ISSN:0094-243X    

DOI:10.1063/1.33187

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A variety of topics are presented almost all related to work we have done over the past several years on the generation, propagation and interaction of non‐linear, dispersive waves. Although, ultimately, the main interest is in internal waves in the ocean, we also include studies of solitary Rossby waves in planetary atmospheres and free surface waves on shallow water.

ISSN:0094-243X    

DOI:10.1063/1.33182

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

This overview will attempt to touch upon a variety of theoretical approaches to the dynamics of the oceanic internal wave field. Further, I will note observations down to scales of molecular dissipation which provide a broader view for overall processes. Theoretical methods to consider will include: (1) wave‐wave interactions, (2) wave‐mean flow interactions (3) turbulent cascades, (4) scattering from finestructure And (5) ’’breaking’’ and saturation. Two suggestions for further work are indicated: wave interaction equations should be derived non‐perturbatively from the Eulerian field equations. For completeness this requires including zero frequency geostrophic modes at all scales. Spectral transport equations should then systematically include transfer due to off‐resonant as well as on‐resonant triads. In addition to theoretical approaches, numerical experiments both in two and three dimensions will be most valuable.

ISSN:0094-243X    

DOI:10.1063/1.33197

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

We report on the results of direct numerical simulations of the decay of homogeneous turbulence in density‐stratified fluids. The effects of stratification are examined by performing simulations for a range of different initial Froude numbers. Also two theoretical approaches are presented, which are applicable in the later stages of decay. For the conditions computed, although the stritification introduced definite wave‐like characteristics into the flow‐fields, the non‐linearity, as expressed for example by the spectral energy transfer, remained strong. However, the selective suppression of certain types of transfer was observed to occur.

ISSN:0094-243X    

DOI:10.1063/1.33198

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A review of current understanding of energy redistribution processes within the ocean internal wave field will be given. Relaxation rates for ’’test waves’’ in a Garrett‐Munk model ocean have mainly been calculated using Hasselmann transport theory or related methods. Computations show that GM76 is approximately a steady state spectrum for 3‐wave interactions except for frequencies near the inertial frequency and at the lowest vertical mode‐numbers. The lack of variation of the internal wave coupling coefficients allows discussion of results in terms of McComas and Bretherton’s three limiting mechanisms; Induced Diffusion, Elastic Scattering and Parametric Subharmonic Instability. In the high vertical modenumber regime Induced Diffusion provides the most significant contribution. Transfer rates are high here and there has been concern for the validity of the Hasselmann theory. However, recent calculations by Meiss and Watson which relate Induced Diffusion to the Taylor Goldstein equation yield relaxation rates which are valid over a much extended domain.

ISSN:0094-243X    

DOI:10.1063/1.33181

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Numerical integration of the dynamical equations for three wave interactions can, in principle, determine the validity of the approximations used in the derivation of relaxation and transport equations. To provide a simple dynamic model, we use the test wave system. This consists of a single mode interacting with a spectrum of ambient modes; the ambient modes, however, do not interact among themselves. These ’’direct’’ interactions are just those required for the derivation of relaxation rates by lowest order perturbation theories. The test wave model, however, has been shown to be an integrable Hamiltonian system and therefore it is necessary to consider an ensemble average to obtain statistical behavior. Numerical computations of the test wave action, averaged over initial conditions of the ambient waves, do indeed exhibit relaxation to a steady state. It is necessary to include non‐resonant triads in the system: to obtain a converged value for the relaxation rate, &ngr;, triads with resonance mismatch, &Dgr;, of order several times &ngr; must be included. Validity of the relaxation rate computations hinges on the degree a continuous set of ambient modes is approximated by the discrete set of modes in the computation. We discuss the effects of discreteness for our computations, and attempt to encourage further work along this line.

ISSN:0094-243X    

DOI:10.1063/1.33183

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Numerical simulations have been conducted to investigate the absorption of internal waves at a critical level. A comparison of linear with nonlinear computational results provides insight into nonlinear processes occurring during wave absorption. In particular, nonlinear processes, including wave breaking, are shown to produce net velocity and density changes that could be described as thin turbulent mixed layers. Additional results are presented to illustrate the influence of wave amplitude on the absorption process.

ISSN:0094-243X    

DOI:10.1063/1.33184

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Measurements of small scale temperature and velocity fluctuations in the ocean show that in most layers the turbulence occurs in isolated patches occupying a small fraction of the fluid volume. Most of the temperature microstructure patches observed are partially mixed remnants of previous overturning turbulence events after the turbulence has been damped out. The fluctuations are often partially restratified at the larger scales of the original turbulence. The smaller scales are more isotropic. An equilibrium microscale is preserved by the laminar straining motions of either the ambient internal waves, remnant internal waves produced when the turbulence was damped, or by laminar restratification. Such remnant waves and remnant temperature fluctuations are referred to as fossil turbulence. Fossil turbulence can be distinguished from active turbulence because active turbulence can only exist when inertial forces of the eddy motions are larger than either buoyancy or viscous forces: this requires 1.2 LR?&lgr;?15 LK, where &lgr; is the wavelength of possible turbulent motions. In the ocean the buoyancy scale 1.2 LRis often found to be less than the viscous scale 15 LK: this implies no active turbulence at any scale in the microstructure. Kinetic energy of the turbulence which produced the microstructure is converted to internal wave energy. A spectral description of the process is presented, and the role of turbulence as a source of internal waves is discussed. For available oceanic data, the buoyancy scale 1.2 LRis always found to be less than the original overturning scale 1.2 LR0= 3.5 (D C0/N)1/2, which implies that the microstructure of active turbulence unaffected by buoyancy (o‐subscripts) has not been observed. A turbulence activity parameter ATis used to classify the available microstructure observations according to hydrodynamic state: all the data indicates various degrees of fossilization, with restratification anisotropy associated with the most advanced stages. Estimates of space‐time average velocity and temperature dissipation rates in the ocean from microstructure measurement are complicated by the extreme patchiness in space and intermittency in time of the dissipation events. Based on recent evidence of the degree of horizontal patchiness, underestimates by several orders of magnitude are probable if short data records are used. Such undersampling errors may account for the large discrepancy which exists between towed body and dropsonde estimates of mean dissipation rates in the core layer of the equatorial undercurrents and a similar discrepancy between dropsonde estimates of vertical diffusivity compared to canonical values inferred from mean properties in the main thermocline. Some corrections for time intermittency may be possible using information of previous dissipation activity preserved by the fossil turbulence.

ISSN:0094-243X    

DOI:10.1063/1.33185

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

We review recent progress in explaining the observed kinematical structure of the mid‐latitude deep ocean internal wave field as a result of dynamic processes of generation, tranfer and dissipation. The nonlinear transfers are fairly well understood; the main generation and dissipation processes have not been yet identified. A model assuming generation at low and dissipation at high wavenumbers with an inertial cascade in between can quantitatively explain the frequency and wavenumber slopes and the high wavenumber cut‐off of the observed spectrum. The universality of the total energy and shear has been qualitatively explained by near dissipation‐free propagation in physical space and by saturation of small scale waves.

ISSN:0094-243X    

DOI:10.1063/1.33186

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

McComas and Bretherton have described ’’elastic scattering’’ as a mechanism contributing to internal wave transport processes. The Taylor‐Goldstein equation is shown here to provide a basis for calculating the contribution for elastic scattering to internal wave transport. In contrast to the case of induced diffusion the elastic scattering mechanism is weak enough to be described by two‐time perturbation theory. Meiss and Watson have shown that induced diffusion is also described by the Taylor‐Goldstein equation, but requires a more elaborate analysis.

ISSN:0094-243X    

DOI:10.1063/1.33188

出版商:AIP    

年代:1981

数据来源: AIP