Power spectral lineshapes of microwave radiation backscattered from sea surfaces at small grazing angles

Analysis of recent small-grazing-angle microwave backscattering data indicate that the Bragg mechanism alone cannot explain the experimental results. The authors investigate the issue of scattering mechanisms by studying the lineshapes of the backscattered microwave power spectra. It is found that spectral lineshapes can be decomposed into physically meaningful basis functions which are Gaussian, Lorentzian or Voigtian. Generally, the spectral component corresponding to the lower frequency peak in the power spectral density (PSD) of the vertical polarisation is well described by a Gaussian profile, while the spectral component due to the higher frequency peak in the horizontal polarisation PSD is well described by either a Lorentzian or a Voigtian profile, depending on the size of the radar footprint. These quantitative results provide compelling evidence that several entirely different mechanisms, corresponding to different surface features of the dynamic sea surface, are active in contributing to microwave backscattering returns. These mechanisms comprise: first, scattering from free Bragg waves, characterised by a Gaussian distribution in scatterer speeds and a Gaussian component in the PSD; secondly, scattering from sporadically appearing, fast moving, short lifetime, facet-like scatterers, characterised by an exponential distribution in scatterer lifetime and a Lorentzian component in the PSD; and finally, scattering from fast-to-intermediate speed bound-Bragg waves, or not-so-fast facets, characterised by a convolution of the Gaussian and Lorentzian processes, resulting in a Voigtian component in the PSD