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1.
Summary An analysis of mesoscale gravity wave events during the severe weather outbreak in the Red River Valley on 10–11 April 1979 is presented utilizing surface pressure data and the 3 h rawinsonde data from the AVE-SESAMEI special network. The unique data set provided by the SESAME field experiment makes it possible to relate the wavelike characteristics observed at the surface to the variability of the temperature, humidity, and wind fields over a deep tropospheric layer that act to initiate and sustain the waves over long distances and time periods.Three different wave events (A, B, and C) were identified via spectral analysis and cross-correlation techniques. They all have similar periods, approximately 3 h, but different phase velocities. All three wave events are generated and propagate in the exit region or anticyclonic side of upperlevel jet streaks. Convection and wind shear are shown to be unlikely contributors to the generation of event A, which is probably related to the development of a strong divergent field in association with an upper-tropospheric jet streak and to the ensuing mass adjustment process. Events B and C also appear in a region of strong ageostrophic motion associated with an upper-level jet streak. However, the low values of the Richardson number (Ri) at the critical levels of these two waves suggest vertical wind shear as a likely contributor to their generation and/or maintenance. A linear stability analysis confirms, with unprecedent spatial and temporal resolution, that a modal structure is present in the atmosphere whose characteristics are consistent with those of waves B and C.Three-hourly rawinsonde data show strong temporal and spatial variability throughout the troposphere in the wind, temperature, and humidity fields when the waves are present. Convective systems, as detected by radar, are closely linked to the waves, although not in a consistent manner: cells intensify or develop at the passage of a wave trough in event A, at the passage of a wave ridge in event C, and at the passage of a wave trough or ridge in event B, depending on the geographic location of the cells. For all three events, maximum rainfall recorded at the surface is associated with a wave ridge with a time lag of approximately 1 h.With 20 Figures  相似文献   

2.
An FM/CW radar sounding system designed and built by one of us (Richter, 1969) reveals atmospheric wave structure in unparalleled detail.The most outstanding features evident in the record are; internal gravity waves; features resembling Kelvin/Helmholtz instability structures; and multiple layering, often with lamina only a few meters thick.This paper shows a variety of atmospheric structural patterns and compares them with several hypothetical models of internal waves to obtain more insight into the atmospheric processes at work. Special attention is given to the distribution of the Richardson number in trapped and untrapped gravity waves. It is proposed that the multiple layers result from untrapped internal gravity waves whose propagation vector is directed nearly vertically within very stable height regions. It is argued that the layers are caused by dynamic instability resulting from reduction in the Richardson number due to wave induced shear and to some background wind shear when the amplitude-to-wavelength ratio grows during propagation into thermally stable height regions of the atmosphere.  相似文献   

3.
与水平风切变强度不均匀相联系的CISK惯性重力波   总被引：3，自引：1，他引：3       下载免费PDF全文
在虚拟高度坐标系中, 用一个简单的线性模式初步研究了水平风切变强度不均匀分布对长江流域梅雨锋附近贯穿整个对流层的深厚惯性重力波发生发展的影响。结果表明:水平风切变强度不均匀对CISK惯性重力波不稳定有重要作用。在一般干的层结大气中, 实际可能出现再强的水平风切变的影响也难以使惯性重力波变得不稳定; 只有在积云对流潜热参与, 原为弱稳定条件下, 水平风切变强度不均匀能促使低空急流北侧不稳定扰动的发生发展。而水平风切变强度不均匀对不稳定贡献最大的区域是梅雨锋南侧的急流轴附近。  相似文献   

4.
The simultaneous operation of a three-axis Doppler sodar system in the centralurban area of Rome and two similar systems in the suburban area, forming atriangle about 20 km on each side, provided evidence of solitary-type wavesin the urban boundary layer. Three events, each lasting from a few minutes toabout 30 min, and ranging in depth from the minimum range of the sodar (39 m) to over 500 m, are reported here. Two events were recognizable onall three sodar records while the third event could be observed at the urbanlocation only. Time-height acoustic echo intensity records showed no-echoregions within the wave indicating transport of trapped recirculating air.This is typical of large amplitude solitary waves. The time series plots ofsodar-derived vertical wind velocity revealed a maximum peak-to-peakvariation of about 5 m s-1 during periods of wave-associated disturbance.The vertical velocity is found to increase with height up to the top of the closedcirculation within the wave and decreases further above. The normalisedamplitude-wavelength relationship for the two events indicates that theobserved waves are close to a strongly nonlinear regime.  相似文献   

5.

The nocturnal low-level jet (LLJ) and orographic (gravity) waves play an important role in the generation of turbulence and pollutant dispersion and can affect the energy production by wind turbines. Additionally, gravity waves have an influence on the local mixing and turbulence within the surface layer and the vertical flux of mass into the lower atmosphere. On 25 September 2017, during a field campaign, a persistent easterly LLJ and gravity waves were observed simultaneously in a coastal area in the north of France. We explore the variability of the wind speed, turbulent eddies, and turbulence kinetic energy in the time–frequency and space domain using an ultrasonic anemometer and a scanning wind lidar. The results reveal a significant enhancement of the turbulence-kinetic-energy dissipation (by?50%) due to gravity waves in the LLJ shear layer (below the jet core) during the period of wave propagation. Large magnitudes of zonal and vertical components of the shear stress (approximately 0.4 and 1.5 m2 s?2, respectively) are found during that period. Large eddies (scales of 110 to 280 m) matching the high-wind-speed regime are found to propagate the momentum downwards, which enhances the mass transport from the LLJ shear layer to the roughness layer. Furthermore, these large-scale eddies are associated with the crests while comparatively small-scale eddies are associated with the troughs of the gravity wave.

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6.
A stability analysis of the coupled ocean–atmosphere is presented which shows that the potential energy (PE) of the upper layer of the ocean is available to generate coupled growing planetary waves. An independent analysis suggests that the growth of these waves would be maintained in the presence of oceanic friction. The growing waves are a consequence of relaxing the rigid lid approximation on the ocean, thus allowing an upward transfer of energy across the sea surface. Using a two and a half layer model consisting of an atmospheric planetary boundary layer, coupled with a two layer ocean comprising an active upper layer and a lower layer in which the velocity perturbation is vanishingly small, it is shown that coupled unstable waves are generated, which extract PE from the main thermocline. The instability analysis is an extension of earlier work [Tellus 44A (1992) 67], which considered the coupled instability of an atmospheric planetary boundary layer coupled with an oceanic mixed layer, in which unstable waves were generated which extract PE from the seasonal thermocline. The unstable wave is an atmospheric divergent barotropic Rossby wave, which is steered by the zonal wind velocity, and has a wavelength of about 6000 km, and propagates eastward at the speed of the deep ocean current. It is argued that this instability, which has a multidecadal growth time constant, may be generated in the Southern Ocean, and that its properties are similar to observations of the Antarctic Circumpolar Wave (ACW).  相似文献   

7.
The generation mechanisms of convective gravity waves in the stratosphere are investigated in a three-dimensional framework by conducting numerical simulations of four ideal storms under different environmental conditions: one un-sheared and three constant low-level sheared basic-state winds with the depth of the shear layer of 6 km and the surface wind speeds (Us) of 8, 18, and 28 m s?1, using the Advanced Regional Prediction System (ARPS) model. The storms simulated under the un-sheared (Us = 0 m s?1), weakly sheared (Us = 8 and 18ms?1), and strongly sheared (Us = 28ms?1) basicstate winds are classified into single-cell, multicell, and supercell storms, respectively. For each storm, the wave perturbations in a control simulation, including nonlinearity and microphysical processes, are compared with those in quasi-linear dry simulations forced by diabatic forcing and nonlinear forcing that are obtained from the control simulation. The gravity waves generated by the two forcing terms in the quasi-linear dry simulations are out of phase with each other for all of the storms. The gravity waves in the control simulation are represented by a linear sum of the wave perturbations generated by the nonlinear forcing and diabatic forcing. This result is consistent with the results of previous studies in a two-dimensional framework. This implies that both forcing mechanisms are important for generating the convective gravity waves in the three-dimensional framework as well. The characteristics of the three-dimensional gravity waves in the stratosphere were determined by the spectral combination of the forcing terms and the wave-filtering and resonance factor that is determined from the basic-state wind and stability as well as the vertical structure of the forcing.  相似文献   

8.
Properties and Stability of a Meso-Scale Line-Form Disturbance   总被引：1，自引：0，他引：1  
By using the 3D dynamic equations for small- and meso-scale disturbances, an investigation is performed on the heterotropic instability (including symmetric instability and traversal-type instability) of a zonal line-like disturbance moving at any angle with respect to basic flow, arriving at the following results: (1) with linear shear available, the heterotropic instability of the disturbance will occur only when flow shearing happens in the direction of the line-like disturbance movement or in the direction perpendicular to the disturbance movement, with the heterotropic instability showing the instability of the internal inertial gravity wave; (2) in the presence of second-order non-linear shear, the disturbance of the heterotropic instability includes internal inertial gravity and vortex Rossby waves. For the zonal line-form disturbance under study, the vortex Rossby wave has its source in the second-order shear of meridional basic wind speed in the flow and propagates unidirectionally with respect to the meridional basic flow. As a mesoscale heterotropic instable disturbance, the vortex Rossby wave has its origin from the second shear of the flow in the direction perpendicular to the line-form disturbance and is independent of the condition in the direction parallel to the flow; (3) for general zonal line-like disturbances, if the second-order shear happens in the meridional wind speed, i.e., the second shear of the flow in the direction perpendicular to the line-form disturbance, then the heterotropic instability of the disturbance is likely to be the instability of a mixed Rossby–internal inertial gravity wave; (4) the symmetric instability is actually the instability of the internal inertial gravity wave. The second-order shear in the flow represents an instable factor for a symmetric-type disturbance; (5) the instability of a traversal-type disturbance is the instability of the internal inertial gravity wave when the basic flow is constant or only linearly sheared. With a second or nonlinear vertical shear of the basic flow taken into account, the instability of a traversal-type disturbance may be the instability of a mixed vortex Rossby – gravity wave.  相似文献   

9.
Wavelike motions within a strong morning inversion of the planetary boundary layer were investigated experimentally using two atmospheric research aircraft: an Aerocommander 280FL and a Cessna 206. The Aerocommander aircraft, instrumented for the measurement of rapid fluctuations of temperature, water vapour density and air velocities, was flown horizontally at different levels within the inversion layer in order to document adequate data on the wave motion. An example of such motions observed on 8 June, 1974 is described and analyzed in the present paper. The aircraft records obtained within the inversion layer at about 600 m above the ground show large fluctuations of the meteorological variables with well-defined amplitudes and wavelengths.Spectra and cross-spectra of temperature, water vapour density and air velocities were computed and analyzed to determine characteristics of gravity waves according to the method described by Metcalf (1975). These spectra exhibit a sharp maximum associated with high coherences at a particular wavelength. In this narrow spectral band, phase angles ±90 ° are obtained between vertical velocity and temperature as well as between vertical and horizontal velocities. These features suggest that observed motions are horizontally propagating trapped or evanescent waves. They enable us to estimate true wavelengths (500 m), wave vector azimuths, intrinsic frequencies and phase velocities of these waves. These results appear to be mutually consistent. Furthermore, it is possible to confirm these latter with the detailed vertical profiles of the boundary layer provided by the Cessna aircraft making spiral soundings. In this regard, the vertical structure of the Brunt-Väisälä frequency confirms that the waves are everywhere evanescent except within a thin highly stable layer between the diurnal mixed layer and the overlapping inversion. Moreover, examination of the wind profiles reveals that the interfacial vertical wind shear might be a relevant parameter reducing phase velocities. Such a conclusion is also supported by the observed wave vector directions which appear to be closely parallel to the wind shear vectors at the 600-m level. Additional confirmation is found by comparing the observed wavelengths to those predicted by applying the hydrodynamical stability model of Hazel (1972) to the measured profiles. Although the wind shear clearly plays a role in wave development, local heat flux and temperature variance values show that in this case the instability is only a marginal and sporadic event embedded in nearly neutral waves. Accordingly, it is argued that the observed motions are interfacial waves at the inversion base level, the amplitude and wave vector azimuth of which are controlled by the vertical wind shear.
Ondes de gravite interfaciales marginalement instables dans la couche limite planetaire
Résumé Des ondes de gravité interfaciales progressives sont étudiées à l'aide de deux avions dans la couche limite planétaire. Les spectres des vitesses et de la température fournissent les directions et vitesses de propagation. La comparaison des résultats aux profils verticaux du vent et de la température montre que ces ondes sont marginalement instables sous l'effet du cisaillement vertical du vent.

IOPG, 12, avenue des Landais - 63001 Clermont-Ferrand Cedex.

Complexe Scientifique des Cézeaux BP 45 - 63170 Aubiere.  相似文献   

10.
The instability of a symmetric jet moving horizontally, in which two shear layers with opposite shear of the same strength are separated by a central irrotational layer and are adjoined by unbounded, irrotational outer layers, is studied.First, the fluid is assumed to be homogeneous. Two unstable modes are found, the central wave one-quarter wave length out of phase with the outer wave. Mode I consists of central waves being in phase and outer waves being in phase. Mode II consists of central waves being in opposite phase and outer waves being in opposite phase. For a given width of the jet, the thicker the central irrotational layer, the stronger the shear of the shear layers, the stronger the instability. For a fixed ratio of the thickness of central layer to that of the shear layers, mode I is more unstable than mode II.Next, a density jump across the outer interface levels and another density jump across the central interface levels are introduced. The effect of these density jumps on mode I is to reduce the growth of the wave. The wave with equal density jump across every interface level grows somewhat slower than the waves with the entire density jump across outer or central interface levels. For an idealized velocity profile with isentropic layers with an overall Richardson number of 4.9, the linear theory predicts that the amplitude of the wave doubles in about 5 min and the wave-length is 241 m, which compares favorably with 320m obtained in the boundary layer by Gossard et al. (1970). For atmospheric parameters with an overall Richardson number of unity, linear theory predicts that the amplitude of the wave doubles in about % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaGOmamaale% aaleaacaaIXaaabaGaaG4maaaaaaa!383C!\[2{\textstyle{1 \over 3}}\] min and the wave-length is about 510 m, which is only slightly larger than the width of the jet.A physical argument is invoked to explain the evolution of finite-amplitude waves.  相似文献   

11.
A case study of Kelvin-Helmholtz waves which were observed by two aircraft in a warm off-shore stable boundary-layer flow over the North Sea is presented. During the one-hour flight mission within an area of 40 × 40 km2, the waves were intermittent both in space and time. They were centered around two levels, at 90 and 330m, where inflection points in the mean profile of the cross-wave wind component occurred together with Richardson numbers smaller than the critical value of 0.25. Observed wave amplitudes were on the order of 0.1 K for the potential temperature, 0.15ms-1 for the vertical wind component, 0.3ms-1 for the cross-wave wind component and 0.15ms-1 for the along-wave wind component. Horizontally averaged vertical wave transports were down-gradient.Based on the observed wind and temperature profiles, wave simulations with a linear model are performed. Different diffusion coefficient estimates are tested. The model produces two types of Kelvin-Helmholtz waves with maximum amplitudes at the above mentioned two heights. The modeled wavelengths are about 30% shorter than the observed ones. Adjusting the modeled to the observed temperature variations, the modeled vertical wind variance and the vertical transports agree well with the observations, whereas the modeled horizontal wind variances are smaller than the observed ones.  相似文献   

12.
The phase-averaged characteristics of the turbulent velocity fields beneath steep short wind waves are investigated. A scheme was developed to compute the phase of individual wind waves using spatial surface displacement data. This information was used to analyze the two-dimensional velocity data acquired using particle image velocimetry (PIV) in a wind-wave tank. The experiments were conducted at a fetch of 5.5m and at wind speeds that ranged from 4 to 10ms−1. Under these conditions previous studies have shown that a significant percentage of the waves are microscale breaking waves. An analysis of the phase-averaged results suggests under these conditions (short fetches and moderate wind speeds) a wind-driven water surface can be divided into three regions based on the intensity of the turbulence. These are the crests of microscale breaking waves, the crests of non-breaking waves and the troughs of all waves. The turbulence is most intense beneath the crests of microscale breaking waves. In the crest region of microscale breaking waves coherent structures were observed that were stronger and occurred more frequently than beneath the crests of non-breaking waves. Beneath the crests of non-breaking waves the turbulence is a factor of two to three times weaker and beneath the wave troughs it is a factor of six weaker. These findings provide additional support for the hypothesis that approximately two-thirds of the gas and heat fluxes occur across the turbulent wakes produced by microscale breaking waves.  相似文献   

13.
Remote sensing of the lower planetary boundary layer in the vicinity of a meteorological tower on many occasions reveals the existence of shear instability (Kelvin-Helmholtz) waves. In general, such waves are found within shallow strata which are marked by strong thermal stability and large vertical wind shear. The independent and concurrent measurements of the vector wind and temperature, made on a 152-m high tower, allow the construction of wind and temperature profiles. From such measurements, the Richardson number profile is constructed as well as the instability regime according to Drazin's criterion. The results show that regions of shear-instability waves as depicted by the remote sensor (an acoustic sounder) agree well with Drazin's instability regime, and that within such regions the Richardson number is indeed 0.25.  相似文献   

14.
Abstract

Analysis of current, temperature and salinity records in the nearshore region of the Scotian Shelf during the Canadian Atlantic Storms Program (CASP), reveals that the inertial wave field is highly intermittent, with comparable amplitudes in the surface and deep layers. Clockwise current energy in the surface layer is concentrated at a frequency slightly below inertial, consistent with Doppler shifting by the strong mean current and/or straining by the mean flow shear, whereas the spectral peak in deep water is at the local inertial frequency. Clockwise coherence is high (γ2 ≥ 0.8) horizontally over the scale of the array (60 km × 120 km) and in the vertical, with upward phase propagation rates of 0.15–0.50 × 10?12 ms?1, inversely proportional to the local value of the Brunt Väisälä frequency. Clockwise current energy decreases in the onshore direction and appears to be completely inhibited on the 60‐m isobath.

A case study of the response to the CASP IOP 14 storm indicates that the inertial waves may be generated by a strong wind shift propagating onshore at a speed of 10 ms?1. On the eastern side of the array (Liscomb line), clockwise current oscillations propagate onshore in the surface layer at a rate (8.1 ± 0.9 m s?1) comparable with the speed of the atmospheric front, while waves in the pycnocline move offshore at a lower (internal wave) speed (1.8 m s?1). Furthermore the temperature and salinity fluctuations are in (out) of phase with longshore current in the deep (surface) layer. However, on the western side of the array (Halifax line), the inertial waves are more complex. A sharp steepening of phase lines at the coast indicates that the phase speed of clockwise current oscillations is considerably reduced and the evidence for offshore propagation of internal waves is less clear. The discrepancies between observations on the two lines suggest that the internal wave field is three‐dimensional.

Results of simple mixed‐layer models indicate that the inertial response near the surface is sensitive to the accurate definition of the local wind field, but not to certain model physics, such as the form of the decay term. The observations also show some qualitative similarities with models for two‐dimensional response to a moving front (e.g. Kundu, 1986), but the actual forcing terms are more complicated, based on IOP 14 wind measurements.  相似文献   

15.
为了研究风场对背风波的影响,针对边界层附近为弱稳定层结的背风波,建立了一个三维三层的理论模型和线性计算模式,分析了各层中风速和风向的变化对背风波特征的影响,揭示了气流过孤立山脉产生背风波的有利风场条件。结果表明：背风波的波长、振幅等特征对各层风速和风向的变化具有相当的敏感性,波长随着低、高层风速的增大而增大,随着中层风速的增大先减小后增大;振幅随着低、中层风速的增大先增大后减小,随着高层风速的增大而增大。此外,风速和上下层风向切变的增大均使背风波的形态逐渐由横波型转为辐散型,但是上下层风向的切变对背风波形态的影响比风速更为显著。  相似文献   

16.
The momentum flux of stratospheric gravity waves generated by Typhoon Ewiniar (2006) is examined using a Weather Research and Forecasting (WRF) model. In the stratosphere, zonal momentum flux with a positive sign by eastward-propagating waves is significant during the northward moving of the typhoon, while both zonal and meridional momentum fluxes with positive signs are significant during the typhoon decaying stage in which the typhoon moves northeastward. The magnitude of the momentum flux is greater during the mature stage of the typhoon than the decaying stage, and the phase speeds of the dominant momentum flux are less than 30 m s?1 with a peak at 10–16 m s?1. Positive momentum flux decreases with height overall in the stratosphere for both zonal and meridional components. The resultant gravity-wave drag forcing plays a role to decelerate the easterly background wind in the stratosphere. This drag forcing is relatively large above z = 40 km and below z = 20 km, and lower stratospheric wave drag is expected to affect the typhoon dynamics by modifying the background wind shear and inducing the secondary circulation in the troposphere.  相似文献   

17.
黄平  黄荣辉 《大气科学》2009,33(1):1-15
利用合成分析方法, 分析了1971～2003年间7个El Niño事件发生时南、北半球大气环流异常的对称与反对称特性。分析结果表明, El Niño事件发生期间, 在热带地区15°S～15°N大气环流异常以对称性为主, 但也有一定的反对称分量: 高度场异常和纬向风异常的对称性较强, 而经向风异常的反对称较强; 同时在El Niño事件演变的各阶段, 高度场异常和纬向风异常的变化较明显, 而经向风的变化较小, 高度场和风场异常在低层 (700 hPa) 和高层 (200 hPa) 呈明显的反位相分布。并且, 为探讨El Niño事件对南、北半球大气环流异常的非对称性影响的机理, 本研究进一步利用ERA－40逐日资料, 应用Hough函数分析了El Niño事件发生期间这些赤道波动的演变特征。结果表明, El Niño事件发生期间, 热带地区大气环流异常对称性较强的主要原因是对称性Rossby波异常较强, 而Rossby重力混合波异常对经向风场的反对称分量有重要作用; 并且, 研究结果还表明, 在El Niño事件发生期间, 热带东太平洋的海温正异常能够激发由对称性Rossby波和Kelvin波组成的Gill型环流异常。此外, 分析表明Hough函数在分析热带大气波动的对称性及反对称性上具有明显的优越性。  相似文献   

18.
Miles' inviscid theory of surface wave generation by wind is (a) modified by replacing the logarithmic shear velocity profile with one which applies right down to the wave surface and which exhibits an explicit dependence on the roughness of the surface, and (b) extended to include the effects of the interaction of wave with air flow turbulence by considering the wave-modified mean flow as the mean of the actual turbulent air flow over water waves and using this in a mixing-length model.The surface pressure is shown to depend significantly on the flow conditions being aerodynamically smooth or rough. Its component in phase with the surface elevation is practically unaffected by the wave-turbulence interaction. However, such interaction tends to increase the rate of energy input ß from wind to waves travelling in the same direction, e.g., the increase is 2gk 2 for aerodynamically rough flow, where gk is the Von Karman constant. It also provides damping of waves in an adverse wind which can be about 10% of the growth rate in a favourable wind.  相似文献   

19.
Mean wind speed profiles were measured by tracking radiosondes in the unstable atmospheric boundary layer (ABL) over the forested Landes region in southwestern France. New Monin-Obukhov stability correction functions, recently proposed following an, analysis by Kader and Yaglom, as well as the Businger-Dyer stability formulation were tested, with wind speeds in the surface sublayer to calculate the regional shear stress. These profile-derived shear stresses were compared with eddy correlation measurements gathered above a mature forest stand, at a location roughly, 4.5 km from the radiosonde launch site. The shear stress values obtained by means of the newly proposed stability function were in slightly better agreement with the eddy correlation values than those obtained by means of a Businger-Dyer type stability function. The general robustness of the profile method can be attributed in part to prior knowledge of the regional surface roughness (z 0=1.2 m) and the momentum displacement height (d 0=6.0 m), which were determined from neutral wind profile analysis. The 100 m drag coefficient for the unstable conditions above this broken forest surface was found to beu * 2 /V 100 2 =0.0173.  相似文献   

20.
The influence of an accelerating shear flow on the propagation of an internal gravity wave in a continuously stratified fluid is studied by means of two-dimensional numerical simulations. These are motivated by earlier laboratory experiments [Thorpe, S.A. 1978b. On internal gravity waves in an accelerating shear flow, Vol. 88. J. Fluid Mech. pp. 623–639]. In these experiments the mean flow is an accelerated Couette flow and the mean density profile is linear. The laboratory experiments revealed the striking effect of the unsteady shear flow in the evolution of an internal gravity wave leading to the wave focusing in a region where the flow is extremum. This phenomenon is associated with the growth of small scale density fluctuations. As a result density overturns are sometimes observed. This behaviour is well reproduced by the numerical simulations. We provide insights on the flow dynamics in particular on the possible occurrence of wavebreaking. We show that the dynamics is characterized by two competitive mechanisms that is a damping of the wave and a local enhancement of its steepness leading sometimes to density overturns. The budget for the energy of the wave reveals that the initial damping of the wave results from wave-mean flow interactions. These interactions lead to the development of a fine scale vertical density structure which is associated with high vertical shear. We find that in some cases wavebreaking occurs as a result of shear instability. The value of the acceleration of the mean flow is very likely to influence the onset of the instability. The scaling laws of the wave evolution, in particular the rate of decrease of its energy, are determined. From these laws the lifetime of the wave is found as a function of the acceleration of the shear. It may be expected that, in the ocean, this development will result in the largest fluctuations derived from wave-flow interactions occurring where the mean flow in the wave direction is greatest. Waves travelling normal to a two-dimensional shear flow will be unchanged. Waves travelling parallel will be damped. This may have particular application at the continental shelf where flow, mainly parallel to the isobaths, will damp waves travelling along-slope, but allows waves travelling normal to the isobaths (e.g., directly across the shelf-break) to be transmitted without attenuation. Similar effects are expected for the evolution of a high frequency wave interacting with a lower frequency (e.g., near inertial) motion.  相似文献   

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