[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.Citation: Hwang, P. A., and M. A. Sletten (2008), Energy dissipation of wind-generated waves and whitecap coverage, J. Geophys.

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[1] Low-grazing angle backscattering data collected by a coherent dual-polarized radar installed on a fixed tower in the ocean are analyzed to investigate the properties of sea spikes attributable to wave breaking. The distribution of breaking wave speed is narrow-banded with an average speed between 2.0 and 2.6 m/s in mixed seas with wind speeds between 7 and 14.5 m/s. The corresponding breaking wavelength is between 2.5 and 4.3 m. The length or velocity scale of wave breaking is not proportional to the length or velocity scale of the dominant wave. This observation reflects the localized nature of the breaking process and may have significant implications on quantifying various breaking properties such as the energy dissipation or area of turnover by breaking waves. The fraction of sea spike coverage generally increases with wind speed but the trend of increase is modified by the intensity and relative direction of background swell. Parameterizations of sea spike coverage needs to take into consideration both wind and wave factors. Similarities and differences between sea spikes and whitecaps are discussed. In particular, while both quantities show a similar power law dependence on wind speed, the fraction of sea spike coverage is considerably higher than that of whitecap coverage. This result reflects the prevalence of steep features that produce quasi-specular facets and short-scale waves bounded to intermediate waves during breaking. These quasi-specular facets and bound waves contribute significantly to enhancing the radar sea return but may not entrain air to produce whitecap signature.

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[1] The technique for extracting wave period and wave direction from radar backscattering intensity is well developed, but the determination of spectral density or wave height is hindered by the complex nature of the modulation transfer function. In contrast to backscattering intensity, the Doppler signal of a coherent radar is originated from the radial velocity of the scattering objects. Its oscillatory component is contributed by ocean waves. The peak component of the Doppler velocity spectrum can be used to obtain the spectral peak wave period and the significant wave height can be calculated from the variance of the Doppler velocity. Analyses of coherent radar measurements collected from the ocean show that with radar range coverage on the order of 10 dominant wavelengths, a good estimate of peak wave period and significant wave height is achievable with radar data as short as a few seconds.Citation: Hwang, P. A., M. A. Sletten, and J. V. Toporkov (2010), A note on Doppler processing of coherent radar backscatter from the water surface: With application to ocean surface wave measurements,

[1] The anomaly of radar sea spikes, defined here as the non-Bragg scattering events with backscattering cross-section of horizontal polarization exceeding that of the vertical polarization, has been associated with steep wave features possibly going through wavebreaking process, with or without whitecap manifestation. This property is exploited for using a dual polarized radar as a remote sensing breaking wave detector. Field data collected in the ocean covering wind speeds from 7 to 15 m/s, grazing angles from 1.4 to 5.5°, and with different levels of background swell influence are analyzed to quantify the radar cross-section and Doppler velocity from sea surfaces with and without wave breaking. Key results of the breaking effects are increasing significantly the Doppler velocity of both polarizations (about 50% faster), enhancing the horizontally polarized backscattering cross-section drastically (with 10-15 dB increase), and producing relatively small change in the vertically polarized cross-section (about 1-2 dB increase). The presence of swell (in the same direction of wind waves) reduces both the radar backscatter and the impact of breaking waves on radar return. By inference, the swell presence decreases the ocean surface roughness and breaking activity. These results are consistent with earlier in-situ surface wave measurements and our expectation of swell modification of breaking process due to interaction of short waves and the orbital velocity of long swell.

[1] Repeat sampling on hourly time scales using an airborne synthetic aperture radar (SAR) is used to investigate the occurrence and evolving characteristics of spiral-shaped slick patterns, commonly presumed to be indicators of submesoscale ocean eddies, in the area around Santa Catalina Island, California (∼33.4°N, 118.4°W). Simultaneous SAR imagery and boat survey data are examined over two ∼5 h long periods spaced 3 days apart in April 2003. The SAR imagery reveals several spiral-like patterns, roughly 5 km in diameter, occurring downstream of the western end of Catalina. We believe that the most likely formation mechanism for these patterns is current-wake instability related to the flow of the Southern California Countercurrent along the north shore of Catalina. In one case, there is an observed cold-core eddy and vortex sheet attached to the tip of the island, similar to island-wake simulations done by Dong and McWilliams (2007). In another case, the SAR imagery shows a series of slick patterns that, at least initially, resemble spiral eddies, but the data show no clear evidence of actual ocean eddies being present either at depth or through a rotating surface expression. A speculation is that such features signify island-wake eddies that are relatively weak and dissipate quickly. An unexpected finding was how quickly a spiral slick pattern could deteriorate, suggesting a time scale for the surface feature of the order of only several hours. An implication of this result is that care is needed when interpreting a single satellite SAR imagery for evidence of active submesoscale eddies. Recommendations are made for future field studies.

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