow mountains part ii numerical modeling
Journal of the Atmospheric Sciences, 2015Co-Authors: Vanda Grubisic, Stefano Serafin, Lukas Strauss, Samuel Haimov, Jeffrey R French, Larry D OolmanAbstract:AbstractMountain 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 differe...
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Wave induced boundary layer separation in the Lee of the medicine bow mountains part i observations
Journal of the Atmospheric Sciences, 2015Co-Authors: Jeffrey R French, Vanda Grubisic, Stefano Serafin, Samuel Haimov, Larry D Oolman, Lukas StraussAbstract:AbstractTwo cases of mountain Waves, rotors, and the associated turbulence in the Lee of the Medicine Bow Mountains in southeastern Wyoming are investigated in a two-part study using aircraft observations and numerical simulations. In Part I, observations from in situ instruments and high-resolution cloud radar on board the University of Wyoming King Air aircraft are presented and analyzed. Measurements from the radar compose the first direct observations of Wave-induced boundary layer separation.The data from these two events show some striking similarities but also significant differences. In both cases, rotors were observed; yet one looks like a classical Lee-Wave rotor, while the other resembles an atmospheric hydraulic jump with midtropospheric gravity Wave breaking aloft. High-resolution (30 × 30 m2) dual-Doppler syntheses of the two-dimensional velocity fields in the vertical plane beneath the aircraft reveal the boundary layer separation, the scale and structure of the attendant rotors, and downsl...
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lower tropospheric Waves and Wave induced turbulence zones insights from t rex
13th Conference on Mountain Meteorology 17th Conference on Applied Climatology (11–15 August 2008), 2008Co-Authors: Vanda GrubisicAbstract:During the Terrain-induced Rotor Experiment (T-REX) in March/April 2006 highly turbulent flows in the Lee of the Sierra Nevada were probed by the University of Wyoming King Air (UWKA) aircraft. In situ thermodynamic and kinematic data was obtained by UWKA on rotor and Wave structures over Owens Valley in a number of research missions under strong Lee-Wave conditions. In situ measurements by the UWKA have been used to examine strongly turbulent flow regions documented by the UWKA and their relationship to the flow structures over Owens Valley. Wave-induced pressure perturbations determined from the aircraft measurements are compared with the surface pressure perturbations derived from the network of surface pressure sensors in Owens Valley in order to detect Wave-induced boundary-layer separation.
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the intense Lee Wave rotor event of sierra rotors iop 8
Journal of the Atmospheric Sciences, 2007Co-Authors: Vanda Grubisic, Brian J BillingsAbstract: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 l...
Masao Fukasawa - One of the best experts on this subject based on the ideXlab platform.
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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.
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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:Abstract 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 s−1. This quite intense mixing reaches do...
Miguel A. C. Teixeira - One of the best experts on this subject based on the ideXlab platform.
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correction teixeira m a c diagnosing Lee Wave rotor onset using a linear model including a boundary layer atmosphere 2017 8 5
Algorithmic Approaches for Transportation Modeling Optimization and Systems, 2018Co-Authors: Miguel A. C. TeixeiraAbstract:The author would like to correct a published article by Teixeira [1], in which there is a factor of 2 missing from his Equation (24).[...]
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diagnosing Lee Wave rotor onset using a linear model including a boundary layer
Atmosphere, 2017Co-Authors: Miguel A. C. TeixeiraAbstract:A linear model is used to diagnose the onset of rotors in flow over 2D hills, for atmospheres that are neutrally stratified near the surface and stably stratified aloft, with a sharp temperature inversion in between, where trapped Lee Waves may propagate. This is achieved by coupling an inviscid two-layer mountain-Wave model and a bulk boundary-layer model. The full model shows some ability to diagnose flow stagnation associated with rotors as a function of key input parameters, such as the Froude number and the height of the inversion, in numerical simulations and laboratory experiments carried out by previous authors. While calculations including only the effects of mean flow attenuation and velocity perturbation amplification within the surface layer represent flow stagnation fairly well in the more non-hydrostatic cases, only the full model, taking into account the feedback of the surface layer on the inviscid flow, satisfactorily predicts flow stagnation in the most hydrostatic case, although the corresponding condition is unable to discriminate between rotors and hydraulic jumps. Versions of the model not including this feedback severely underestimate the amplitude of trapped Lee Waves in that case, where the Fourier transform of the hill has zeros, showing that those Waves are not forced directly by the orography.
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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.
Lukas Strauss - One of the best experts on this subject based on the ideXlab platform.
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Wave induced boundary layer separation in the Lee of the medicine bow mountains part ii numerical modeling
Journal of the Atmospheric Sciences, 2015Co-Authors: Vanda Grubisic, Stefano Serafin, Lukas Strauss, Samuel Haimov, Jeffrey R French, Larry D OolmanAbstract:AbstractMountain 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 differe...
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Wave induced boundary layer separation in the Lee of the medicine bow mountains part i observations
Journal of the Atmospheric Sciences, 2015Co-Authors: Jeffrey R French, Vanda Grubisic, Stefano Serafin, Samuel Haimov, Larry D Oolman, Lukas StraussAbstract:AbstractTwo cases of mountain Waves, rotors, and the associated turbulence in the Lee of the Medicine Bow Mountains in southeastern Wyoming are investigated in a two-part study using aircraft observations and numerical simulations. In Part I, observations from in situ instruments and high-resolution cloud radar on board the University of Wyoming King Air aircraft are presented and analyzed. Measurements from the radar compose the first direct observations of Wave-induced boundary layer separation.The data from these two events show some striking similarities but also significant differences. In both cases, rotors were observed; yet one looks like a classical Lee-Wave rotor, while the other resembles an atmospheric hydraulic jump with midtropospheric gravity Wave breaking aloft. High-resolution (30 × 30 m2) dual-Doppler syntheses of the two-dimensional velocity fields in the vertical plane beneath the aircraft reveal the boundary layer separation, the scale and structure of the attendant rotors, and downsl...