Surface Wave Field Research Articles

Wave-Coherent Air–Sea Heat Flux

Abstract Air–sea fluxes of heat and momentum play a crucial role in weather, climate, and the coupled general circulation of the oceans and atmosphere. Much progress has been made to quantify momentum transfer from the atmosphere to the ocean for a wide range of wind and wave conditions. Yet, despite the fact that global heat budgets are now at the forefront of current research in atmospheric, oceanographic, and climate problems and despite the good research progress in recent years, much remains to be done to better understand and quantify air–sea heat transfer. It is well known that ocean-surface waves may support momentum transfer from the atmosphere to the ocean, but the role of the waves in heat transfer has been ambiguous and poorly understood. Here, evidence is presented that there are surface wave–coherent components of both the sensible and the latent heat fluxes. Presented here are data from three field experiments that show modulations of temperature and humidity at the surface and at 10–14 m above the surface, which are coherent with the surface wave field. The authors show that the phase relationship between temperature and surface displacement is a function of wind speed. At a 10–12-m elevation, a wave-coherent heat transfer of O(1) W m−2 is found, dominated by the latent heat transfer, as well as wave-coherent fractional contributions to the total heat flux (the sum of latent and sensible heat fluxes) of up to 7%. For the wind speeds and wave conditions of these experiments, which encompass the range of global averages, this wave contribution to total heat flux is comparable in magnitude to the atmospheric heat fluxes commonly attributed to the effects of greenhouse gases or aerosols. By analogy with momentum transfer, the authors expect the wave-coherent heat transfer to decay with height over scales on the order of k−1, where k is the characteristic surface wavenumber; therefore, it is also expected that measurements at elevations of O(10) m may underestimate the contribution of the wave-induced heat flux to the atmosphere.

Ultrasound-initiated structural transformations in liquid crystals (A review)

The results of the studies of structural transformations in liquid crystal layers with homeotropic and planar macrostructures in the fields of longitudinal, surface, and shear ultrasonic waves are systematized and generalized.

High harmonic fast wave heating efficiency enhancement and current drive at longer wavelength on the National Spherical Torus Experiment

High harmonic fast wave heating and current drive (CD) are being developed on the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 41, 1435 (2001)] for supporting startup and sustainment of the spherical torus plasma. Considerable enhancement of the core heating efficiency (η) from 44% to 65% has been obtained for CD phasing of the antenna (strap-to-strap ϕ=−90°, kϕ=−8m−1) by increasing the magnetic field from 4.5to5.5kG. This increase in efficiency is strongly correlated to moving the location of the onset density for perpendicular fast wave propagation (nonset∝B×k∥2∕ω) away from the antenna face and wall, and hence reducing the propagating surface wave fields. Radio frequency (RF) waves propagating close to the wall at lower B and k∥ can enhance power losses from both the parametric decay instability (PDI) and wave dissipation in sheaths and structures around the machine. The improved efficiency found here is attributed to a reduction in the latter, as PDI losses are little changed at the higher magnetic field. Under these conditions of higher coupling efficiency, initial measurements of localized CD effects have been made and compared with advanced RF code simulations.

A General Multisegment Artificial Neural Network Architecture for the Efficient Evaluation of Electromagnetic Plane-Wave Wedge Diffraction

A multisegment artificial neural network (ANN) is proposed as an interpolation technique for the evaluation of the electromagnetic field diffracted at the edge of anisotropic impedance wedges under plane wave illumination at oblique incidence. Multisegmentation is needed as the high-frequency wedge diffracted field is characterized by a number of discontinuities at the shadow boundaries of the geometrical optics and surface wave fields. The proposed approach is applied, as a test case, to the problem of an anisotropic impedance right-angled wedge illuminated by a skewly incident plane wave. Some exact analytical solutions valid for specific surface impedance tensors are used to obtain numerical data for the ANN training phase as well as to show the interpolation capabilities of the implemented ANN. Nevertheless, the proposed ANN structure is general and can be trained with data obtained from other available solutions (analytical, perturbative, numerical) valid for more general wedge configurations, eventually leading to a single software tool encompassing all of them and providing accurate approximations of the wedge diffracted field in a relatively short time, comparable to that of a closed form analytical solution.

A Uniform Geometrical Theory of Diffraction for predicting fields of sources near or on thin planar positive/negative material discontinuities

Relatively simple and accurate closed form Uniform Geometrical Theory of Diffraction (UTD) solutions are obtained for describing the radiated and surface wave fields, respectively, which are excited by sources near or on thin, planar, canonical two‐dimensional (2‐D) double positive/double negative (DPS/DNG) material discontinuities. Unlike most previous works, which analyze the plane wave scattering by such DPS structures via the Wiener‐Hopf (W‐H) or Maliuzhinets methods, the present development can also treat problems of the radiation by and coupling between antennas near or on finite material coatings on large metallic platforms. The latter is made possible mainly through the introduction of important higher‐order UTD slope diffraction terms which are developed here in addition to first‐order UTD. The present solutions are simpler to use because, in part, they do not contain the complicated split functions of the W‐H solutions nor the complex Maliuzhinets functions. Unlike the latter methods based on approximate boundary conditions, the present solutions, which are developed via a heuristic spectral synthesis approach, recover the proper local plane wave Fresnel reflection and transmission coefficients and surface wave constants of the DPS/DNG material. They also include the presence of backward surface waves in DNG media. Besides being asymptotic solutions of the wave equation, the present UTD diffracted fields satisfy reciprocity, the radiation condition, boundary conditions on the conductor, and the Karp‐Karal lemma which dictates that the first‐order UTD space waves vanish on a material interface.

Estimation of tissue’s elasticity with surface wave speed

The mechanical response of tissues to external forces has gained considerable interest in medical diagnosis. One approach to imaging tissue elastic properties is to apply an external force on the surface of the body. Another approach is to generate a localized force inside the tissue with the radiation force of ultrasound. In this paper, a new method is developed to estimate tissue's elasticity based on surface wave speed measurement. The theory of surface wave speed is developed for estimating tissue's elasticity. Experiments are carried out on a tissue-mimicking ultrasound phantom. An amplitude modulated ultrasound signal of a few hundred hertz is used to generate a localized force in the phantom. The surface wave fields are measured with a laser vibrometer. It shows that the surface wave speed can be used for inversely estimating tissue's elastic properties based on tissue's surface measurement.

Evaluation of the scattering coefficient for high-frequency sound scattered from the sea surface

The effect of the deviation of the real rough sea surface from the Gaussian isotropic surface on the scattering coefficient of high-frequency sound is analyzed. The analysis relies on the data on the sea surface slopes, obtained from field experiments in the Black Sea with the use of a two-dimensional laser slopemeter. It is shown that the effect of the anisotropy of the surface wave field on the scattering coefficient is small when the angle of incidence is small, but this effect rapidly increases as the angle of incidence grows. The deviation of the real statistical moments of the sea surface slopes from those corresponding to the Gaussian distribution results in a ±20% error in the theoretical values of the scattering coefficient.

Numerical method for calculating the surface wave field in the presence of caustics

Due to the lateral heterogeneity of the upper layers of the Earth, paths of surface waves deviate from arcs of great circles. Because of the sphericity of the Earth, the paths intersect on a hemisphere opposite to the epicenter and form caustics consisting of two branches, with their tangent point being a cusp. For this reason, the field of surface waves cannot be analyzed in terms of the ray theory at distances larger than 90°. The asymptotic approach to the analysis of the field in the vicinity of such caustics is very ill-suited for numerical implementation. The difficulties of such an approach to the field calculation are aggravated by the fact that such caustics are superimposed in some regions. Therefore, it is suggested to use the theorem of representation, according to which the field within a certain contour is expressed as an integral whose integrand contains values of the function itself, its derivative along the normal to the contour, and Green’s function. The field on the contour (the circle bounding a hemisphere centered at the epicenter) is calculated by the ray method because rays do not intersect on this hemisphere. These data are used for the construction of the field on the opposite hemisphere assumed to be homogeneous, which enables the construction of Green’s function for this hemisphere. This limitation is not very stringent because the configuration of rays and caustics on this hemisphere is mainly determined by the field on the circle. The integral in the representation theorem is calculated numerically. Numerical examples are presented for models in which one caustic or two superimposed caustics form. These calculations yield constraints on variations in the amplitude and phase of the wave. Rayleigh wave fields are also calculated for a model of the real Earth. It is shown that, at some points, the Rayleigh wave spectrum can be strongly distorted because caustics corresponding to different periods differ in shape.

Physical response of the coastal ocean to Hurricane Isabel near landfall

Abstract. Hurricane Isabel made landfall near Drum Inlet, North Carolina on 18 September 2003. In nearby Onslow Bay an array of 5 moorings captured the response of the coastal ocean to the passage of the storm by measuring currents, surface waves, bottom pressure, temperature and salinity. Temperatures across the continental shelf decreased by 1–3°C, consistent with a surface heat flux estimate of 750 W/m2. Salinity decreased at most mooring locations. A calculation at one of the moorings estimates rainfall of 11 cm and a net addition of fresh water at the surface of 8 cm. The low-pass current field shows a shelf-wide movement of water, first to the southwest, with an abrupt reversal to the northeast along the shelf after landfall. Close analysis of this reversal shows it to be a disturbance propagating offshore at a speed somewhat less than the local shallow water wave speed. The high-pass current field at one of the moorings shows a significant increase in kinetic energy at periods between 10 min and 2 h during the approach of the storm. This high-pass flow is isotropic and has a short (<5 m) vertical decorrelation scale. It appears to be closely associated with the winds, Finally we examined the surface wave field at one of the moorings. It shows the swell energy peaking well before the wind waves. At the height of the storm, as the winds rotated rapidly in the cyclonic sense, the wind wave direction rotated as well, with a lag of 45–90°.

Deeper insight into surface waves excitation at edge of arbitrary impedance wedge

In the paper, main features of surface waves excited at the edge of an anisotropic impedance wedge under plane-wave illumination are analysed. In particular, explicit closed-form expressions for the surface-wave propagation vectors are given, which are valid for an arbitrary geometrical and electrical wedge configuration. From the point of view of applications, this allows effective hybrid analytical-numerical methods to be derived, accounting for the presence of surface-wave field contributions excited by the diffraction phenomenon at the edge of the wedge.

Caveats in Multi-modal Inversion of Seismic Surface Wavefields

We consider several examples demonstrating that the formal modal representation of surface wavefields often does not describe adequately observable wave parameters, such as the phase and group velocity dispersion of higher modes. The main reason for this is the existence in the medium of several waveguides or weakly coupled wavefields in the same waveguide. In such cases the separation of neighboring higher modes may be impossible, and observed dispersion curves may significantly differ from the ones predicted by the theory. From the example related to the studies of the crustal and upper mantle structure we found that the difficulty in the separation of first and second crustal higher modes can be overcome by applying a special inversion procedure. This procedure ignores the existence of a low velocity layer in the upper mantle when fitting the observable higher-mode dispersion curve to the one predicted by the model.

Effect of the cryolithic zone electrical properties on the medium-frequency surface wave at high latitudes

Result of the measurements of the cryolithic zone electrical properties by the radio impedance sounding method are presented. The surface impedance and geoelectric section of the Arctic permafrost have been determined. The experiments have been performed near Cherskii in the continental zone. The surface wave attenuation function is considered. The accuracy of the surface wave field calculations at high latitudes has been estimated based on the local electrical characteristics of the underlying cryogenic medium. The effect of the Arctic Ocean on the operating zone of navigation and communication facilities in the kilometer wave band has been experimentally estimated.

Internal and surface wave effects on inverted echo sounder measurements

Inverted echo-sounders operating at 10 kHz, long used to observe mesoscale synoptic variability from the sea-floor through changes in acoustic travel time and pressure, were recently deployed in the South China Sea to explore their potential for detecting nonlinear internal waves. The observations, based on a 6-s transmission rate, reveal both the internal tide and high-frequency nonlinear internal wave packets and are consistent with remotely sensed images and nearby temperature time series. The results motivate exploration of a refinement in instrument operation in which the raw signal is digitized and stored at the full transducer bandwidth, rather than the present scheme of narrow band filtering and threshold detection. If the raw signal is available, it will be related to the surface wave field, leading to the prospect of ocean surface environmental measurements from the sea-floor. An intriguing possibility is that internal wave modulation of the surface wave field, so evident in remotely sensed images, including the consequence of surface wave breaking in convergence zones, might be detectable in this way from the ocean floor, thus linking internal wave observations directly with surface wave modulation. Model calculations of surface scatter are used to gain insight on this approach.

Changes in ocean infrasonic energy density and vector intensity with changes in wind and ocean surface wave conditions as measured by a freely drifting vector sensor

During an experiment in 1.3-km-deep water in the Southern California Bight, the wind speed decreased from about 7 to 1 m/s over a 10-h period, and then increased to 9 m/s over the next 8-h period. Measurements made by a neutrally buoyant, freely drifting vector sensor at 920-m depth show that the corresponding changes in the potential and kinetic energy density levels of the infrasonic sound field have a frequency-dependent delay. The levels in a 0.1-Hz-wide bin at 3 Hz respond to wind-speed changes with approximately a 2-h delay, and the delay in response increases with decreasing frequency at a rate of about 1 h per hertz down to 1 Hz. The corresponding evolution in directionality of the active and reactive acoustic intensity vectors in these same frequency bins and their relationship to the surrounding bathymetry will be presented. The directional spectra of the ocean surface wave field during the experiment were reconstructed using back-refraction modeling with wave data from shore stations and offshore platforms archived by the Coastal Data Information Program to obtain estimates of the temporal variability of the nonlinear wave-wave interactions generating the ocean infrasound. [Work supported by ONR.]

Sub-surface crack in an inhomogeneous half-plane: Wave scattering phenomena by BEM

Elastic wave propagation in a cracked inhomogeneous half-plane is studied herein with the aid of the Boundary Element Method (BEM). More specifically, inhomogeneity arises in the half-plane because of spatial dependence of its material parameters on the depth coordinate. Furthermore, conditions of plane strain are assumed to hold, while the external load is an incident time-harmonic pressure (P) wave. More specifically, inhomogeneity is restricted to the case where both shear modulus and density profiles are quadratic functions of depth, but vary proportionally to each other, while Poisson's ratio is fixed at one-quarter. This way, body wave speeds remain macroscopically constant and it becomes possible to recover appropriate fundamental solution using a functional transformation method in conjunction with the Radon transform. Subsequently, a non-hypersingular, traction-based BEM is developed for solution of this particular problem. The usual quadratic boundary elements are used for discretization of all surfaces, supplemented by special edge-type boundary elements to model crack-tips. The present methodology is validated against standard examples appearing in the literature, and is subsequently used to study boundary-value problems (BVP) involving a sub-surface crack in an inhomogeneous half-plane swept by time-harmonic P-waves. The results of this detailed parametric study demonstrate that surface wave-fields are sensitive to the degree of material inhomogeneity, to the characteristics of the incident wave and especially to the relative position of crack versus free-surface.

Temporal and spatial variability of fecal indicator bacteria in the surf zone off Huntington Beach, CA

Fecal indicator bacteria concentrations measured in the surf zone off Huntington Beach, CA from July 1998-December 2001 were analyzed with respect to their spatial patterns along 23 km of beach, and temporal variability on time scales from hourly to fortnightly. The majority of samples had bacterial concentrations less than, or equal to, the minimum detection limit, but a small percentage exceeded the California recreational water standards. Areas where coliform bacteria exceeded standards were more prevalent north of the Santa Ana River, whereas enterococci exceedances covered a broad area both north and south of the river. Higher concentrations of bacteria were associated with spring tides. No temporal correspondence was found between these bacterial events and either the timing of cold water pulses near shore due to internal tides, or the presence of southerly swell in the surface wave field. All three fecal indicator bacteria exhibited a diel cycle, but enterococci rebounded to high nighttime values almost as soon as the sun went down, whereas coliform levels were highest near the nighttime low tide, which was also the lower low tide.

Automated trinocular stereo imaging system for three-dimensional surface wave measurements

A novel automated trinocular stereo imaging system (ATSIS) is developed for non-intrusively measuring the temporal evolution of three-dimensional wave characteristics. The system consists of three progressive digital cameras to provide three independent stereo-pairs, i.e. left–right, left–center, and center–right, for accurately estimating depth of a scene. A third camera assists to resolve correspondence problems due to specular reflection on the water surface and provides additional constraints on image matching, dramatically reducing the chance of a mismatch. An oblique configuration for the trinocular system effectively increases spatial coverage, allowing observations of wave phenomena over a broad range of spatial scales. The height resolution is increased with the optical axes of the cameras pointed at an oblique angle with respect to vertical surface wave displacements. A new exterior calibration procedure is developed in this paper to determine the orientation of cameras in the field. Field experiments demonstrate that ATSIS can robustly measure hundreds of matched image points in seconds, allowing fast extraction of the temporal evolution of a three-dimensional surface wave field.

Directional Wavelet Analysis of Inhomogeneity in the Surface Wave Field from Aerial Laser Scanning Data

Abstract Modern measurement techniques such as aerial laser scanning allow for rapid determination of the spatial variation of sea surface elevation. Wave fields obtained from such data show spatial inhomogeneity associated with the presence of wave groups. A method based on two-dimensional directional wavelet analysis is described by which such inhomogeneity can be characterized in the spatial and wavenumber domains. The directional wavelet method has been applied to aerial laser scanning measurements of nearshore wave conditions off the east coast of New Zealand’s South Island. A high level of spatial variability was observed, with evidence of ensembles of wave-group envelopes of quasi-Gaussian form. These envelopes occur, with variations in spatial location, across a range of wavelet scales and directions.

Shorted elliptical patch antennas with reduced surface waves on two frequency bands

In this paper, a study on the surface waves emission of a shorted annular elliptical patch (SAEP) antenna is presented. This radiator is constituted of an annular elliptical patch having the inner border shorted to the ground plane. The surface wave field excited by the SAEP antenna has been evaluated considering an equivalent magnetic current distribution flowing onto the outer boundary of the patch. It will be demonstrated that the dimensions of the outer elliptical contour can be chosen in such a way that the excitation of the TM/sub 0/ surface wave mode is suppressed on two frequencies. This characteristic makes possible the design of SAEP antennas with reduced surface wave (RSW) emissions on two frequency bands. To validate this result both numerical simulations and measurements will be presented. In particular, it will be shown that when the RSW-SAEP is considered the field strength within the substrate undergoes a faster decay resulting in a reduced mutual coupling in dual band array applications. Furthermore, the effective reduction of the surface waves power minimizes the scattering from the ground plane edges leading in smoother radiation patterns.

Simulation of geometrical effects on surface wave discharges

Surface wave discharges (SWDs) are sustained by electromagnetic waves propagating along the plasma surface. We present results of a two-dimensional model, based on the self-consistent numerical solution of Maxwell's equations for the surface wave fields and a set of fluid equations for the plasma, that explain observed effects of the SWD geometry.