Fetch and wind speed dependence of Doppler spectra


  • J. R. Duncan,

  • W. C. Keller,

  • J. W. Wright


Previous measurements of Doppler spectra in microwave backscattering in a wind wave tank at 9.375 GHz and 23.9 GHz have been extended to 70.1 GHz at 3 m fetch and to longer fetches and higher winds at the two lower microwave frequencies. At high depression angles, the scatterers appear to be specular points moving at approximately the speed of the dominant wind wave. At lower depression angles, the scatterers can be thought of as comprising systems which are respectively bound to and free of the dominant wave. The free scatterers are low-order Bragg scatterers and it is natural to treat them as free waves or free wave systems in their own right. The bound scatterers are the familiar parasitic capillary waves at low winds, but at higher winds they appear to be associated with the breaking water at the crest of the dominant wave.