Analysis of the variability of hydrological characteristics of the north subpolar front using the model of frontal-trapped baroclinic Rossby waves

Solutions, termed frontal-trapped Rossby waves, have been found for the long-wave equation in a model of a two-layer fluid with the frontal boundary given in the form of an exponential function. Data from a hydrological survey of a subpolar hydrological front in the vicinity of Newfoundland are analysed. A frontal-trapped wave model is shown to describe about 40% of the dispersion of the initial seawater density field. Translated by V. Puchkin.     

Studies of backscattered sea return with a CW, dual-frequency,X-band radar

A coherent, CW, dual-frequency,X-band radar was used to study microwave sea return from the Chesapeake Bay. It is shown that the product of the backseattered fields depends strongly on long surface wave properties. In particular, a sharp line is found in the product power spectrum whose frequency is that of the water wave whose wavelength is in resonance with the spatial period of the beat frequency between the two transmitted signals and whose wave vector is parallel to the horizontal line of sight. Thus, gravity wave dispersion relations can be obtained with the system. Furthermore, the degree of modulation of short waves by long ones is given by the intensity of the line. A broad background corresponding to the convolution of the single-frequency Doppler spectra is also seen in the product power spectrum. These results are shown to be interpretable by composite surface scattering theory.     

Edge waves under ice cover at a straight coast with a sloping beach

Edge waves in an ice-covered sea at a straight coast with a sloping beach are analyzed within the linearized theory. Such waves propagate along the coast with an amplitude which exponentially decays offshore. The problem is examined without using the hydrostatic assumption. The seawater is considered to be a homogeneous, inviscid, nonrotating, and incompressible fluid. Ice with a uniform thickness is considered, with constant values of density, cylindrical rigidity, Poisson ratio, and compressive stress in the ice. The normal velocity at the bottom is zero; the linearized kinematic and dynamic boundary conditions are satisfied at the lower surface of the ice. Explicit solutions for the edge flexural-gravity waves and the corresponding dispersion equations are obtained and analyzed.     

Theory of underwater LIDAR with a complex modulated illumination beam

The main effects accompanying the propagation of an initially narrow complex modulated light beam in sea water are studied. Sea water, with respect to modulation waves, is shown to be similar to a medium with frequency dispersion with respect to electromagnetic waves; in the medium, the effects of the time focusing of a wave packet are possible. Based on the self-similar small-angle solution to the equation of radiation transfer, we estimate spatial-temporal characteristics of pulse-signal amplitude modulated by a complex signal. We propose a scheme for constructing underwater LIDAR using a complex modulated illumination beam and the processing of a received echo signal. The processing allows the separation of the modulated component and its matched filtration. Values of possible delays and broadening of the separated echo signal caused by dispersion properties of sea water are estimated.     

Distribution of eddy scales for wave current combined flow

This paper illustrates the modulation of the eddy scale distribution due to superimposition of surface wave on only current flow. Time series data of three-dimensional velocity components were measured in a laboratory flume by a three-dimensional (3D) 16-MHz micro-acoustic Doppler velocimeter (Micro-ADV). The velocity time series of only current case and waves following the current were analysed to obtain the phase-averaged mean velocities, turbulent intensities, and Reynolds stress. The probability density function of phase-averaged stream-wise and vertical velocity fluctuations showed bimodal oscillations towards the free surface for higher frequency surface waves. It was revealed that surface waves along the current effectively decrease the intermittency of turbulence of the only current flow. Surface wave changed the intermittent structure of only current flow by modulation of the energy cascade mechanism of the only current flow by introduction of wave induced length scales. Also the scale of the finer dissipative eddies were prominently enhanced by the increase in surface wave frequency. Wavelet analysis of time series of velocity signals provided information on the eddy scale and their frequency of occurrence. It was found that the large eddies are carried by the crest regions of the progressive wave while the small scale eddies are carried by the trough regions.     

海底载流管道轴对称波频散特性分析

海底载流管道是典型的土-固-液三相耦合问题.利用Flügge壳体运动方程,结合弹性动力学方程,建立海底载流管道轴对称运动时的频散方程,分析自由振动波的性质及其随频率变化的规律.结果表明管道周围的土壤介质对其自由振动波的传播特性影响显著,而土壤介质刚度的增大使得自由振动波的截止频率随之增加.     

The form of the asymptotic depth-limited wind-wave spectrum: Part III — Directional spreading

The directional spreading of both the wavenumber and frequency spectra of finite-depth wind generated waves at the asymptotic depth limit are examined. The analysis uses the Wavelet Directional Method, removing the need to assume a form for the dispersion relationship. The paper shows that both the wavenumber and frequency forms are narrowest at the spectral peak and broaden at wavenumbers (frequencies) both above and below the peak. The directional spreading of the wavenumber spectrum is bi-modal above the spectral peak. In contrast, the frequency spectrum is uni-modal. This difference is shown to be the result of energy in the wind direction being displaced from the linear dispersion shell. A full parametric relationship for the directional spreading of the wavenumber spectrum is developed. The analysis clearly shows that typical dispersion relationships are questionable at high frequencies and that such effects can be significant. This result supports greater attention being focussed on the routine recording of wavenumber spectra, rather than frequency spectra.     

Wave group analysis of natural wind waves based on modulational instability theory

This paper discusses wave grouping of wind waves observed at Lake Biwa from the physical viewpoint of wave modulational instability. Amplitude modulation periods obtained from the smoothed instantaneous wave energy history (SIWEH) of the observed data proposed by Funke and Mansard are compared with the values predicted by the modulational instability theory using the Zakharov equation for a finite constant water depth derived by Stiassnie and Shemer. The modulation period normalized by the typical wave period corresponds to the length of total run. It is shown that the amplitude modulation periods of the observed data agree satisfactorily with the predicted values. Thus, it is concluded that the modulational instability is a hydrodynamical cause of grouping of high waves.     

On an inverse problem of ship-induced internal waves

An inverse problem for trapped internal waves is considered in an attempt to provide a practical tool for estimating the density stratification in the sea from the wake pattern behind a moving vessel. The ambient stratification is represented by Barber's (1993) exponential series and the coefficients are found by matching the first mode dispersion relation to the one found from the wake data. A fast algorithm for calculating the dispersion relation is derived. It is shown that when the series converges with a low number of coefficients, the inverse profile is adequate, as happens for example for typical sea loch profiles. In more general circumstances the predicted maximum stratification still provides a reasonable approximation as a result of Barber's (1993) theorem.     

Downward transfer of momentum by wind-driven waves

A model for the downward transfer of wind momentum is derived for growing waves. It is shown that waves, which grow due to an uneven pressure distribution on the water surface or a wave-coherent surface shear stress have horizontal velocities out of phase with the surface elevation. Further, if the waves grow in the x-direction, while the motion is perhaps time-periodic at any fixed point, the Reynolds stresses associated with the organized motion are positive. This is in agreement with several field and laboratory measurements which were previously unexplained, and the new theory successfully links measured wave growth rates and measured sub-surface Reynolds stresses. Wave coherent air pressure (and/or surface shear stress) is shown to change the speed of wave propagation as well as inducing growth or decay. From air pressure variations that are in phase with the surface elevation, the influence on the waves is simply a phase speed increase. For pressure variations out of phase with surface elevation, both growth (or decay) and phase speed changes occur. The theory is initially developed for long waves, after which the velocity potential and dispersion relation for linear waves in arbitrary depth are given. The model enables a sounder model for the transfer to storm surges or currents of momentum from breaking waves in that it does not rely entirely on ad-hoc turbulent diffusion. Future models of atmosphere-ocean exchanges should also acknowledge that momentum is transferred partly by the organized wave motion, while other species, like heat and gasses, may rely totally on turbulent diffusion. The fact that growing wind waves do in fact not generally obey the dispersion relation for free waves may need to be considered in future wind wave development models.     

基于微结构图像的风生微尺度波频率谱初探

利用海面微结构光学测量装置实验获取到的微尺度波图像来给出风生微尺度波的波数谱,在此基础上利用频散关系建立微尺度波波数谱和频率谱之间的关系,最终得到微尺度波频率谱.对微尺度波的频率谱随频率变化的响应进行了研究,发现频率谱与频率的a次方成正比关系,a的值随风速的增加而增加,同一风速下a几乎为定值.     

On an inverse problem of ship-induced internal waves

An inverse problem for trapped internal waves is considered in an attempt to provide a practical tool for estimating the density stratification in the sea from the wake pattern behind a moving vessel. The ambient stratification is represented by Barber's (1993) exponential series and the coefficients are found by matching the first mode dispersion relation to the one found from the wake data. A fast algorithm for calculating the dispersion relation is derived. It is shown that when the series converges with a low number of coefficients, the inverse profile is adequate, as happens for example for typical sea loch profiles. In more general circumstances the predicted maximum stratification still provides a reasonable approximation as a result of Barber's (1993) theorem.     

Spectral characteristics of Rossby waves in the Northwestern Pacific based on satellite altimetry

Using satellite altimetry measurement data for 1993–2013, we study the spectral characteristics of Rossby waves in the Northwestern Pacific (25°–50° N, 140°–180° E). For each latitude degree, we draw integral plots of spectral power density calculated with a two-dimensional Fourier transform (2D-FFT). We compare the dispersion equations of Rossby waves calculated from the WKB-approximation and an approximation of a two-layer ocean model with the empirical velocities determined by the slope of isopleths by the Radon method; also, we compare the dispersion equations with the spectral distributions of level variations. It is shown that the main energy of Rossby waves in the Northwestern Pacific corresponds to the first baroclinic mode. At almost all latitudes, there is good agreement between the empirical phase velocities calculated by isopleths by the Radon method and the theoretical values; also, the spectral peaks correspond to graphs of the dispersion equations for the first baroclinic mode Rossby waves, except for the Kuroshio region, where some peaks correspond to the second mode.     

Cramer-Rao lower bounds for sonar broad-band modulation parameters

A role of passive sonar signal processing is the detection and estimation of the parameters associated with amplitude modulated broad-band signals. An example of such signals is propeller noise. Discrete frequency lines occur at the rotational frequency of the propulsion shaft and at the blade frequency. This correspondence provides expressions for the Cramer-Rao lower bounds for the estimates of broad-band signal power, modulation level, modulation frequency, and modulation phase. It is shown that for low broad-band-signal-to-broad-band-noise ratios, the estimates of power and modulation level are uncoupled from the estimates of modulation frequency and phase     

Internal flow structure of short wind waves

For wind waves generated in a wind-wave tunnel, the surface pressure and also the pressure distribution along the internal streamlines were calculated from the measured internal velocity field. In distinct waves, with wave height comparable with or larger than the mean, the surface pressure is found to vary drastically in a narrow region around the crest, showing a dominant minimum near the crest. On the other hand, the pressure distribution along the streamline shows systematic variations that are nearly in phase with the streamline profile. It is shown that the occurrence of the pressure in phase with the streamline profile is linked with the internal vorticity distribution, especially with the presence of a high vorticity region below the crest described in Part I of this study. As a result of the occurrence of such pressure variations, the dispersion relation is modified by about 10% from that for linear irrotational waves. It is argued from the present measurements that the dispersion relation and also the energy transfer from wind into wind waves are strongly affected by the internal vortical structure so that the assumption of irrotational gravity waves cannot be applied to the wind waves being studied.     

海南岛新村湾营养负荷对海菖蒲的影响研究

对海南岛新村湾海草床的优势种——海菖蒲(Enhalus acoroides)的茎枝特征、茎枝密度、生物量和不同组织营养素含量进行了研究,并探讨了海水和沉积物间隙水营养负荷对其的影响。结果表明,(1)海菖蒲的茎枝特征(叶长、宽和枝重)、茎枝密度和地上生物量存在显著的空间差异,这些因子与海水和沉积物间隙水DIN含量呈负相关;(2)海菖蒲TN、TP质量分数随着样区的变化而产生显著差异,海菖蒲TN质量分数与其所在样区海水和沉积物间隙水DIN浓度呈显著正相关;(3)随着水体N负荷的增加,海菖蒲叶单位面积附着藻类生物量显著增加。本次研究的结果表明,网箱养殖引起的营养负荷是导致海草衰亡的潜在原因之一,引起了新村湾网箱养殖区海草床的退化。     

Radiophysical investigations of sea roughness (radiooceanography) at the Ukrainian Academy of Sciences

Experiments on the scattering of radio waves in the range 200 m to 3 cm from a rough sea surface are described. Amplitude, frequency, and space-time characteristics of scattered radio signals at different states of the sea surface are presented. It is shown that the problem of the short and medium wave scattering from the sea can be solved by the perturbance method. In this case the mechanism of scattering is of "resonant" character. The intensity of the backscatter signals is proportional to the density of the spatial spectrum on the half-length of the radio waves. The high frequency radio wave scattering is well described by a two-scale model of the scattering surface, "ripple on the large wave." The intensity of scattered radio signals is also proportional to the spectrum density of "ripples" whose length is approximately equal to half a radio wave. The effect of the large waves is to modulate the amplitude of a scattered radio signal and to broaden its frequency spectrum. Methods of solution of the reverse problem were considered. This allowed determination of parameters of sea roughness by characteristics of scattered radio signals. The principles of design of the corresponding equipment are described.     

Influence of currents on weakly nonlinear topographic waves

In the Boussinesq approximation, we consider trapped topographic waves in an inhomogeneous current directed along isobaths. The influence of the current on the dispersion properties of trapped topographic waves in the Norwegian Sea is studied. We determine the mean currents and nonoscillatory (on the time scale of the waves) density corrections induced by the waves due to their nonlinearity. It is shown that the influence of currents is significant in the short-wave region. Its influence leads to a decrease in the wavelength for the constant period of waves, whereas the mean current caused by nonlinearity noticeably varies, especially in the bottom layer.     

Air-Sea Interactions under the Existence of Opposing Swell

Data of a comprehensive laboratory study on the coexistent system of wind waves and opposing swell (Mitsuyasu and Yoshida, 1989) have been reanalyzed to clarify the air-sea interaction phenomena under the coexistence of wind waves and swell. It is shown that the magnitude of the decay rate of swell due to an opposing wind is almost the same as that of the growth rate of swell caused by a following wind, as measured by Mitsuyasu and Honda (1982). The decay rate is much smaller than that obtained recently by Peirson et al. (2003), but the reason for the disagreement is not clear at present. The effect of an opposing swell on wind waves is very different from that of a following swell; wind waves are intensified by an opposing swell while they are attenuated by a following one. The phenomenon contradicts the model of Phillips and Banner (1974), but the reason for this is not clear at this time. The high-frequency spectrum of wind waves shows a small increase of the spectral density. Wind shear stress increases a little due to the effect of opposing swell. The intensification of wind waves by opposing swell and the small increase of the spectral density in a high-frequency region can be attributed to the increase of wind shear stress. Such organized phenomena lead to the conclusion that the hypothesis of local equilibrium for pure wind waves (Toba, 1972) can also be satisfied for wind waves that coexist with opposing swell. The recent finding of Hanson and Phillips (1999) can be explained by this mechanism.     

Wave forces on a seawater intake caisson

The wave force on a seawater intake structure consisting of a perforated square caisson of 400 mm×400 mm size encircling a vertical suction pipe of 160-mm diameter is investigated using physical model studies. The porosity of caisson was varied from 1.6 to 16.9%. Regular and random waves of wide range of heights and periods were used. It is found that the force ratio (ratio of the force on perforated caisson to the force on caisson with zero percent porosity) reduces to an extent of up to 60% with increase in porosity of the caisson from 1.6 to 16.9%. The force ratio was found to increase with increase in relative wave height and reduces with increase in relative width. Multiple regression analysis of the measured data points was carried out and predictive equations for wave force ratios are obtained both for regular and random waves. The results of this investigation can be used in the hydrodynamic design of perforated caissons, which are widely used as seawater intake structures.