مرکز منطقه ای اطلاع رساني علوم و فناوري - High resolution doppler spectra from short gravity-capillary wind waves

Abstract :

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 $\\Delta f = f_{d} \\bigg[ 1 - {c_{g}(k_{B}) \\over c_{d}} \\bigg]$ where $C_{g}(k_{B})$ is the group speed of the first order Bragg wave and $f_{d}$ and $C_{d}$ 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.