X-band backscatter from the ocean at low-grazing angles

In 2005 and 2006, we mounted an X- band Doppler radar on ships that operated in the South China Sea and off the coast of New Jersey, respectively. The measurements were made only at W polarization in 2005 but at both HH and W polarization in 2006. On average, VV normalized radar cross sections, sigma₀(VV), behaved at grazing angles between 1 and 2 degrees in much the same manner that they do at higher grazing angles. In particular, they showed the second-harmonic dependence on the angle between the antenna-look direction and the wind direction that is characteristic of scatterometry. sigma₀(HH), on the other hand, showed little evidence of the second harmonic component, maximizing with the antenna looking into the wind and minimizing in the opposite direction. For both polarizations, sigma₀ was generally well above that expected from Bragg scattering and the polarization ratio sigma₀(VV) / sigma₀(HH) was much smaller. Surface signatures of internal waves (IWs) off the New Jersey coast were much weaker when the antennas looked in the direction of internal wave propagation than when they looked opposite this direction. Interestingly, for the nonlinear internal waves found in the South China Sea, the opposite phenomenon occurred: W signatures were stronger looking in the direction of IW propagation than opposite to it.