Ocean Wave Measurements Research Articles

Study on the wave climate variation to the renewable wave energy assessment

To reduce the dependence on fossil fuel and imported energy resources, Taiwan has ever-increasing needs of renewable energy. With the rapid development of the technologies of wave energy converter, the wave energy source will be able to meet parts the demand. The Energy Research Laboratories of the Industrial Technology Research Institute, Taiwan (2005), based on the statistic of one-year wave data, stated that the mean wave energy at the northeast coast of Taiwan reaches 11.56 kW/m, giving it the potential of wave power utilization. However, one of the major obstacles with the wave energy utilization is lack of long-term ocean wave measurements. The long-term variations in wave parameters impose changes in wave energy converter outputs. Lack of long-term data makes it difficult to assess the cost-benefit of wave energy conversion projects for the policy and decision makers. The present study aims to quantitatively evaluate the wave climate variations of the northwestern Pacific and the Taiwan Waters based on long-term wave data base. Wave observations around Taiwan have been performed since 1998, thus, earlier data of wave climate are not available. This study reconstructs the wave data of the northwest Pacific over the past three decades based on the SWAN numerical wave model that driven by NECP global reanalysis wind fields. The simulation results are compared and validated with measured data. The results show that the long-term wave climate variations around Taiwan consist of oscillations of three different periods, i.e. the seasonal, inter-annual and decade oscillations. The seasonal oscillation has significant amplitude that leads the wave energy one order magnitude greater in winter than in summer. In addition to seasonal changes, the wave energy features inter-annual variations, which are highly related to the El Nino and La Nina phenomena. In the La Nina years, the annual averaged wave energy could be double than in El Nino years. Finally, this study adopted the Man-Kendall Non-Parametric Test and the Hilbert Huang EMD method to analyze the long-term wave variation trends. The results showed that the wave height experienced climbing trends during 1976–1985 and 1997–2006, and a descending trend during 1985–1997. The reasons for wave climate oscillations in the decadal variation should be further investigation.

Wave Measurements Using GPS Velocity Signals

This study presents the idea of using GPS-output velocity signals to obtain wave measurement data. The application of the transformation from a velocity spectrum to a displacement spectrum in conjunction with the directional wave spectral theory are the core concepts in this study. Laboratory experiments were conducted to verify the accuracy of the inversed displacement of the surface of the sea. A GPS device was installed on a moored accelerometer buoy to verify the GPS-derived wave parameters. It was determined that loss or drifting of the GPS signal, as well as energy spikes occurring in the low frequency band led to erroneous measurements. Through the application of moving average skill and a process of frequency cut-off to the GPS output velocity, correlations between GPS-derived, and accelerometer buoy-measured significant wave heights and periods were both improved to 0.95. The GPS-derived one-dimensional and directional wave spectra were in agreement with the measurements. Despite the direction verification showing a 10° bias, this exercise still provided useful information with sufficient accuracy for a number of specific purposes. The results presented in this study indicate that using GPS output velocity is a reasonable alternative for the measurement of ocean waves.

Estimation of Maximum Wave Height of Observation Data and its Application to Freak Wave Prediction

A linear envelope assumption and quasi-resonant four-wave interactions for deep-water random waves may influence the statistical properties of deep-water surface gravity waves. As a consequence, the exceedance probability of wave height substantially deviates from the expected distribution obtained by assuming linear narrow banded spectrum waves. Here the occurrence probability of maximum wave heights is compared with ocean wave measurements during the yearsd 2001-2005 around Japan. The maximum wave height distribution and probability of occurrence of freak waves are compared with theoretical expectations and are adjusted by the measured data. The proposed distribution well predicts extreme waves in nonlinear random wave fields.

Validation of RADARSAT‐2 fully polarimetric SAR measurements of ocean surface waves

C band RADARSAT‐2 fully polarimetric (fine quad‐polarization mode, HH+VV+HV+VH) synthetic aperture radar (SAR) images are used to validate ocean surface waves measurements using the polarimetric SAR wave retrieval algorithm, without estimating the complex hydrodynamic modulation transfer function, even under large radar incidence angles. The linearly polarized radar backscatter cross sections (RBCS) are first calculated with the copolarization (HH, VV) and cross‐polarization (HV, VH) RBCS and the polarization orientation angle. Subsequently, in the azimuth direction, the vertically and linearly polarized RBCS are used to measure the wave slopes. In the range direction, we combine horizontally and vertically polarized RBCS to estimate wave slopes. Taken together, wave slope spectra can be derived using estimated wave slopes in azimuth and range directions. Wave parameters extracted from the resultant wave slope spectra are validated with colocated National Data Buoy Center (NDBC) buoy measurements (wave periods, wavelengths, wave directions, and significant wave heights) and are shown to be in good agreement.

Ice mélange dynamics and implications for terminus stability, Jakobshavn Isbræ, Greenland

We used time‐lapse imagery, seismic and audio recordings, iceberg and glacier velocities, ocean wave measurements, and simple theoretical considerations to investigate the interactions between Jakobshavn Isbræ and its proglacial ice mélange. The mélange behaves as a weak, granular ice shelf whose rheology varies seasonally. Sea ice growth in winter stiffens the mélange matrix by binding iceberg clasts together, ultimately preventing the calving of full‐glacier‐thickness icebergs (the dominant style of calving) and enabling a several kilometer terminus advance. Each summer the mélange weakens and the terminus retreats. The mélange remains strong enough, however, to be largely unaffected by ocean currents (except during calving events) and to influence the timing and sequence of calving events. Furthermore, motion of the mélange is highly episodic: between calving events, including the entire winter, it is pushed down fjord by the advancing terminus (at ∼40 m d−1), whereas during calving events it can move in excess of 50 × 103 m d−1 for more than 10 min. By influencing the timing of calving events, the mélange contributes to the glacier's several kilometer seasonal advance and retreat; the associated geometric changes of the terminus area affect glacier flow. Furthermore, a force balance analysis shows that large‐scale calving is only possible from a terminus that is near floatation, especially in the presence of a resistive ice mélange. The net annual retreat of the glacier is therefore limited by its proximity to floatation, potentially providing a physical mechanism for a previously described near‐floatation criterion for calving.

Observing Directional Properties of Ocean Swell with an Acoustic Doppler Current Profiler (ADCP)

Abstract Acoustic Doppler current profilers (ADCPs) are widely used for routine measurements of ocean currents and waves in coastal environments. These instruments have the basic capability to measure surface wave frequency–directional spectra, but the quality of the estimates is not well understood because of the relatively high noise levels in the velocity measurements. In this study, wave data are evaluated from two 600-kHz ADCP instruments deployed at 20- and 45-m depths on the Southern California continental shelf. A simple parametric estimation technique is presented that provides robust estimates of the gross directional wave properties, even when the data quality is marginal, as was often the case in this benign wave environment. Good agreement of mean direction and (to a lesser degree) directional spreading estimates with measurements from a nearby surface-following buoy confirms that reliable wave information can generally be extracted from ADCP measurements on the continental shelf, supporting the instrument’s suitability for routine wave-monitoring applications.

Wave measurements by pressure transducers using artificial neural networks

Underwater ultrasonic acoustic transducers are frequently used in ocean wave measurements as they measure surface level using acoustic waves. However, their effectiveness can be severely affected in rough sea conditions, when bubbles generated by breaking waves interfere with their acoustic signals. When the seas are rough, one therefore often has to rely on a pressure transducer, which is generally used as a back-up for the acoustic wave gauge. A pressure transfer function is then used to obtain the surface wave information. Alternatively, the present study employed an artificial neural network to convert the pressure signal into significant wave height, significant wave period, maximum wave height, and spectral peakedness parameter using data obtained from various water depths. The results showed that, for water depths greater than 20 m, the wave parameters obtained from the artificial neural network were significantly closer to those obtained by the acoustic measurements than those obtained by using a linear pressure transfer function. Moreover, for a given water depth, the wave heights estimated by the network model from pressure data were not as good as those estimated by linear wave theory for large wave heights (above a 4 m significant wave height in this study). This can be improved if the training data set has more records with large wave heights.

Ocean wave measurements based on satellite synthetic aperture radar (SAR) and numerical wave model (WAM) data – extreme sea state and cross sea analysis

In the present paper new algorithms to derive sea state developed at the German Aerospace Center are briefly introduced and used to determine SAR derived spectral sea state and individual parameters. As an example of SAR measurements for a severe North Pacific storm, which produced significant wave height (SWH) above 10 m are compared to ECMWF model results and Radar Altimeter. The results are intercompared and validated. Further an example of a cross sea is analyzed. It occurred in the South East Pacific and was captured clearly by consecutive ERS-2 SAR wave mode images acquired on August 10, 2000. The WAM model spectra, SAR cross spectra and nonlinear SAR retrieved spectra together with derived integral parameters of the spectra are used to analyze the generation and development of the cross sea. The swell dissipation is observed to increase in the cross sea.

HF Radar Sea-echo from Shallow Water.

HF radar systems are widely and routinely used for the measurement of ocean surface currents and waves. Analysis methods presently in use are based on the assumption of infinite water depth, and may therefore be inadequate close to shore where the radar echo is strongest. In this paper, we treat the situation when the radar echo is returned from ocean waves that interact with the ocean floor. Simulations are described which demonstrate the effect of shallow water on radar sea-echo. These are used to investigate limits on the existing theory and to define water depths at which shallow-water effects become significant. The second-order spectral energy increases relative to the first-order as the water depth decreases, resulting in spectral saturation when the waveheight exceeds a limit defined by the radar transmit frequency. This effect is particularly marked for lower radar transmit frequencies. The saturation limit on waveheight is less for shallow water. Shallow water affects second-order spectra (which gives wave information) far more than first-order (which gives information on current velocities), the latter being significantly affected only for the lowest radar transmit frequencies for extremely shallow water. We describe analysis of radar echo from shallow water measured by a Rutgers University HF radar system to give ocean wave spectral estimates. Radar-derived wave height, period and direction are compared with simultaneous shallow-water in-situ measurements.

Nonintrusive Measurement of Ocean Waves: Lidar Wave Gauge

Abstract In December 1999, a nonintrusive directional lidar wave gauge (LWG) was field tested at the Field Research Facility (FRF) in North Carolina. The LWG uses proven lidar technology to directly measure water surface elevation from above the water’s surface. Therefore, unlike bottom-mounted gauges, the surface elevation measurement from the LWG is independent of other hydrodynamic processes. The LWG prototype consists of four rangefinders, where each collects water surface elevation time series at a rate of 10 Hz. During the field test, ground truth data were collected concurrently with the LWG data using a biaxial current and pressure meter mounted directly beneath the LWG. Additional ground truth data were available from the existing FRF wave-gauging infrastructure. This paper describes principles of LWG operation and field experiment setup, LWG environmental and technical constraints, and LWG performance in measuring spectral wave parameters.

A fourth‐order compact finite volume scheme for fully nonlinear and weakly dispersive Boussinesq‐type equations. Part II: boundary conditions and validation

AbstractThis paper supplements the validation of the fourth‐order compact finite volume Boussinesq‐type model presented by Cienfuegos et al. (Int. J. Numer. Meth. Fluids 2006, in press). We discuss several issues related to the application of the model for realistic wave propagation problems where boundary conditions and uneven bathymetries must be considered. We implement a moving shoreline boundary condition following the lines given by Lynett et al. (Coastal Eng. 2002; 46:89–107), while an absorbing‐generating seaward boundary and an impermeable vertical wall boundary are approximated using a characteristic decomposition of the Serre equations. Using several benchmark tests, both numerical and experimental, we show that the new finite volume model is able to correctly describe nonlinear wave processes from shallow waters and up to wavelengths which correspond to the theoretical deep water limit. The results compare favourably with those reported using former fully nonlinear and weakly dispersive Boussinesq‐type solvers even when time integration is conducted with Courant numbers greater than 1.0. Furthermore, excellent nonlinear performance is observed when numerical computations are compared with several experimental tests on solitary waves shoaling over planar beaches up to breaking. A preliminary test including the wave‐breaking parameterization described by Cienfuegos (Fifth International Symposium on Ocean Wave Measurement Analysis, Madrid, Spain, 2005) shows that the Boussinesq model can be extended to deal with surf zone waves. Finally, practical aspects related to the application of a high‐order implicit filter as given by Gaitonde et al. (Int. J. Numer. Methods Engng 1999; 45:1849–1869) to damp out unphysical wavelengths, and the numerical robustness of the finite volume scheme are also discussed. Copyright © 2006 John Wiley & Sons, Ltd.

Doppler frequency shift in ocean wave measurements: Frequency downshift of a fixed spectral wave number component by advection of wave orbital velocity

Because the Jacobian connecting wave number to frequency conversion is nonlinear, in the presence of background waves even without a mean current a wave number spectral component does not appear at the expected intrinsic frequency in the frequency spectrum measured by a stationary probe. The advection of the wave number component by the orbital current of background waves produces a net downshift in the encounter frequency. Comparison with laboratory measurements shows that the effect is important in low‐wind conditions. As wind speed increases, the frequency downshift due to orbital advection becomes less recognizable.

Simultaneous Measurement of Ocean Winds and Waves with an Airborne Coherent Real Aperture Radar

AbstractA coherent, X-band airborne radar has been developed to measure wind speed and direction simultaneously with directional wave spectra on the ocean. The coherent real aperture radar (CORAR) measures received power, mean Doppler shifts, and mean Doppler bandwidths from small-resolution cells on the ocean surface and converts them into measurements of winds and waves. The system operates with two sets of antennas, one rotating and one looking to the side of the airplane. The rotating antennas yield neutral wind vectors at a height of 10 m above the ocean surface using a scatterometer model function to relate measured cross sections to wind speed and direction. The side-looking antennas produce maps of normalized radar cross section and line-of-sight velocity from which directional ocean wave spectra may be obtained. Capabilities of CORAR for wind and wave measurement are illustrated using data taken during the Shoaling Waves Experiment (SHOWEX) sponsored by the Office of Naval Research. Wind vectors measured by CORAR agree well with those measured by nearby buoys. Directional wave spectra obtained by CORAR also agree with buoy measurements and illustrate that offshore winds can produce dominant waves at an angle to the wind vector that are in good agreement with the measurements. The best agreement is produced using the Joint North Sea Wave Project (JONSWAP) parameterizations of the development of wave height and period with fetch.

Factors Affecting the Accuracy of SHOWEX HF Radar Wave Measurements

Abstract Ocean Surface Current Radar (OSCR) HF radar measurements of ocean waves and currents were made during the Shoaling Waves Experiment (SHOWEX) in the fall of 1999. During some periods, at some locations, good quality wave measurements were obtained. Limitations in the wave measurement capability due to OSCR hardware, deployment configuration, signal-to-noise ratio, and antenna sidelobes are identified and discussed. A short period of large currents in the presence of antenna pattern distortion is identified as the source of the main errors in the wave measurements.

Improved joint probability distribution for ocean wave heights and periods

Expressions have been derived, from the standpoint of Gaussian random theory, for the joint and marginal distributions of wave envelope amplitude and local wave period when the sampling frequency is equal to the local wave frequency. The new marginal p.d.f. for wave envelope amplitudes shows substantial non-zero probability density at very small amplitudes. The new joint and marginal distributions are found to compare favourably with data obtained from both high frequency measurements of real ocean waves in extreme storms and from measurements of numerical simulations of moderately broadbanded processes with Pierson-Moskowitz spectra. The new p.d.f. of wave envelope amplitudes is found to provide a better approximation to the p.d.f.s of the simulated zero down-crossing wave amplitude than either the traditional Rayleigh p.d.f., applicable for infinitesimal bandwidth; or the narrow bandwidth approximation given by M. S. Longuet-Higgins (Proc. R. Soc. Lond. A, 389: 241-258, 1983). The reason for this improvement is that our method takes into account that small waves are likely to have shorter periods than large waves, rather than assuming a constant wave period. There are, however, limitations to the approach adopted which assumes that the individual wave amplitudes can be obtained from the amplitude of the wave envelope. These limitations become more severe as the bandwidth increases. The results obtained apply not only to sea waves, but to any Gaussian linear random process.

Measurements of ocean surface waves and currents using l- and c-band along-track interferometric sar

Along-track interferometric synthetic aperture radar (ATI-SAR) is an active coherent imaging system, utilizing two antennas separated along the platform flight direction. The phase information of ATI-SAR from the Doppler shift of the backscattered signal represents the line-of-sight velocity of the water scatterers. While the advent of ATI-SAR provided us with a potentially powerful technique for ocean surface current and wave mapping, the surface current has not been measured exactly from the ATI-SAR velocity because the Doppler shift is not simply proportional to the component of the mean surface current. It also includes other types of contributions associated with the phase velocity of the Bragg waves and orbital motions of all ocean waves that are longer than Bragg waves. In this paper, we review how the phase difference measured by ATI-SAR is related to the mean Doppler frequency, and we develop a new and practically useful method to extract the surface current component utilizing simultaneously measured L- and C-band ATI-SAR data. Since the measured ATI-SAR velocity shows a different value at a different radar-frequency, we investigate the influence of Bragg-resonant waves and long ocean wave motions on the ATI-SAR velocity according to the radar frequency. The Bragg-wave phase velocity component, which is a significant source of error for extracting the surface current, can be effectively eliminated by using L- and C-band ATI-SAR. The method is applied to L- and C-band ATI-SAR measurements acquired at the Ulsan coast in the southeastern part of the Korean peninsula. The resulting ocean surface current vectors are compared with in situ measurements collected by recording current meter. We furthermore extract ocean surface wave information from the ATI-SAR phase image using a quasi-linear transform.

Simultaneous Ocean Wave Measurements by the Jason and Topex Satellites, with Buoy and Model ComparisonsSpecial Issue: Jason-1 Calibration/Validation

The verification phase of the Jason-1 satellite altimeter mission presents a unique opportunity for comparing near-simultaneous, independent satellite measurements. Here we examine simultaneous significant wave height measurements by the Jason-1 and TOPEX/Poseidon altimeters. These data are also compared with in situ measurements from deep-ocean buoys and with predicted wave heights from the Wave Watch III operational model. The rms difference between Jason and TOPEX wave heights is 28 cm, and this can be lowered by half through improved outlier editing and filtering of high-frequency noise. Noise is slightly larger in the Jason dataset, exceeding TOPEX by about 7 cm rms at frequencies above 0.05 Hz, which is the frequency at which the coherence between TOPEX and Jason measurements drops to zero. Jason wave heights are more prone to outliers, especially during periods of moderate to high backscatter. Buoy comparisons confirm previous reports that TOPEX wave heights are roughly 5% smaller than buoy measurements for waves between 2 and 5 m; Jason heights in general are 3% smaller than TOPEX. Spurious dips in the TOPEX density function for 3- and 6-m waves, a problem that has existed since the beginning of the mission, can be solved by waveform retracking.

A linear functional relationship model for circular data with an application to the assessment of ocean wave measurements

This article suggests a linear functional relationship model for comparing two sets of circular data subject to unobservable errors. Unlike the corresponding and relatively well-studied model for linear data, maximum likelihood estimation for this model is very complicated and no explicit solutions are possible. Using a numerical approximation, we are able to solve the likelihood equations approximately, and to obtain good approximations to the likelihood estimates of the parameters. The quality of our estimates and the feasibility of the estimation method are illustrated via simulation. By establishing a parallel with the model for linear data, we are able to explain the various problems occurring in the process of estimation and to substantiate our numerical results. The interest in the model arose in connection with the study of ocean wave data; an application to such data is also given.

A Linear Functional Relationship Model for Circular Data with an Application to the Assessment of Ocean Wave Measurements

A Linear Functional Relationship Model for Circular Data with an Application to the Assessment of Ocean Wave Measurements

Estimation of wind friction velocity and direction at the ocean surface from physical models and space‐borne radar scatterometer measurements

This paper presents a new technique for estimating the wind friction velocity at the ocean surface from C‐band radar scatterometer measurements. This technique uses physical models of ocean surface waves and electromagnetic backscattering from a rough surface at intermediate angles of incidence to generate predictions of the normalized radar cross section (NRCS, or σ0) of the ocean surface for a given wind friction velocity and observational geometry. The ocean spectral model used in this technique has been developed specifically for this application. It combines in situ wave measurements at low wave numbers with the Phillips [1985] equilibrium spectral model. This choice of ocean wave model is supported by a set of open ocean wave measurements summarized in this paper. A suite of models, derived from both in situ and remote measurements of the sea surface, is used to characterize the directional spreading of ocean waves relative to the wind direction. The resulting two‐dimensional ocean wave spectra are used with a composite surface model to predict radar backscattering from the ocean surface at C‐band. These radar cross‐section predictions are combined with ERS‐1 scatterometer measurements in a cost function minimization scheme to yield estimates of the friction velocity vector at the ocean surface. We present examples of this technique and compare friction velocity retrievals obtained via this scheme with buoy‐based measurements under a variety of wind and wave conditions. On the basis of the analysis of a limited number of cases, this technique yielded friction velocity estimates for which the magnitude was within 22% and the direction was within ±25°. Given that scientific applications require magnitude estimates within 10–15% and directional estimates within ±20° of in situ measurements, these preliminary results suggest that this is a promising approach to wind retrieval.