Directional and Frequency Spread of Surface Ocean Waves From SWIM Measurements

Le Merle, Eva ; Hauser, Danièle ; Peureux, Charles ; Aouf, Lotfi ; Schippers, Patricia ; Dufour, Christophe ; Dalphinet, Alice

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<p align=justify>The "China France Oceanography Satellite" (CFOSAT) launched in 2018 now routinely provides directional ocean wave spectra at the global scale. It consists of analyzing the normalized radar cross-section measured by the near-nadir pointing Ku-Band real-aperture scanning radar SWIM (Surface Waves Investigation and Monitoring). The significant wave height, dominant wavelength and direction are provided as the main parameters, but here, we analyze additional parameters, namely the frequency width of the omni-directional spectra, the directional spread of the dominant waves, and the related Benjamin-Feir index. This latter was proposed in the literature to estimate the probability of extreme waves. We discuss the geographical distributions of these parameters, their relation with sea-state conditions, and their similarities and differences with respect to the same parameters obtained from the MFWAM numerical wave model and buoy data. We find that the SWIM omni-directional spectra are narrower and more peaked than the model spectra and that these differences are more obvious in the high sea-state conditions encountered in the Southern Ocean. We find that under the intense conditions of the Southern Ocean, the SWIM directional spread at the peak is the smallest for swell, the largest for young wind seas, and takes intermediate values for mature wind seas. The directional Benjamin-Feir index is similar for SWIM and MFWAM, but this is mainly due to compensating effects in the parameters contributing to this index. The results indicate that these shape parameters may be used in the future to better describe the wave space-time evolution.</p>
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