Showing papers on "Internal wave published in 1981"

Turbulence and stress owing to gravity wave and tidal breakdown

Abstract: It has been suggested (Lindzen, 1967, 1968a, b; Lindzen and Blake, 1971; Hodges, 1969) that turbulence in the upper mesosphere arises from the unstable breakdown of tides and gravity waves. Crudely speaking, it was expected that sufficient turbulence would be generated to prevent the growth of wave amplitude with height (roughly as (basic pressure)−1/2). This work has been extended to allow for the generation of turbulence by smaller amplitude waves, the effects of mean winds on the waves, and the effects of the waves on the mean momentum budget. The effects of mean winds, while of relatively small importance for tides, are crucial for internal gravity waves originating in the troposphere. Winds in the troposphere and stratosphere sharply limit the phase speeds of waves capable of reaching the upper mesosphere. In addition, the existence of critical levels in the mesosphere significantly limits the ability of gravity waves to generate turbulence, while the breakdown of gravity waves contributes to the development of critical levels. The results of the present study suggest that at middle latitudes in winter, eddy coefficients may peak at relatively low altitudes (50 km) and at higher altitudes in summer and during sudden warmings (70–80 km), and decrease with height rather sharply above these levels. Rocket observations are used to estimate momentum deposition by gravity waves. Accelerations of about 100 m/s/day are suggested. Such accelerations are entirely capable of producing the warm winter and cold summer mesopauses.

Array measurements of atmospheric pressure fluctuations above surface gravity waves

Abstract: A joint experiment to study microscale fluctuations of atmospheric pressure above surface gravity waves was conducted in the Bight of Abaco, Bahamas, during November and December 1974. Field hardware included a three-dimensional array of six wave sensors and seven air-pressure sensors, one of which was mounted on a wave follower. The primary objectives of the study were to resolve differences in previous field measurements by Dobson (1971), Elliott (1972b) and Snyder (1974), and to estimate the vertical profile of wave-induced pressure and the corresponding input of energy and momentum to the wave field.Analysis of a pre-experiment intercalibration of instruments and of 30 h of field data partially removes the discrepancy between the previous measurements of the wave-induced component of the pressure and gives a consistent picture of the profile of this pressure over a limited range of dimensionless height and wind speed. Over this range the pressure decays approximately exponentially without change of phase; the decay is slightly less steep than predicted by potential theory. The corresponding momentum transfer is positive for wind speeds exceeding the phase speed. Extrapolation of present results to higher frequencies suggests that the total transfer is a significant fraction of the wind stress (0·1 to 1·0, depending on dimensionless fetch).Analysis of the turbulent component of the atmospheric pressure shows that the ‘intrinsic’ downwind coherence scale is typically an order-of-magnitude greater than the crosswind scale, consistent with a ‘frozen’ turbulence hypothesis. These and earlier data of Priestley (1965) and Elliott (1972c) suggest a horizontally isotropic ‘intrinsic’ turbulent pressure spectrum which decays as k−ν where k is the (horizontal) wave-number and ν is typically −2 to −3; estimates of this spectrum are computed for the present data. The implications of these findings for Phillips’ (1957) theory of wave growth are examined.

The dynamic balance of internal waves

Abstract: For oceanic internal waves with vertical scales larger than 1 m the evolution of the spectrum is adequately described by weak-interaction theory. Based on simple physical arguments, a model for internal-wave energy dissipation predicts dissipation as weak ever the some scales, for reasonable values of the total dissipation. Assuming dissipation at small scales, such as in our proposed model, and generation at large scales, a consistent dynamic balance with a constant downscale energy flux under nonlocal nonlinear interactions reproduces the observed spectral dependencies. A small-scale break point at which the total shear reaches a given value, and beyond which dissipation is important, is determined by the level and bandwidth of the energy-containing waves.

Observations and models of inertial waves in the deep ocean

Abstract: A study of the structure of the inertial peak in deep ocean kinetic energy is presented, based on records taken from Polymode arrays deployed in the western North Atlantic Ocean. Results are interpreted in terms of both local sources and turning point effects on internal waves generated at lower latitudes, and it is found that three classes of environment and their corresponding spectra emerge from peak height variations: (1) the 1500-m level near the Mid-Atlantic Ridge, with the greatest peak height of 18 dB; (2) the upper and deep ocean over rough topography and the deep ocean underneath the Gulf Stream, with the intermediate peak height of 11.5 dB; and (3) the deep ocean over smooth topography, with the lowest peak height of 7.5 dB. Using the globally valid wave functions obtained by Munk and Phillips (1968), frequency spectra near f are calculated numerically. The model is latitudinally dependent, with the frequency shift and bandwidth of the inertial peak decreasing with latitude.

Gravity waves on water with non-uniform depth and current

Abstract: A mathematical model for the combined refraction-diffraction of linear periodic gravity waves on water is developed, in which the influence of inhomogeneities of depth and current is taken into account. The model is used to compute partial reflection of waves a gully or an undersea slope, with influence of a current. The model is also applied to prismatic wave channels with reflecting side-walls. For a gully bounded by shallows the model predicts the decay of wave height due to radiation of energy in lateral direction. For practical application in regions with arbitrary bottom and current topography a parabolic approximation of the model is derived. This is used as a basis for numerical calculation of waves in a sea region near the coast.

Lidar observation of gravity and tidal waves in the stratosphere and mesosphere

Abstract: Lidar measurements of atmospheric density and temperature in the altitude range 30-to 80 km have been performed during the last 2 years from the Observatory of Haute-Provence (latitude 44°N, longitude, 6°E). The potential of this technique for studying the middle atmospheric structure is presented and preliminary results on wave propagation are discussed. It is shown that wave-like structures are observed systematically in this height range. Fourier analysis indicates that most of the energy is transported by waves of vertical wavelengths on the order of 8 to 15 km. The amplitude of the density variations is shown to follow a ρ−l/2 law up to 70 km. The characteristics of the observed density waves suggest that they are caused by a superposition of internal gravity waves propagating upward from the troposphere and a diurnal tide component in the range 30–50 km. Such waves are able to induce quite significant perturbations in atmospheric density and therefore temperature on an hourly basis. The Lidar technique is able to monitor those variations for the first time from a ground station operating continuously.

Inertial Oscillations on the Continental Shelf of the Gulf of Lions—Observations and Theory

Abstract: Observations in the Gulf of Lions (northwestern Mediterranean Sea) in summer have shown that gusts of wind lasting a few days generate transient upwellings and inertial motions. Oscillations at the inertial frequency were observed in current meter data near the shore and at a frequency 10% greater in the temperature data. Vertical coherences in current meter data show a strong baroclinic mode at frequencies greater than inertial frequency. A simple one-dimensional two-layer transient model suggests that these motions are associated with two different physical processes. The first process describes the local response of the ocean to meteorological forcing, the second is associated with the propagation of long internal waves generated in the transient phase of the geostrophic adjustment process. As suggested by the model, the direction of propagation of the internal waves is computed from current and temperature data measured at one point and it is found that the shore is the source zone.

Dynamical cooling induced by dissipating internal gravity waves

Abstract: In this analysis we show that vertically propagating internal gravity waves induce a downward transfer of sensible heat from regions of wave dissipation, and that this transfer of heat may result in a net cooling of regions of the upper atmosphere.

Slope turbulence, internal waves and phytoplankton growth at the Celtic Sea shelf-break

Abstract: The edge of the Celtic Sea shelf is characterized during the summer by a band of cold water ( ca . 100 km broad), which is generally conspicuous in high resolution infrared images from satellites, particularly under high pressure atmospheric conditions with clear skies. Preliminary studies of mixing in this region were made in 1972, 1973 and 1974 and were followed by more detailed interdisciplinary studies in 1976, 1979 and 1980 relating phytoplankton growth to the ways in which turbulence in the environment controls the availability of nutrients and light energy. The results have shown the cooler water to be about 1-2 °C colder than the adjacent surface waters of the Celtic Sea and Atlantic Ocean. This cold band also exhibits higher than background surface values of inorganic nitrate and chlorophyll a . Although these values are generally low compared with the values that have been observed near the neighbouring shelf tidal fronts, the increased surface values along the shelf break in summer appear to be significant. The observed increases of chlorophyll a are thought to be related to physical processes associated with the slopes, ridges and canyons where enhanced mixing, particularly due to internal waves or upwelling, results in nutrient renewal and subsequent phytoplankton growth along the shelf-break region of the Celtic Sea.

Variability in the upper ocean during MILE. Part I: The heat and momentum balances

Abstract: The Mixed Layer Experiment (MILE) was an examination of the upper ocean carried out near Ocean Weather Station P during a 20-day period in the autumn of 1977 characterized by two wind events. In this paper the variability of temperature and velocity observed at two moorings are described and related to the surface heat flux and wind forcing. A one-dimensional upper layer heat budget is found to close acceptably and it is shown that this success depends on having well-sampled temperature records and a method of accounting for vertical velocities in the seasonal thermocline. A similar budget of momentum is also reasonably accurate, but it is necessary to account for the effects of quasi-geostrophic velocities in the upper layer inferentially. Internal waves are found to be a major source of variability, dominating velocity records even at 5-m depth, well within the mixed layer. In the seasonal thermocline there is a high-frequency cut-off of internal wave energy well below the local buoyancy frequency. High-frequency internal wave ‘events’ are identified in the seasonal thermocline. Shear in the upper layer, and its response to wind, have some features in accord with models taking slab flow in the mixed layer, but the agreement is far from complete. Observations in the thermocline indicate that velocity differences in the vertical are maintained only if there is sufficient density stratification to make the Richardson number exceed a critical value.

Time Scales of Resonant Interactions Among Oceanic Internal Waves

Abstract: Transfer rates and times of nonlinear resonant interactions within the oceanic internal wave field are evaluated analytically for the mechanisms which dominate the transfers within the Garrett-Munk spectral models (the elastic scattering, the induced diffusion, and the parametric subharmonic-instability mechanism). The analytic transfer rates assume that the interacting wave components are widely separated in wavenumber and/or frequency, and they are shown to agree well with the exact numerically calculated transfer rates. The analytic expressions are used to discuss conveniently and explicitly possible equilibrium states and the extent to which high-wavenumber internal waves can be treated in the weak-interaction limit. The Garrett-Munk spectral models are in equilibrium with respect to the elastic scattering, close to equilibrium with respect to the induced diffusion, and not in equilibrium with respect to the parametric subharmonic-instability mechanism. For an overall dissipation time scale o...

Internal gravity waves in the solar atmosphere. I - Adiabatic waves in the chromosphere

Abstract: The properties of adiabatic and linear internal gravity waves propagating in a solar wind model are discussed, using nonlinearity criteria unique to gravity waves to estimate wave-breaking heights. The results are used to deduce information on the possible role of gravity waves in the chromospheric energy balance. Maximum vertical velocity amplitudes for gravity waves are estimated to be on the order of 2 km/sec or less, and maximum horizontal velocity amplitudes are less than 6 km/sec, with temperature perturbations as large as 1000-2000 K. It is also estimated that gravity waves with an incident energy flux of one million ergs/sq cm-sec can propagate upward to a maximum height of 900-1000 km above the visible surface before nonlinearities lead to wave breaking, while those with an energy flux of 100,000 ergs/sq cm-sec can reach maximum heights of 1400-1600 km.

The Propagation of Internal Waves in a Geostrophic Current

Abstract: The paper presents the WKB theory of internal wave propagation in a large-scale geostrophic mean flow with vertical as well as horizontal shear. As an application a mean flow with isopycnals having constant slope but arbitrary spacing is considered and the behavior of waves at turning points and critical layers is discussed. In particular, it is shown that horizontal variations of the mean flow shift the critical layer to the interior of the wave guide, i.e., away from ω02 = f2, where ω0 is the intrinsic frequency, and produces a valve effect at the critical layer which can be penetrated by a wave incident from one side while incidence from the other side results in absorption.

The instability of atmospheric gravity waves through wave‐wave interactions

Abstract: We investigate the possibility of a propagating primary wave decaying into secondary waves through three-wave interactions. Such a possibility exists in an inviscid isothermal atmosphere when the vertical wavelength is small compared to the scale height. Initially, the amplitude of the secondary waves grows exponentially, and this growth time decreases exponentially with height. This means that as the primary wave propagates upward its energy is continuously transfered to the secondary waves. This transfer becomes increasingly rapid with increasing height. It is shown that this process can take place even when the local Richardson number is more than 100, indicating dynamic stability.

A preliminary investigation of the interaction of internal gravity waves with a steady shearing motion

Abstract: Preliminary experimental results are presented which describe the interaction of an internal-wave field with a steady shearing motion. The results are primarily qualitative and presented in the form of photographs of shadowgraph images. Several internal-wave sources are used, and both critical- and non-critical-layer flows are examined. The results of these observations are interpreted in terms of several existing theories. For critical-layer flows the primary result is that virtually none of the internal-wave momentum flux penetrates the critical-level region, and under certain conditions a critical-layer instability develops resulting in the generation of turbulence. Such wave-induced turbulence is also observed for certain non-critical-layer flows and is believed to be the result of a convective instability.

Wave Transience in a Compressible Atmosphere. Part I: Transient Internal Wave, Mean-Flow Interaction

Abstract: Vertically propagating internal waves give rise to mean flow accelerations in an atmosphere due to the effects of wave transience resulting from compressibility and vertical group velocity feedback. Such accelerations appear to culminate in the spontaneous formation and descent of regions of strong mean wind shear. Both analytical and numerical solutions are obtained in an approximate quasi-linear model which describes this effect. The numerical solutions display mean flow accelerations due to Kelvin waves in the equatorial stratosphere. Wave absorption alters the transience mechanism in some significant respects, particularly in causing the upper atmospheric mean flow acceleration to be very sensitive to the precise magnitude and distribution of the damping mechanisms. Part II of this series discusses numerical simulations of transient equatorial waves in the quasi-biennial oscillation. These results are of sufficient qualitative interest to merit attention in this paper, and this is done with t...

Internal waves in fluids with rapidly varying density

Abstract: © Scuola Normale Superiore, Pisa, 1981, tous droits réservés. L’accès aux archives de la revue « Annali della Scuola Normale Superiore di Pisa, Classe di Scienze » (http://www.sns.it/it/edizioni/riviste/annaliscienze/) implique l’accord avec les conditions générales d’utilisation (http://www.numdam.org/conditions). Toute utilisation commerciale ou impression systématique est constitutive d’une infraction pénale. Toute copie ou impression de ce fichier doit contenir la présente mention de copyright.

Disqualifying Two Candidates for the Energy Balance of Oceanic Internal Waves

Abstract: It is shown that spreading of tidal energy across the wave continuum and bottom scattering play no role in the spectral energetics of ocean internal waves.

Near-Inertial Internal Waves Observed on the Outer Shelf in the Middle Atlantic Bight in the Wake of Hurricane Belle

Abstract: On 10 August 1976 Hurricane Belle passed rapidly over the highly stratified shelf of the New York Bight. Records from Aanderaa current-meter moorings show that the response to the hurricane depended strongly on bathymetry. At deeper stations (∼70 m depth), intense, first-mode, internal near-inertial oscillations were generated at frequencies ∼1% less than the local inertial frequency. At shallower stations (∼50 m depth), only weak, heavily damped second-mode oscillations were observed in the current records, with no corresponding inertial signals in temperature. In the Hudson Shelf Valley, inertial motion occurred only near the surface. This was probably due to topographic effects. The divergence and curl of the wind stress contributed equally to the forcing. The response at the deeper stations is consistent with Geisler's (1970) theory for the open ocean in which a hurricane leaves a wake of internal-inertial oscillations if it travels faster than the internal phase speed and if its horizontal s...

The structure of short gravity waves on the ocean surface

Abstract: A brief review is given of the salient properties of short gravity waves and the way in which their structure is modified by longer waves or swell, by variable currents, and by internal waves. It is noted that an underlying swell produces a mottled pattern in synthetic aperture radar (SAR) imagery, and an expression is derived giving the fractional modulation in backscattering cross section of the ocean surface in terms of the slope of an underlying swell, the wind direction, and the direction of swell propagation relative to the angle of observation. The expression provides the possibility in appropriate circumstances of estimating the swell slope by remote sensing, in addition to the wavelength and direction of propagation, which can be measured directly from the imagery. An account is also given of the refraction of waves in variable currents and internal waves.

Observations of the Horizontal Interactions between the Internal Wave Field and the Mesoscale Flow

Abstract: Momentum and energy transfers from the mesoscale horizontal velocity shear to the internal wave field have been deduced from an analysis of a closely spaced, 25 km, moored current-meter array. The correlation between the low-frequency horizontal shear and internal-wave-field continuum effective stress implies a significant horizontal eddy viscosity of O (106 cm2 s−1), somewhat larger than predicted by Muller (1976). A simple steady-state energy balance for the internal wave field using the observed correlation between the internal wave kinetic energy and the square of the low-frequency shear implies a 10-day relaxation time for the internal-wave Acid and a combined vertical viscosity and horizontal diffusivity not significantly different from zero. These estimates are within the experimental uncertainty of previous observational analyses.

Satellite observations of internal waves on the Australian north-west shelf

Abstract: This paper describes the results of a search of the existing Landsat (1 and 2) imagery catalogue for tidally generated internal waves over the continental shelves of Australia and New Zealand. The only area where such waves were observed was the Australian north-west shelf where they were found in abundance from the North West Cape to the latitude of Darwin. Wavelengths were in the range 300-1000 m with typical speeds of 0.5-1 m s-1, and spacing between wave packets of 25 km.

On the propagation of a three-dimensional packet of weakly non-linear internal gravity waves

Abstract: The evolution of a three-dimensional packet of weakly non-linear internal gravity waves propagating obliquely at an arbitrary angle to the vertical line is considered Two coupled non-linear equations connecting variations of a packet amplitude and induced flows are derived three-dimensionality of the packet having been found responsible for the non-linearity of the system Explicit formulae for the induced flow vertical component and the mean density field variation caused by packet propagation have been obtained The plane wave is shown to be unstable at any arbitrary slope of the wave vector The non-linear equation describing the evolution of the two-dimensional packet is derived in the subsequent order of the asymptotic scheme It has been found possible for the packet to collapse The collapse of internal waves packets may be one of the possible mechanisms of “blini”-shaped regions of mixed waters formation in the ocean