Note on the directional properties of meter-scale gravity waves
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 <...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-01-01
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| Series: | Ocean Science |
| Online Access: | https://www.ocean-sci.net/14/41/2018/os-14-41-2018.pdf |
| Summary: | 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 <i>f</i><sub>p</sub> = 0.18 Hz and a wind speed of
10.7 m s<sup>−1</sup> 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
<i>k</i><sub>p</sub> and increasing up to 45 <i>k</i><sub>p</sub>. 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 <i>k</i>∕<i>k</i><sub>p</sub>, up to a value of 6 at <i>k</i>∕<i>k</i><sub>p</sub> <mo form="infix">≃</mo> 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. |
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| ISSN: | 1812-0784 1812-0792 |