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Ocean Sci., 14, 41-52, 2018
https://doi.org/10.5194/os-14-41-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
22 Jan 2018
Note on the directional properties of meter-scale gravity waves
Charles Peureux1, Alvise Benetazzo2, and Fabrice Ardhuin1 1Laboratoire d'Océanographie Physique et Spatiale, Univ. Brest, CNRS, Ifremer, IRD, 29200 Plouzané, France
2Institute of Marine Sciences, Italian National Research Council, Venice, Italy
Abstract. The directional distribution of the energy of young waves is bimodal for frequencies above twice the peak frequency; i.e., their directional distribution exhibits two peaks in different directions and a minimum between. Here we analyze in detail a typical case measured with a peak frequency fp = 0.18 Hz and a wind speed of 10.7 m s−1 using a stereo-video system. This technique allows for the separation of free waves from the spectrum of the sea-surface elevation. The latter indeed tend to reduce the contrast between the two peaks and the background. The directional distribution for a given wavenumber is nearly symmetric, with the angle distance between the two peaks growing with frequency, reaching 150° at 35 times the peak wavenumber kp and increasing up to 45 kp. When considering only free waves, the lobe ratio, the ratio of oblique peak energy density over energy in the wind direction, increases linearly with the non-dimensional wavenumber kkp, up to a value of 6 at kkp 22, and possibly more for shorter components. These observations extend to shorter components' previous measurements, and have important consequences for wave properties sensitive to the directional distribution, such as surface slopes, Stokes drift or microseism sources.

Citation: Peureux, C., Benetazzo, A., and Ardhuin, F.: Note on the directional properties of meter-scale gravity waves, Ocean Sci., 14, 41-52, https://doi.org/10.5194/os-14-41-2018, 2018.
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Short summary
Little is known on the short ocean wave (1 to 20 m wave length typically) directional distribution. It has been measured from a platform in the Adriatic Sea using a three-dimensional reconstruction technique, used for the first time for this purpose. In this record, while longer waves propagate along the wind direction, shorter waves have been found to propagate mainly along two oblique directions, more and more separated towards smaller scales.
Little is known on the short ocean wave (1 to 20 m wave length typically) directional...
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