A new ocean SAR cross-spectral parameter: definition and directional property using the global Sentinel-1 measurements

Space‐borne synthetic aperture radar (SAR) measurements have already been proven to be invaluable in detecting and quantifying properties of ocean swell systems. With the improved spatial resolution, Sentinel‐1 wave mode measurements can further be extended towards shorter scale waves, i.e. within the surface wave equilibrium range. In this study, a new parameter is derived from filtering SAR image cross‐spectra around range‐traveling intermediate wind waves (wavelengths 15‐20 m). This parameter captures both the radar cross section variability and its time evolution. Given the dependence of intermediate waves on local wind field, this parameter is statistically documented to confirm its sensitivity to both wind speed and wind direction. Comparable to Doppler estimate, the signed parameter can be used to reduce the wind direction ambiguity in the inversion of high‐resolution wind fields from SAR imagery. In addition, under complex environmental situations over a Polar Low event, this parameter is expected to better capture the dynamics of the surface wind. Globally, our analysis demonstrates regional and seasonal variations of this parameter, associated with those of wind/wave patterns. In particular, its directionality reveals the seasonal migration of intertropical convergence zone. This parameter is valuable to various further applications, for example, to help develop wind retrieval scheme from SAR measurements or to map space‐time variability of ocean waves at various wavelengths.

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Our paper entitled 'A new ocean SAR cross‐spectral parameter: definition and directional property using the global Sentinel‐1 wave mode measurements' defines a new radar parameter to highlight the properties of intermediate scale ocean waves (wavelength of 15m) based on high‐resolution spaceborne synthetic aperture radar (SAR) images. This new parameter is found to be a signed quantity, sensitive to both wind speed and direction. At global scale, we use a one‐year data to demonstrate the distribution and variation of this parameter. It is found that this parameter not only provides directional information of global winds, but exhibits distinct seasonal signatures of regional wind/wave. A case study over a Polar Low event also proves the capability of our new parameter in carrying surface wind information.

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Li Huimin, Chapron Bertrand, Mouche Alexis, Stopa Justin (2019). A new ocean SAR cross-spectral parameter: definition and directional property using the global Sentinel-1 measurements. Journal Of Geophysical Research-oceans. 124 (3). 1566-1577. https://doi.org/10.1029/2018JC014638, https://archimer.ifremer.fr/doc/00482/59394/

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