High resolution doppler spectra from short gravity-capillary wind waves

Microwave CW Doppler backscatter spectra from wind waves have been measured in a controlled laboratory environment at 4.283, 9.375, and 23.9 GHz. Independent variables and their ranges were: windspeed, 0 to 15 m/s; fetch, 1 to 10 m; and Bragg water wavelength, 7 to 0.7 cm. The use of parabolic antennas focused in the near field, and digital signal processing and averaging techniques has yielded considerable improvement in Doppler frequency resolution over that previously achieved[1]. The observed spectra contain considerable fine structure at intermediate and higher winds. At the intermediate winds, this is largely explainable on the basis of second order scattering and hydrodynamic effects[2]. The second order splitting is found to be given by where is the group speed of the first order Bragg wave and and are the frequency and phase speed of the dominant wave.At higher winds a transition to two scale scattering is clearly discernible in the measured spectra. Crest speeds and optical slope spectra measurements were made to allow comparison of the measured spectra with a two scale model.