Showing papers on "Breaking wave published in 2005"

Spectral wave dissipation over a barrier reef

Abstract: [1] A 2 week field experiment was conducted to measure surface wave dissipation on a barrier reef at Kaneohe Bay, Oahu, Hawaii. Wave heights and velocities were measured at several locations on the fore reef and the reef flat, which were used to estimate rates of dissipation by wave breaking and bottom friction. Dissipation on the reef flat was found to be dominated by friction at rates that are significantly larger than those typically observed at sandy beach sites. This is attributed to the rough surface generated by the reef organisms, which makes the reef highly efficient at dissipating energy by bottom friction. Results were compared to a spectral wave friction model, which showed that the variation in frictional dissipation among the different frequency components could be described using a single hydraulic roughness length scale. Surveys of the bottom roughness conducted on the reef flat showed that this hydraulic roughness length was comparable to the physical roughness measured at this site. On the fore reef, dissipation was due to the combined effect of frictional dissipation and wave breaking. However, in this region the magnitude of dissipation by bottom friction was comparable to wave breaking, despite the existence of a well-defined surf zone there. Under typical wave conditions the bulk of the total wave energy incident on Kaneohe Bay is dissipated by bottom friction, not wave breaking, as is often assumed for sandy beach sites and other coral reefs.

Introduction to nearshore hydrodynamics

Abstract: Hydrodynamical Background Theory of Linear Waves Energy Balance for Non-Breaking and Breaking Waves Wave Breaking Wave Models Based on Linear Wave Theory Nonlinear Waves: Analysis of Parameters Stokes Wave Theory, Stream Function Theory Long Wave Theory, Boussinesq Waves Wave Boundary Layers The Equations for Nearshore Circulation Cross-Shore Currents, Undertow Quasi-3D Nearshore Circulation Models Other Nearshore Phenomena.

Characteristics of the Atlantic Storm-track Eddy Activity and its Relation With the North Atlantic Oscillation

Abstract: This study focuses on feedbacks of the high-frequency eddy activity onto the quasi-stationary circulation, particularly with regard to the North Atlantic Oscillation (NAO). The methodology consists of analyzing NCEP–NCAR reanalysis data and sensitivity runs from a high-resolution nonhydrostatic regional model. Consistent with recent studies, results show that the jet displacement characteristic of the NAO phenomenon depends strongly on the dynamics of the synoptic-scale waves and the way they break. Positive and negative phases of the NAO are closely related to anticyclonic and cyclonic wave breaking, respectively. Indeed, the high-frequency momentum flux whose sign is directly related to the type of wave breaking is correlated with the NAO index over the Atlantic. The peak of the momentum flux signal precedes that of the NAO by a few days suggesting that wave breaking is triggering NAO events. Two examples illustrate the significant impact of single storms, in particular those occurring in the e...

The degeneration of internal waves in lakes with sloping topography

Abstract: In a laboratory study, we quantified the temporal energy flux associated with the degeneration of basin-scale internal waves in closed basins. The system is two-layer stratified and subjected to a single forcing event creating available potential energy at time zero. A downscale energy transfer was observed from the wind-forced basin-scale motions to the turbulent motions, where energy was lost due to high-frequency internal wave breaking along sloping topography. Under moderate forcing conditions, steepening of nonlinear basin-scale wave components was found to produce a high-frequency solitary wave packet that contained as much as 20% of the available potential energy introduced by the initial condition. The characteristic lengthscale of a particular solitary wave was less than the characteristic slope length, leading to wave breaking along the sloping boundary. The ratio of the steepening timescale required for the evolution of the solitary waves to the travel time until the waves shoaled controlled their development and degeneration within the domain. The energy loss along the slope, the mixing efficiency, and the breaker type were modeled using appropriate forms of an internal Iribarren number, defined as the ratio of the boundary slope to the wave slope (amplitude/wavelength). This parameter allows generalization to the oceanographic context. Analysis of field data shows the portion of the internal wave spectrum for lakes, between motions at the basin and buoyancy scales, to be composed of progressive waves: both weakly nonlinear waves (sinusoidal profile with frequencies near 10 24 Hz) and strongly nonlinear waves (hyperbolic‐secant-squared profile with frequencies near 10 23 Hz). The results suggest that a periodically forced system may sustain a quasi-steady flux of 20% of the potential energy introduced by the surface wind stress to the benthic boundary layer at the depth of the pycnocline.

On radar imaging of current features: 1. Model and comparison with observations

Abstract: [1] A new radar imaging model of ocean current features is proposed. The simulated normalized radar cross section (NRCS) takes into account scattering from ‘‘regular’’ surfaces (by means of resonant Bragg scattering and specular reflections) and scattering from breaking waves. The description of background wind waves and their transformation in nonuniform medium is based on solution of the wave action conservation equation. Wave breaking plays a key role in the radar imaging model. Breaking waves scatter radio waves (thus directly contributing to the NRCS), provide energy dissipation in wind waves (thus defining the wave spectrum of intermediate scale waves), and generate short surface waves (thus affecting Bragg scattering). Surface current, surfactants accumulated in the convergence zone, and varying wind field are considered as the main sources for the NRCS manifestations of current features. The latter source can result from transformation of atmospheric boundary layer over the sea surface temperature front. It is shown that modulation of wave breaking significantly influences both radar returns and short wind waves. In the range of short gravity waves related to Ku- X-, and C-bands, the modulation of Bragg waves through wave breaking is the governing mechanism. The model is tested against well-controlled experiments including JOWIP, SARSEX, and CoastWatch-95. A reasonably good agreement between model and observations is obtained.

Parametrization of gas transfer velocities and sea‐state‐dependent wave breaking

Abstract: Both experimental estimates and different parametrizations of the transfer velocity of poorly soluble gases exhibit a very broad range of values at a given wind speed. Transfer velocities also appear to depend non-linearly on wind speed, and for high wind speeds this non-linearity is widely attributed to the influence of wave breaking. Both theoretical and experimental studies suggest that wave breaking, and associated whitecapping, is not simply dependent on wind speed but depends also on sea state. New parametrizations of gas transfer velocity based on an existing model of the dependence of transfer velocity on wind stress and whitecapping, supplemented by two sea-state-dependent parametrizations of whitecapping, are developed. These new models predict a diversity of transfer velocities at a given wind speed comparable to the diversity of existing parametrizations. Further, the results suggest that some of the existing parametrizations of transfer velocity reflect in part the wind fetch and sea state typical of the experiments used as a basis of the parametrization. It is suggested that transfer velocities may be estimated much more accurately through satellite retrieval of both wind speed and significant wave height than by wind speed alone.

Three-dimensional vortex structures under breaking waves

Abstract: The large-scale vortex structures under spilling and plunging breakers are investigated, using a fully three-dimensional large-eddy simulation (LES). When an overturning jet projecting from the crest in a breaking wave rebounds from the water surface ahead, the vorticity becomes unstable in a saddle region of strain between the rebounding jet and a primary spanwise vortex, resulting in spanwise undulations of the vorticity. The undulations are amplified on a braid in this saddle region, leading to a vortex loop with counter-rotating vorticity. This vortex loop consequently envelops adjacent primary vortices, to form a typical rib structure. This rib component (the stretched vortex loop) in the large-scale vortex structure, which intensifies in the strains associated with the multiple primary vortices generated throughout the splash-up cycle, appears to be the previously found obliquely descending eddy.

Modulation instability of Stokes wave → freak wave

Abstract: Formation of waves of large amplitude (freak waves, killer waves) at the surface of the ocean is studied numerically. We have observed that freak waves have the same ratio of the wave height to the wave length as limiting Stokes waves. When a freak wave reaches this limiting state, it breaks. The physical mechanism of freak wave formation is discussed.

Use of bubble image velocimetry for measurement of plunging wave impinging on structure and associated greenwater

Abstract: The measurement of velocity fields of a plunging wave impacting on a structure in a two-dimensional wave tank was investigated experimentally. As the wave impinged and overtopped the structure, a large highly aerated region was created in front of the structure and on top of the structure. The broken wave in front of the structure and associated greenwater on top of the structure are highly aerated containing not only a large number of bubbles but also very large sizes of bubbles. The highly aerated bubbly flow caused the traditional particle image velocimetry (PIV) technique to fail due to the uncontrollable scattering of laser light. A modified PIV method, called bubble image velocimetry (BIV), was introduced by directly using bubbles as the tracer and measuring the bubble velocity by correlating the 'texture' of the bubble images. No laser light sheet was needed while the depth of field was limited to minimize the error. Velocity measurements using BIV and fibre optic reflectometer were compared to validate the BIV technique. While the fluid velocity in the region where no or few bubbles exist can be successfully obtained using PIV, the velocity in the high void fraction region can be measured using BIV. Therefore, BIV can be seen as a complementary technique for PIV. The use of BIV is essential in the studied problem here due to the fact that in the vicinity of the structure the flow is almost entirely bubbly flow. From both the PIV and BIV measurements, it was found that the maximum fluid particle velocity as well as the bubble velocity in front of the structure during the impinging process is about 1.5 times the phase speed of the waves.

Large-Amplitude Mountain Wave Breaking over Greenland

Abstract: A large-amplitude mountain wave generated by strong southwesterly flow over southern Greenland was observed during the Fronts and Atlantic Storm-Track Experiment (FASTEX) on 29 January 1997 by the NOAA G-IV research aircraft. Dropwindsondes deployed every 50 km and flight level data depict a vertically propagating large-amplitude wave with deep convectively unstable layers, potential temperature perturbations of 25 K that deformed the tropopause and lower stratosphere, and a vertical velocity maximum of nearly 10 m s−1 in the stratosphere. The wave breaking was associated with a large vertical flux of horizontal momentum and dominated by quasi-isotropic turbulence. The Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) nonhydrostatic model with four-nested grid meshes with a minimum resolution of 1.7 km accurately simulates the amplitude, location, and timing of the mountain wave and turbulent breakdown. Finescale low-velocity plumes that resemble wakelike structures emanate from highl...

The energetics of large-scale internal wave degeneration in lakes

Abstract: Field observations in lakes, where the effects of the Earth's rotation can be neglected, suggest that the basin-scale internal wave field may be decomposed into a standing seiche, a progressive nonlinear surge and a dispersive solitary wave packet. In this study we use laboratory experiments to quantify the temporal energy distribution and flux between these three component internal wave modes. The system is subjected to a single forcing event creating available potential energy at time zero (APE). During the first horizontal mode one basin-scale wave period (T i ), as much as 10 % and 20 % of the APE may be found in the solitary waves and surge, respectively. The remainder is contained in the horizontal mode one seiche or lost to viscous dissipation. These findings suggest that linear analytical solutions, which consider only basin-scale wave motions, may significantly underestimate the total energy contained in the internal wave field. Furthermore, linear theories prohibit the development of the progressive nonlinear surge, which serves as a vital link between basin-scale and sub-basin-scale motions. The surge receives up to 20 % of the APE during a nonlinear steepening phase and, in turn, conveys this energy to the smaller-scale solitary waves as dispersion becomes significant. This temporal energy flux may be quantified in terms of the ratio of the linear and nonlinear terms in the nonlinear non-dispersive wave equation. Through estimation of the viscous energy loss, it was established that all inter-modal energy flux occurred before 2T i ; the modes being independently damped thereafter. The solitary wave energy remained available to propagate to the basin perimeter, where although it is beyond the scope of this study, wave breaking is expected. These results suggest that a periodically forced system with sloping topography, such as a typical lake, may sustain a quasi-steady flux of 20 % of APE to the benthic boundary layer at the depth of the metalimnion.

On radar imaging of current features: 2. Mesoscale eddy and current front detection

Abstract: [1] The surface signatures of meandering fronts and eddies have been regularly observed and documented in synthetic aperture radar (SAR) images. Wave-current interactions, the suppression of short wind waves by natural film, and the varying wind field resulting from atmospheric boundary layer changes across an oceanic temperature front all contribute to the radar image manifestation of such mesoscale features. The corresponding imaging mechanisms are quantitatively explored using a new radar imaging model (Kudryavtsev et al., 2005) that solves the energy balance equation where wind forcing, viscous and wave breaking dissipation, wave-wave interactions, and generation of short waves by breaking waves are taken into account. High-quality and synoptic in situ observations of the surface conditions should ideally be used in this model. However, such data are rarely available. Instead, the fields of temperature and ocean current are herein derived from two distinct numerical ocean models. SAR image expressions of current fronts and eddies are then simulated based on these fields. The comparison of simulated images with European Remote Sensing (ERS) SAR and Envisat advanced SAR (ASAR) images is favorable. We consequently believe that the new radar imaging model provides promising capabilities for advancing the quantitative interpretation of current features manifested in SAR images.

Lagrangian particle method for simulation of wave overtopping on a vertical seawall

Abstract: A particle method, or a gridless Lagrangian method, shows the high performance in describing the complicated behavior of water surface with the fragmentation and coalescence of water. In this paper...

Nearshore subtidal bathymetry from time-exposure video images

Abstract: Time-averaged (over many wave periods) nearshore video observations show the process of wave breaking as one or more white alongshore bands of high intensity. Across a known depth profile, similar bands of dissipation can be predicted with a model describing the time-averaged cross-shore evolution of organized wave and roller energy. This close correspondence between observed and modeled dissipation proxies is used to develop a new remote sensing technique, termed Subtidal Beach Mapper (SBM), to estimate nearshore bathymetry. SBM operates on a time series of cross-shore intensity profiles to resolve the pattern in depth change on a morphological timescale (including overall gain or loss of sediment) rather than to focus on the particular change induced by a single intensity profile. From each intensity profile, the breaking-induced component is isolated by removing the contribution of background illumination and persistent foam. The depth profile is updated based on a comparison between this video-derived dissipation proxy and a cross-shore profile of the dissipation of the roller energy. This updating is implemented through time-dependent mass balance equations for the seabed and a buffer layer above the bed. SBM was tested using 1 year of hourly video data collected at Egmond aan Zee, Netherlands. The dominant morphological changes observed from ground truth data were reproduced reasonably well, including the shoreward migration of the outer bar and the net sediment gain in the profile. Root-mean square differences between surveyed and SBM derived depth after 1 year of video-based depth updating with an average of about 70 intensity profiles per month were smallest (~0.2 m) on the inner bar and largest (~0.6 m) in the outer bar trough, with a profile average value of about 0.4 m. Despite the many processes included in SBM, the implementation of a heuristic scaling function in the mass balance equations to spatially adjust morphological growth rates was essential to these results, in particular near the shoreline, where otherwise the profile is prone to an unrealistic deepening.

Large-scale laboratory observations of turbulence on a fixed barred beach

Abstract: The details of a large-scale laboratory experiment to study the turbulence generated by waves breaking on a fixed barred beach are presented. The data set includes comprehensive measurements of free surface displacement and fluid velocity for one random and one regular wave case. Observations of the time-averaged turbulent kinetic energy per unit mass, , show that the turbulence generated by wave breaking was greatest at the bar crest and did not fully dissipate prior to reaching the bed. This indicates that, even in a time-averaged sense, wave breaking turbulence may be important for near-bed processes. Onshore of the bar, turbulence was generally confined to the upper part of the water column and had dissipated once the waves reformed (approximately 1.5 wavelengths onshore of the bar crest). The turbulent structure was the same in the random and regular wave cases; however, the magnitude of was much less in the random wave case, despite similar offshore wave conditions. Additionally, three methods were used to separate the wave-induced and turbulent components of velocity:. ensemble averaging, high-pass filtering and a differencing method proposed by Trowbridge (1998 J. Atmos. Ocean. Technol. 15 290–8). The magnitude of varied by as much as a factor of 5 among these methods, but qualitatively, the cross-shore and vertical structure were independent of the method used. The differencing method agreed closely with ensemble averaging in terms of the magnitude and structure of time-averaged quantities and in the signature of the time-dependent turbulent kinetic energy. Given this agreement, the differencing method appears to be the most suitable for application to random waves, such as those observed in the field.

The instability and breaking of long internal waves

Abstract: Laboratory experiments are carried out to determine the nature of internal wave breaking and the limiting wave steepness for progressive, periodic, lowest-mode internal waves in a two-layer, miscible density stratification. Shoaling effects are not considered. The waves investigated here are long relative to the thickness of the density interface separating the two fluid layers. Planar laser-induced fluoresence (PLIF) flow visualization shows that wave breaking most closely resembles a Kelvin–Helmholtz shear instability originating in the high-shear wave crest and trough regions. However, this instability is strongly temporally and spatially modified by the oscillations of the driving wave shear. Unlike a steady stratified shear layer, the wave instability discussed here is not governed by the canonical stability limit. Instead, the wave time scale (the time scale of the destabilizing shear) imposes an additional constraint on instability, lowering the critical Richardson number below 1/4. Experiments were carried out to quantify this instability threshold, and show that, for the range of wavenumbers considered in this study, the critical wave steepness at which the wave breaking occurs is wavenumber-dependent (unlike surface waves). The corresponding critical wave Richardson numbers at incipient wave breaking are well below 1/4, in consonance with a modified instability analysis based on results from stratified shear flow instability theory.

Wave-Follower Field Measurements of the Wind-Input Spectral Function. Part I: Measurements and Calibrations

Abstract: An experimental study of wind energy and momentum input into finite-depth wind waves was undertaken at Lake George, New South Wales, Australia. To measure microscale oscillations of induced pressure above surface waves, a high-precision wave-follower system was developed at the University of Miami, Florida. The principal sensing hardware included Elliott pressure probes, hot-film anemometers, and Pitot tubes. The wave-follower recordings were supplemented by a complete set of relevant measurements in the atmospheric boundary layer, on the surface, and in the water body. This paper is dedicated to technical aspects of the measurement procedure and data analysis. The precision of the feedback wave-following mechanism did not impose any restrictions on the measurement accuracy in the range of wave heights and frequencies relevant to the problem. Thorough calibrations of the pressure transducers and moving Elliott probes were conducted. It is shown that the response of the air column in the connecting tubes provides a frequency-dependent phase shift, which must be accounted for to recover the low-level induced pressure signal. In the finite-depth environment of Lake George, breaking waves play an important role in the momentum exchange between wind and waves, as will be shown in a subsequent paper.

The Effect of Wave Breaking on Surf-Zone Turbulence and Alongshore Currents: A Modeling Study

Abstract: The effect of breaking-wave-generated turbulence on the mean circulation, turbulence, and bottom stress in the surf zone is poorly understood. A one-dimensional vertical coupled turbulence (k–) and mean-flow model is developed that incorporates the effect of wave breaking with a time-dependent surface turbulence flux and uses existing (published) model closures. No model parameters are tuned to optimize model–data agreement. The model qualitatively reproduces the mean dissipation and production during the most energetic breaking-wave conditions in 4.5-m water depth off of a sandy beach and slightly underpredicts the mean alongshore current. By modeling a cross-shore transect case example from the Duck94 field experiment, the observed surf-zone dissipation depth scaling and the observed mean alongshore current (although slightly underpredicted) are generally reproduced. Wave breaking significantly reduces the modeled vertical shear, suggesting that surf-zone bottom stress cannot be estimated by fitting a logarithmic current profile to alongshore current observations. Model-inferred drag coefficients follow parameterizations (Manning– Strickler) that depend on the bed roughness and inversely on the water depth, although the inverse depth dependence is likely a proxy for some other effect such as wave breaking. Variations in the bed roughness and the percentage of breaking-wave energy entering the water column have a comparable effect on the mean alongshore current and drag coefficient. However, covarying the wave height, forcing, and dissipation and bed roughness separately results in an alongshore current (drag coefficient) only weakly (strongly) dependent on the bed roughness because of the competing effects of increased turbulence, wave forcing, and orbital wave velocities.

Wave attractors: linear yet nonlinear

Abstract: A number of physical mechanisms give rise to confined linear wave systems whose spatial structure is governed by a hyperbolic equation. These lack the discrete set of regular eigenmodes that are found in classical wave systems governed by an elliptic equation. In most 2D hyperbolic cases the discrete eigenmodes are replaced by a continuous spectrum of wave fields that possess a self-similar spatial structure and have a (point, line or planar) singularity in the interior. These singularities are called wave attractors because they form the attracting limit set of an iterated nonlinear map, which is employed in constructing exact solutions of this hyperbolic equation. While this is an inviscid, ideal fluid result, observations support the physical relevance of wave attractors by showing localization of wave energy onto their predicted locations. It is shown that in 3D, wave attractors may co-exist with a regular kind of trapped wave. Wave attractors are argued to be of potential relevance to fluids that are density-stratified, rotating, or subject to a magnetic field (or a combination of these) all of which apply to geophysical media.

Dynamical coupling of the stratosphere and mesosphere in the 2002 Southern Hemisphere major stratospheric sudden warming

Abstract: [1] NCEP data and a NCAR TIME-GCM simulation are used to explore the dynamical coupling of the stratosphere and mesosphere during the 2002 Southern Hemisphere major stratospheric sudden warming. The analyses suggest the possibility of feedback interactions between the planetary wave forcing and the mesospheric/stratospheric mean state changes. Multiple strong planetary waves before the warming penetrate into the mesosphere and weaken the polar jet. They alter the wave transmission condition in favor of more upward-poleward propagation of the wave energy at progressively lower altitudes. The jet reversal and the planetary wave surf zone also descend from the mesosphere down to the stratosphere, making wave breaking more likely at decreasing altitudes with each wave episode. These changes in the wave transmission and breaking conditions in the mesosphere and stratosphere may be critical for the extensive major stratospheric warming.

Excitation of Transient Rossby Waves on the Stratospheric Polar Vortex and the Barotropic Sudden Warming

Abstract: The excitation of Rossby waves on the edge of the stratospheric polar vortex, due to time-dependent topographic forcing, is studied analytically and numerically in a simple quasigeostrophic f-plane model. When the atmosphere is compressible, the linear response of the vortex is found to have two distinct components. The first is a spectrum of upward-propagating waves that are excited by forcing with temporal frequencies within a fixed “Charney–Drazin” range that depends on the angular velocity at the vortex edge and the vortex Burger number. The second component of the response is a barotropic mode, which is excited by forcing with a fixed temporal frequency outside the Charney–Drazin range. The relative magnitude of the two responses, in terms of total angular pseudomomentum, depends on the ratio of the horizontal scale of the forcing to the Rossby radius. Under typical stratospheric conditions the barotropic response is found to dominate. Nonlinear simulations confirm that the linear results remain relevant to understanding the response in cases when strongly nonlinear Rossby wave breaking ensues. It is shown that a sudden warming, or rapid increase in vortex angular pseudomomentum, can be generated at much lower forcing amplitudes when the barotropic mode is resonantly excited compared to when the upwardpropagating waves are excited. A numerical simulation of a “barotropic sudden warming” due to excitation of the barotropic mode by a relatively weak topographic forcing is described.

MASNUM ocean wave numerical model in spherical coordinates and its application

Abstract: In order to evaluate the wave-induced mixing in the upper ocean and the impact of surface waves on ocean-atmosphere fluxes, a global wave numerical model in spherical coordinates is developed based on the previous LAGFD-WAM regional model The wave energy spectrum balance equation and its complicated characteristic equations are derived in spherical coordinates In these control equations, the modulation of background current to wave evolution and the refraction of waves propagating along great circles are included The characteristic inlaid method is applied to integrating the wave energy spectrum balance equation Primary calibration indicates that the global wave model can describe the dynamical processes of ocean surface gravity waves