On the spectral dissipation of ocean waves due to white capping (original) (raw)
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Energy dissipation of wind-generated waves and whitecap coverage
Journal of Geophysical Research, 2008
1] The energy dissipation per unit area of the ocean surface attributed to fetch-or duration-limited wind-generated waves can be expressed in terms of wind speed, significant wave height and peak wave frequency. Such a parameterization equation can be exploited for obtaining a first order estimation of the rate of energy input through the air-sea interface in the world's oceans using satellite output of wind speed, wave height and wave period. For general wind wave events in the ocean with event duration longer than one hour, the energy dissipation (in W/m 2 ) is equal to the product of the density of air, wind speed cubed and a proportionality coefficient between 0.00037 and 0.00057. Using the equation to calculate the wave energy dissipation, the whitecap coverage is proportional linearly to the energy dissipation. The threshold energy dissipation for whitecap inception is between 0.013 and 0.038 W/m 2 , which corresponds to a threshold wind speed of between 2.5 and 3.6 m/s. The proportionality coefficient is relatively constant for a wide range of wave growth conditions in comparison to the data scatter in the whitecap measurements. This may explain why it is so difficult to establish an unequivocal dependence on the explicit surface wave parameters in the whitecap data. The weak explicit wave signal can be detected after the cubic wind speed dependence is removed.
Journal of Atmospheric and Oceanic Technology, 2012
A new wind-input and wind-breaking dissipation for phase-averaged spectral models of wind-generated surface waves is presented. Both are based on recent field observations in Lake George, New South Wales, Australia, at moderate-to-strong wind-wave conditions. The respective parameterizations are built on quantitative measurements and incorporate new observed physical features, which until very recently were missing in source terms employed in operational models. Two novel features of the wind-input source function are those that account for the effects of full airflow separation (and therefore relative reduction of the input at strong wind forcing) and for nonlinear behavior of this term. The breaking term also incorporates two new features evident from observational studies; the dissipation consists of two parts—a strictly local dissipation term and a cumulative term—and there is a threshold for wave breaking, below which no breaking occurs. Four variants of the dissipation term ar...
Ocean Science Discussions
The performance of two methods for quantifying whitecapping dissipation incorporated in the SWAN wave model is evaluated for waves generated along and off the U.S. East Coast under energetic winter storms with a predominantly westerly wind. Parameterizing the whitecapping effect can be done using the Komen-type schemes, which are based on mean spectral parameters, or the saturation-based (SB) approach of van der Westhuysen (2007), which is based on local wave parameters and the saturation level concept of the wave spectrum (we use "Komen" and 15 "Westhuysen" to denote these two approaches). Observations of wave parameters and frequency spectra at four NDBC buoys are used to evaluate simulation results. Model-data comparisons show that when using the default parameters in SWAN, both Komen and Westhuysen methods underestimate wave height. Simulations of mean wave period using the Komen method agree with observations, but those using the Westhuysen method are substantially lower. Examination of source terms shows that the Westhuysen method underestimates the total energy transferred into the 20 wave action equations, especially in the lower frequency bands that contain higher spectral energy. Several causes for this underestimation are identified. The primary reason is the difference between the wave growth conditions along the East Coast during winter storms and the conditions used for the original whitecapping formula calibration. In addition, some deficiencies in simulation results are caused along the coast by the "slanting fetch" effect that adds low-frequency components to the 2-D wave spectra. These components cannot be simulated partly or entirely by 25 available wind input formulations. Further, the effect of boundary layer instability that is not considered in the Komen and Westhuysen whitecapping wind input formulas may cause additional underestimation. 65 parameters as incorporated in SWAN performed slightly better in the simulation of wave height, period, and frequency spectra than the SB method. This conclusion contradicts the results of previous studies. Although van Velder et al.
Field Observations of Breaking of Dominant Surface Waves
Remote Sensing, 2021
The results of field observations of breaking of surface spectral peak waves, taken from an oceanographic research platform, are presented. Whitecaps generated by breaking surface waves were detected using video recordings of the sea surface, accompanied by co-located measurements of waves and wind velocity. Whitecaps were separated according to the speed of their movement, c, and then described in terms of spectral distributions of their areas and lengths over c. The contribution of dominant waves to the whitecap coverage varies with the wave age and attains more than 50% when seas are young. As found, the whitecap coverage and the total length of whitecaps generated by dominant waves exhibit strong dependence on the dominant wave steepness, ϵp, the former being proportional to ϵp6. This result supports a parameterization of the dissipation term, used in the WAM model. A semi-empirical model of the whitecap coverage, where contributions of breaking of dominant and equilibrium range...
Effect of Surface Waves on Air–Sea Momentum Exchange. Part I: Effect of Mature and Growing Seas
Journal of the Atmospheric Sciences, 2004
The effect of surface waves on air-sea momentum exchange over mature and growing seas is investigated by combining ocean wave models and a wave boundary layer model. The combined model estimates the wind stress by explicitly calculating the wave-induced stress. In the frequency range near the spectral peak, the NOAA/ NCEP surface wave model WAVEWATCH-III is used to estimate the spectra, while the spectra in the equilibrium range are determined by an analytical model. This approach allows for the estimation of the drag coefficient and the equivalent surface roughness for any surface wave fields. Numerical experiments are performed for constant winds from 10 to 45 m s Ϫ1 to investigate the effect of mature and growing seas on air-sea momentum exchange. For mature seas, the Charnock coefficient is estimated to be about 0.01 ϳ 0.02 and the drag coefficient increases as wind speed increases, both of which are within the range of previous observational data. With growing seas, results for winds less than 30 m s Ϫ1 show that the drag coefficient is larger for younger seas, which is consistent with earlier studies. For winds higher than 30 m s Ϫ1 , however, results show a different trend; that is, very young waves yield less drag. This is because the wave-induced stress due to very young waves makes a small contribution to the total wind stress in very high wind conditions.
The Phillips spectrum and a model of wind-wave dissipation
Theoretical and Mathematical Physics, 2020
We consider an extension of the kinetic equation developed by Newell & Zakharov [1]. The new equation takes into account not only the resonant four-wave interactions but also the dissipation associated with the wave breaking. A dissipation function that depends on the spectral energy flux is introduced into the equation. This function is determined up to a functional parameter, which optimal choice should be made based on comparison with experiment. A kinetic equation with this dissipation function describes the transition from the Kolmogorov-Zakharov spectrum E(ω) ∼ ω −4 to the Phillips spectrum E(ω) ∼ ω −5 usually observed experimentally. The version of the dissipation function expressed in terms of the energy spectrum can be used for wave modelling and prediction of sea waves.
A Field Study of Whitecap Coverage and its Modulations by Energy Containing Surface Waves
Geophysical Monograph Series, 2002
A field study of whitecap coverage generated by breaking wind waves has been performed from MHI's Black Sea Research Platform. It is revealed that the main contribution to the whitecap coverage of the sea surface results from breaking of short wind waves, which are more than 3 times shorter than the wavelength of the spectral peak. The energy containing waves strongly modulate the whitecap coverage. Zones of enhanced wave breaking are located on the modulating waves' crests. The effect is described in terms of a modulation transfer function for whitecap coverage. Its magnitude equals about 24, and decreases with the increase of inverse wave age of the energy containing waves.
J. Phys. Oceanogr …, 2011
New parameterizations for the spectral dissipation of wind-generated waves are proposed. The rates of dissipation have no predetermined spectral shapes and are functions of the wave spectrum, in a way consistent with observation of wave breaking and swell dissipation properties. Namely, swell dissipation is nonlinear and proportional to the swell steepness, and wave breaking only affects spectral components such that the non-dimensional spectrum exceeds the threshold at which waves are observed to start breaking. An additional source of short wave dissipation due to long wave breaking is introduced, together with a reduction of wind-wave generation term for short waves, otherwise taken from Janssen (J. Phys. Oceanogr. 1991). These parameterizations are combined and calibrated with the Discrete Interaction Approximation of Hasselmann et al. (J. Phys. Oceangr. 1985) for the nonlinear interactions. Parameters are adjusted to reproduce observed shapes of directional wave spectra, and the variability of spectral moments with wind speed and wave height. The wave energy balance is verified in a wide range of conditions and scales, from the global ocean to coastal settings. Wave height, peak and mean periods, and spectral data are validated using in situ and remote sensing data. Some systematic defects are still present, but the parameterizations probably yield the most accurate overall estimate of wave parameters to date. Perspectives for further improvement are also given.
Modulation of Wind-Wave Breaking by Long Surface Waves
Remote Sensing
This paper reports the results of field measurements of wave breaking modulations by dominant surface waves, taken from the Black Sea research platform at wind speeds ranging from 10 to 20 m/s. Wave breaking events were detected by video recordings of the sea surface synchronized and collocated with the wave gauge measurements. As observed, the main contribution to the fraction of the sea surface covered by whitecaps comes from the breaking of short gravity waves, with phase velocities exceeding 1.25 m/s. Averaging of the wave breaking over the same phases of the dominant long surface waves (LWs, with wavelengths in the range from 32 to 69 m) revealed strong modulation of whitecaps. Wave breaking occurs mainly on the crests of LWs and disappears in their troughs. Data analysis in terms of the modulation transfer function (MTF) shows that the magnitude of the MTF is about 20, it is weakly wind-dependent, and the maximum of whitecapping is windward-shifted from the LW-crest by 15 deg....