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Abstract
We present new results for ocean surface current signatures in dual co- and cross-polarized synthetic aperture radar (SAR) images. C-band RADARSAT-2 quad-polarized SAR ocean scenes are decomposed into resonant Bragg scattering from regular (non-breaking) surface waves and scattering from breaking waves. Surface current signatures in dual co- and cross-polarized SAR images are confirmed to be governed by the modulations due to wave breaking. Due to their small relaxation scale, short Bragg waves are almost insensitive to surface currents. Remarkably, the contrast in sensitivity of the non-polarized contribution to dual co-polarized signals is found to largely exceed, by a factor of about 3, the contrast in sensitivity of the corresponding cross-polarized signals. A possible reason for this result is the co- and cross-polarized distinct scattering mechanisms from breaking waves: for the former, quasi-specular radar returns are dominant, whereas for the latter, quasi-resonant scattering from the rough breaking crests governs the backscatter intensity. Thus, the differing sensitivity can be related to distinct spectral intervals of breaking waves contributing to co- and cross-polarized scattering in the presence of surface currents. Accordingly, routinely observed current signatures in quad-polarized SAR images essentially originate from wave breaking modulations, and polarized contrasts can therefore help quantitatively retrieve the strength of surface current gradients.
Highlights
With the advent of new spaceborne synthetic aperture radar (SAR) systems, more detailed information on the ocean surface signatures of high-resolution ocean–atmosphere processes is becoming available
Results show that cross-polarized scattering from breaking waves can reach 80% of the total normalized radar cross-section (NRCS) for wind speeds up to 20 m/s, which suggests that wave breaking can play a crucial role in cross-polarized radar imaging of the ocean current, like co-polarization
Analysis of the data confirms that the governing mechanism that leads to a manifestation of ocean current features in quad-polarized SAR images is wave breaking modulations by the currents
Summary
With the advent of new spaceborne synthetic aperture radar (SAR) systems, more detailed information on the ocean surface signatures of high-resolution ocean–atmosphere processes is becoming available. The crucial role of wave breaking in radar imaging of ocean currents has been further demonstrated on the basis of previously developed models [17]; as well as dual co- and cross-polarized SAR observations of meso-scale currents and internal waves [9,10,24,25], nonlinear Ekman divergence [26] and sea surface temperature front signatures [27]. As already suggested [17,28] and tested [24,29,30] for particular case-studies, an effective methodology can build on previous work that uses the dual co-polarization (VV and HH) radar data to quantitatively assess contributions of Bragg and non-Bragg scattering (associated with wave breaking) in SAR imaging of different ocean surface phenomena, such as surface slicks, current gradients, and wind field features.
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