Marine winds

SAR images of the ocean surface often show the impact of atmospheric phenomena due to the interaction of the near surface wind field and the ocean surface roughness. In general, the higher the near surface wind speed the rougher the ocean surface becomes, resulting in increased radar backscatter and brighter image tones. This relationship is key to deriving wind information from SAR data.

The microwave signatures for ocean surface roughness with scales on the order of the radar wavelength are due mainly to Bragg scattering. Due to the increased sensitivity of the backscatter to the surface capillary waves, VV polarization is preferred for wind speed estimation when compared to HH. However, imagery obtained with HH polarization may be more suitable for monitoring other oceanic phenomena such as internal waves, currents, and bathymetry (as illustrated in Figure 9-50).

Figure 9-50
Figure 9-50. An illustration of polarization sensitivity to atmospheric and oceanic phenomena from real aperture radar data acquired at 13.3 GHz. a) This VV polarization image is dominated by atmospheric signatures, in this case wind speed variability and convective cells; b) Meanwhile, this simultaneously acquired HH polarization image shows evidence of oceanic phenomena, in this case internal waves (from Ufermann & Romeiser ).

In general, for estimating information on winds and waves, the signal obtained in cross-polarized imagery is very low. This may result in the signal being close to or below the noise floor for spaceborne SAR systems, thus making the cross-polarized channel of little use for observation of marine phenomena. However, the potential of using imagery at HH and VV synergistically for wind and wave estimation has been demonstrated using C-SAR data from the CV-580 (Figure 9-51), as well as for estimation of the C-band co-polarization ratio, (Figure 9-52). The differences in the structures of the wave number spectra from the VV and HH channels, especially along the range wavenumber axis, illustrates the potential for improving ocean information estimation using imagery acquired at two polarizations. These studies showed that the C-VV and C-HH model functions are in reasonable agreement over the RADARSAT-2 incident angle range and that the Kirchoff-based C-band co-polarization ratio agrees well with observations. Thus, the dual channel, like polarization RADARSAT-2 mode may be attractive for ocean surface observation.

CV580,L3P3,27June2000-16:31,Lake Superior,Polarization Ratio,AziLines 2049:4096
Figure 9-52
A = boresight - 20°    B = Far Edge of Nominal Swath    C = CV580 boresight
Figure 9-52. Plot of C-band co-polarization ratio from C-SAR data acquired by the CV-580 over a NOAA buoy on Lake Superior in June 2000, along with a Kirchoff-based scattering C-band co-polarization ratio (courtesy CCRS).