Surface Wave Field Research Articles

Infrared remote sensing of breaking waves

ENERGY dissipation due to deep-water wave breaking plays a critical role in the development and evolution of the ocean surface wave field. Furthermore, the energy lost by the wave field via the breaking process is a source for turbulent mixing and air entrainment, which enhance air–sea heat and gas transfer1–3. But the current lack of reliable methods for measuring energy dissipation associated with wave breaking inhibits the quantitative study of processes occurring at ocean surfaces, and represents a major impediment to the improvement of global wave-prediction models4. Here we present a method for remotely quantifying wave-breaking dynamics which uses an infrared imager to measure the temperature changes associated with the disruption and recovery of the surface thermal boundary layer (skin layer). Although our present results focus on quantifying energy dissipation—in particular, we show that the recovery rate of the skin layer in the wakes of breaking waves is correlated with the energy dissipation rate—future applications of this technique should help to elucidate the nature of important small-scale surface processes contributing to air–sea heat5 and gas6 flux, and lead to a fuller understanding of general ocean–atmosphere interactions.

Wave transmission along a ribbed fluid-loaded membrane

This work studies wave transmission along a fluid-loaded membrane supported by an array of ribs, one of which is driven harmonically. The influence of realistic rib dynamics is considered, including finite mechanical impedance as well as spring and damping forces. Under heavy fluid loading, when the coupling between ribs is dominated by the surface wavefield in the fluid, the system exhibits the usual stop/pass band structure. It is shown that there exists a critical rib mass, depending on drive frequency, at which the disturbance becomes completely localized at the driven rib. For frequencies at a band edge, the solution at the critical mass is no longer unique; in this parameter region, the solution is highly sensitive to small changes in frequency or rib properties. Varying the parameters in different ways gives rise to different limiting behavior at the critical points. Numerical simulations have been carried out to study this system. These simulations confirm the analytical predictions and help to determine when the surface wave approximation breaks down.

Lévy statistics of water wave forces

A newly developed technique for determining the statistical properties of random forces with long-range and long-time correlations is applied to the loading of ships and platforms by wind-generated water waves. Using empirical spectra for the vertical displacement of the sea surface we find that the random forces produced by the surface wave field in high sea states are Lévy stable rather than Gaussian. These Lévy statistics imply a shorter survival time for ships and platforms than had been previously estimated using Gaussian statistics.

Conjugate mirror array performance in the presence of surface and bottom roughness.

Dowling and Jackson [J. Acoust. Soc. Am. 91, 3257–3277 (1992)] provided a study of the temporal/spatial effects of an internal wave field on phase conjugate arrays. In the present work the effects of interface roughness on phase conjugate arrays are simulated. The stochastic surface wave field is generated to include realizations of the sea surface height that evolve in time. The time variability of the surface also includes the effects of the acoustic travel time, so that each realization is ‘‘as viewed’’ in the frame of the acoustic wave from source‐array‐to‐receiving‐array and the return after phase conjugation., The acoustic wave propagation is accomplished using a parabolic equation marching algorithm that includes this random rough surface. Bottom penetration and absorption are included. Coupling of the surface wave field with the bottom/sub‐bottom randomness can produce an increased degradation of the return field. Thus even if the processing time to produce the conjugate field approached zero, deg...

Structure and variability of Langmuir circulation during the Surface Waves Processes Program

A cooperative, multiplatform field experiment was conducted in the eastern North Pacific during February and March of 1990 as part of the Surface Waves Processes Program (SWAPP). One of the experimental objectives was to investigate Langmuir circulation so that its role in the evolution of the oceanic surface boundary layer could be better understood. The concurrent use of different observational techniques, ranging from simple surface drifters to complex Doppler sonar systems, resulted in new information about Langmuir circulation structure and variability. Estimates of Langmuir cell spacing indicated that a broad range of scales, from about 2 to 200 m, was excited during periods of strong surface forcing and that the energy containing scales evolved with time. Estimates of cell spacing based on Doppler velocities from a surface‐scanning sonar directed crosswind showed this scale evolution, but estimates based on backscattered intensity did not. This was attributed to the fact that the intensity‐based estimates were only indirectly related to circulation strength. The near‐surface convergent velocities from the sonar were used to form an objective, quantitative measure of the temporal variations in Langmuir circulation strength. As expected, the circulation strength increased dramatically during strong wind events. However, circulation strength and wind stress did not decrease simultaneously, and Langmuir circulation was detectable for up to a day after abrupt reductions in wind stress. Energy from the surface wave field, which decayed more slowly than the wind, was apparently responsible for maintaining the circulation. The variation of circulation strength was found to be better related to (u*Us)½ than to u*, where u* = (τ/ρ)½ is the friction velocity, τ is the wind stress, and Us is the surface wave Stokes drift. This scaling is consistent with wave‐current interaction theories of Langmuir cell generation.

Directionally Reinforced Random Walks

This paper introduces and analyzes a class of directionally reinforced random walks. The work is motivated by an elementary model for time and space correlations in ocean surface wave fields. We develop some basic properties of these walks. For instance, we investigate recurrence properties and give conditions under which the limiting continuous versions of the walks are Gaussian diffusion processes.

The Role of Surface-Wave Breaking in Air-Sea Interaction

Breaking serves to limit the height of surface waves, mix the surface waters, generate ocean currents, and enhance air-sea fluxes of heat, mass, and momentum through the generation of turbulence and the entrainment of air. Breaking may result from intrinsic instabilities of deep-water waves or through wavewave, wave-current, and wind-wave interactions. Observations in the field are made difficult by the fact that breaking is a strongly nonlinear intermittent process occurring over a wide range of scales. Controlled laboratory studies of breaking have proven useful in measuring the scaling relationships between the surface wave field and the kinematics and dynamics of breaking. Our inability to predict the occurrence and dynamics of breaking is a major impediment to the development of better wind-wave and mixed-layer models. Modern acoustic and electromagnetic oceanographic instrumentation should lead to significantly improved measurements of breaking in the near future.

INTERFACIAL INTERACTIONS AND SECONDARY FLOWS IN STRATIFIED TWO-PHASE FLOW

Abstract In this paper are analysed the interactions between a surface wave field and the kinematic structures above and below the waves, in gas-liquid stratified flow in a rectangular cross sectional channel. The analysis is based on experimental data both on the local structure of the flows and on the deformation of the gas-liquid surface. The basic phenomena that have been observed are: on the one hand, the waves that propagates over the liquid surface can exhibit a crosswise distribution of amplitude; on the other hand, secondary flows can be generated both in the gas and in the liquid. A theoretical attempt is developed to explain the distribution to wave amplitude: in fact, the waves propagate over a non uniform liquid current. On the one hand, the wave field interact with the liquid current to generate secondary flows below the waves. On the other hand, the wave amplitude distribution interacts with the gas flow; the wave distribution can be considered as a non uniform interfacial roughness which g...

Using a Synthetic Continental Array to Study the Earth's Interior.

A major field-based observation program (the Skippy Project) is underway to synthesize an array covering the whole Australian continent, using a series of deployments of around 10 broadband seismometers for periods of 5 months at a time. Such a time period is long enough to exploit the favourable position of Australia relative to world seismicity to assemble a good sampling of global structure. The distance coverage from available events is very good, so that it is possible to build up composite record sections covering propagation from 10 to nearly 180.., with a full coverage of the body and surface wavefields. For deep events at regional ranges, the field arrays also yield good data on mantle reverberation patterns.

Laboratory study of polarized microwave scattering by surface waves at grazing incidence: the influence of long waves

Laboratory measurements of microwave scattering at grazing incidence from superposed wind and weakly nonlinear (AK<0.024) regular long waves are presented. This study is an extension of previous measurements with wind waves only. A dual polarized (VV, HH) coherent pulsed Ku-band (14 GHz) scatterometer with temporal resolution of 3 ns was used to obtain Doppler spectra and the absolute cross section of scattered signals for grazing angles from 6/spl deg/ to 25/spl deg/ and winds in the range 2-12 m/s. A wire wave-gauge array was used to measure the wind-wave field. Measurements of the frequency and amplitude modulation of the scattered signal due to the long waves showed that the data separated into two groups. The first grouping corresponded to HH scattering in the upwind direction and was clearly associated with scattering from the dominant gravity wind-waves on the crests of the long waves. In this case, the wind speed clearly influences the frequency modulation due to long waves. The second grouping corresponded to scattering in the downwind direction and was consistent with Bragg scattering from higher frequency waves. In this case the frequency modulation due to orbital velocity of the long waves was found to be weakly dependent on wind speed over the range of parameters studied. This classification of the electromagnetic scattering was consistent with comparisons of direct and Doppler measurements of the kinematics of the surface wave field.

Quasi‐coherent structures in the marine atmospheric surface layer

This paper presents a study aimed at identifying burst structures in the marine atmospheric surface layer. Standard ejection detection schemes, including the the quadrant, the variable interval time averaging, and the modified u‐level techniques, were applied to turbulent wind data measured over the ocean. It was found that the proportion of contribution to the Reynolds stress from the four quadrants of the u′w′ plane is in close agreement with the corresponding contributions in laboratory flows. Ejection detection by the quadrant technique followed by the grouping of ejections into bursts yielded a mean burst period of 47 s at a height of 8.2 m above the water surface, where the mean wind velocity was 6.74 m/s. This burst period corresponds well with the peaks obtained from the short‐time autocorrelation of the streamwise velocity signal and the first moment of the stress cospectrum. Phase averages of these events show a structure which is similar to the structure of the events detected in laboratory flows. These events do not scale on the “inner” variables, friction velocity u* and the kinematic viscosity ν, as has been suggested for laboratory flows. Both ejection and burst periods decrease with increasing wind speed, however, the grouping of ejections into bursts breaks down at higher wind speeds. These periods showed no dependence on the surface wave field.

Nonlinear permittivity of surface wave produced plasmas

The origin of the nonlinearity of plasma permittivity in high-frequency discharges maintained by ionizing surface waves is studied. It is shown that in a diffusion controlled regime the plasma density at the discharge axis is determined nonlocally in terms of the electric field intensity by accounting for the small nonlinear effect of recombination, along with diffusion in the particle balance equation. The axial self-consistent distribution of the surface wave field intensity and of the plasma density of discharges are obtained. The transition to a 'pure' diffusion and a 'pure' recombination regime is considered. A brief discussion is included regarding the case of a nonlinearity due to a weak dependence of the ionization frequency on the plasma density (e.g. due to stepwise processes).

Laboratory study of polarized scattering by surface waves at grazing incidence. I. Wind waves

Laboratory measurements of Ku-band scattering at grazing incidence are presented. This study was motivated by the need to understand the processes which significantly contribute to scattering at grazing incidence. A dual polarized (VV, HH) coherent pulsed Ku-band scatterometer with good temporal resolution (3 ns) was used to obtain Doppler spectra and the absolute cross-section of scattered signals for grazing angles from 6-12/spl deg/, and winds in the range 2-12 m/s. Wire wave gauges were used to measure the wind-wave field. Measurements of the first few moments of the Doppler spectra (cross-section, central frequency and bandwidth) showed that the data separated into two groups. The first grouping corresponded to HH scattering in the upwind look direction, and was clearly associated with scattering from the dominant gravity wind-waves. The second grouping corresponded to HH scattering in the downwind look direction, and all VV scattering, and was consistent with Bragg scattering from free higher frequency waves. This classification of the electromagnetic scattering was consistent with comparisons of direct and Doppler measurements of the kinematics of the surface wave field. The electromagnetic classification was also consistent with asymmetries in the wave field which increased with increasing wind speed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Non-plane geometries of seismic surface wavefields and their implications for regional surface-wave tomography: W. Friederich, E. Wielandt & S. Stange, Geophysical Journal International, 119(3), 1994, pp 931–948

Non-plane geometries of seismic surface wavefields and their implications for regional surface-wave tomography: W. Friederich, E. Wielandt & S. Stange, Geophysical Journal International, 119(3), 1994, pp 931–948

Multipole radiation of seismic waves from explosions in nonspherical cavities and its application to signal identification

A new method, named ‘‘spherical mapping approximation’’ (SMA), is developed for the evaluation of displacement fields of body waves and surface waves from explosions in nonspherical cavities embedded in elastic media. Under SMA, the explosion-generated stress distribution on the surface of an arbitrary cavity is mapped onto the surface of an equivalent virtual spherical cavity having the volume of the true cavity. The analytical results express the displacement field in terms of a multipole double-sum expansion of spherical eigenvectors with coefficients in the form of a finite Legendre transform of the components of the normal vector of the cavity boundary. These ‘‘cavity integrals’’ can be evaluated exactly for spheroidal and cylindrical inclusions. In the long-wave far-field approximation, symmetric finite cavities are shown to be equivalent to a linear combination of point dipoles directed along the principal cavity axes. The ensuing radiation patterns yield, in general, four-lobe patterns for S waves, two-lobe patterns for P waves, and single to two-lobe Rayleigh-wave pattern, independent of the details of the cavities’ shape. However, all radiation patterns are modulated by a frequency-dependent ‘‘cavity factor’’ that embodies the boundary conditions on the cavity surface. Moreover, it is shown that the radiation pattern for P waves from a nonspherical symmetrical cavity in the long-wave far-field approximation is always dipolar. Since the radiation pattern of radiated P waves from a standard earthquake is always quadrupolar, the cavity explosion behaves like a non-double-couple earthquake. Thus the examination of the deviatoric moment tensor of a given seismic event enables one, in principle, to state whether it is a standard earthquake or perhaps (if the S-wave pattern is quadrupolar) an evasion of the test-ban treaty. It is shown that SMA can be easily fitted to a multilayered elastic half-space if the equivalent source is placed in one of the layers. In that case, Love waves will be generated in addition to P, S, and Rayleigh waves. Displacement patterns for body and surface waves are calculated for spheroidal and cylindrical cavities for a wide range of aspects ratios and corresponding aperture angles, exhibiting the whole gamut of cavity shapes from a line source to a disk. The moments of the equivalent dipoles are shown to depend on the corresponding cavity integrals, the elastic constants of the medium in the neighborhood of the source, and the initial energy injection. All nonspherical cavities generate strong shear waves except for special aperture angles at which a spherical P wave is generated, unaccompanied by S waves. The wave-spectra of body waves (surface waves) exhibit a corner frequency (peak frequency) at a wavelength equal to the radius of the equivalent sphere. This enables one to deduce the size of the cavity from the spectrum of its far-field displacement signals, provided that the explosion is fully decoupled and that the interaction of the shock wave with the medium occurred in the elastic regime. The results of the present research are applicable to the detection and identification of seismic signals from clandestine underground nuclear explosions.

Radiation from one-dimensional dielectric leaky-wave antennas

Dielectric leaky-wave antennas consist of multiple dielectric layers above a ground plane, along with a simple source such as an infinite line current in the one-dimensional case. The layers act as a guiding structure for one or more leaky waves, which are responsible for producing a narrow-beam radiation pattern. Depending on the beam angle, the leaky waves may extend for a considerable distance from the source, and thus experience truncation effects due to a finite-size substrate. One of the purposes of the present investigation is to study efficient methods for calculating the patterns under such conditions. The chirp z-transform (CZT) is shown to be very efficient for this. Simple approximate formulas for calculating the radiation pattern are also investigated. In addition, a study is also made of the different contributions to the total aperture field, including the leaky-wave field, the space-wave field, and the surface-wave field. >

Correlations between Ambient Noise and the Ocean Surface Wave Field

Abstract Measurements of the ambient noise spectrum level N with simultaneous, coincident wind and wave measurements were made from RP FLIP in fall 1991. The measurements were designed to investigate the correlation between the ambient noise and relevant surface wave parameters. The results suggest that wave parameters related to the incidence of wave breaking correlated well with the ambient noise level. The correlation between N and the rms wave amplitude a was found to he poor but that between N and the rms amplitude of the local wind sea aw was comparable to that between wind speed U and N. Similar good correlations were found between the rms wave slope s and N, and the higher frequency surface wave spectral levels and N. Correlations between the surface wave dissipation estimates D based on the Hasselmann and Phillips models and the ambient noise were comparable to those between the wind speed and the ambient noise. The mean square acoustic pressure was found to be proportional to Dn with n in the ra...

INSAR imagery of surface currents, wave fields, and fronts

The authors demonstrate the ability of interferometric radar imagery to determine both relative and absolute surface velocities in the open ocean. Absolute phase calibration is accomplished by noting the azimuthal displacement of range-travelling targets-demonstrating for the first time that under favourable circumstances phase calibration can be achieved in open-ocean in the absence of ground truth. The high resolution of radar imagery permits observation of sharp velocity discontinuities, e.g. the Gulf Stream boundary and the wave field. The recent SIR-C/X-SAR shuttle missions dramatically emphasize the experimental and observational aspects of space-based radar. The combination of absolute velocities, high spatial resolution, and wide-area coverage suggest that interferometric radar imagery can provide a unique and powerful aid both for studies of global circulation patterns and detailed analysis of slope/shelf water interactions with ocean currents. In particular, the authors employ this measurement of the surface currents and wave field near a velocity front to help refine and bound results of their modeling of calculated radar images of the front. The results of this paper are compared with available ground truth.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The Impact of Hurricane Andrew on the Near-Surface Marine Environment in the Bahamas and the Gulf of Mexico

Abstract Hurricane Andrew was a relatively small but intense hurricane that passed through the Bahamas, across the Florida Peninsula, and across the Gulf of Mexico between 23 and 26 August 1992. The characteristics of this hurricane primarily beyond its core are summarized using 1) marine observations from three National Data Buoy Center (NDBC) buoys and three Coastal-Marine Automated Network stations close to the storm track; 2) water levels and storm surge at 15 locations in the Bahamas, around the coast of Florida, and along the northern coast of the Gulf of Mexico; 3) currents, temperatures, and salinities at a depth of 11 m in the northern Gulf; and 4) spatial analyses of sea surface temperature (SST) before and after the passage of Andrew. Sea level pressure, wind direction, wind speed, wind gust, air temperature, and the surface wave field were strongly influenced at locations generally within 100 km of the hurricane track. Maximum sustained winds of 75 m s−1 occurred just north of the storm track ...

Non-Plane Geometries of Seismic Surface Wavefields and Their Implications For Regional Surface-Wave Tomography

Seismic surface waves exhibit much more complicated wavefields than is commonly assumed. We are led to this conclusion after analysing 90 teleseismic events recorded at on average eight broad-band stations in Southern Germany. Large amplitude and phase fluctuations across the network are observed which, as we show, are definitely not due to instrument response or calibration problems. In order to give an impression of how surface wavefields may look in reality, we fit wavefields to the observed network data using basis wavefields derived from Hermite-Gaussian functions. We show both amplitude and phase distributions of several events. Synthetic wavefields, generated by acoustic finite-difference computations in a random medium with realistic correlation length and rms velocity fluctuations, support the interpretation that most of the observed wavefield anomalies have accumulated on the path of the wave from the source to the network. Even if no lateral heterogeneities existed in the region of the network, the large-scale features of the observed wavefields would remain nearly unaltered. We also model the synthetic wavefields using Hermite-Gaussian wavefields and achieve extremely good fits. In view of the large anomalies of the wavefields, we investigate their implications for the regional surface-wave tomography. In this method, one infers regional structure from a set of measurements of phase traveltime between pairs of stations. The basic assumption of the method is that the wavefield incident on the network be plane. Thus, all observed phase traveltime anomalies are interpreted in terms of regional heterogeneous structure. The consequence is a seemingly highly inconsistent data set. We perform several tomographic experiments with realistic synthetic data sets. Our experiments indicate that regional surface-wave tomography may work, if there is an excellent coverage of paths crossing the region and if each path is sampled several times. Under realistic conditions, that is on a sparse network with not much more than one velocity value per path, the imaging power of surface-wave tomography is very poor. Owing to the seeming inconsistency of the phase traveltimes, variance reduction is generally low. With an increasing number of traveltime data it further decreases, while at the same time the imaging power increases. The opposite behaviour is observed if the number of data is reduced—variance reduction increases, but imaging power decreases. Hence, in connection with regional surface-wave tomography, high variance reduction indicates a lack of data rather than the goodness of reconstruction.