POWER SPECTRAL LINESHAPES OF MICROWAVE-RADIATION BACKSCATTERED FROM SEA SURFACES AT SMALL GRAZING ANGLES

Analysis of recent small-grazing-angle microwave backscattering data i ndicate that the Bragg mechanism alone cannot explain the experimental results. The authors investigate the issue of scattering mechanisms b y studying the lineshapes of the backscattered microwave power spectra . It is found that spectral lineshapes can be decomposed into physical ly meaningful basis functions which are Gaussian, Lorentzian or Voigti an. Generally, the spectral component corresponding to the lower frequ ency peak in the power spectral density (PSD) of the vertical polarisa tion is well described by a Gaussian profile, while the spectral compo nent due to the higher frequency peak in the horizontal polarisation P SD is well described by either a Lorentzian or a Voigtian profile, dep ending 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 surfa ce, are active in contributing to microwave backscattering returns. Th ese mechanism comprise: first, scattering from free Bragg waves, chara cterised by a Gaussian distribution in scatterer speeds and a Gaussian component in the PSD; secondly, scattering from sporadically appearin g, fast moving, short lifetime, facet-like scatterers, characterised b y an exponential distribution in scatterer lifetime and a Lorentzian c omponent in the PSD; and finally, scattering from fast-to-intermediate speed bound-Bragg waves, or not-so-fast facets, characterised by a co nvolution of the Gaussian and Lorentzian processes, resulting in a Voi gtian component in the PSD.