A wind-wave tank study of the azimuthal response of a Ka-band scatterometer

The azimuthal response of a 36-GHz scatterometer from wind-generated waves in large wave tanks is presented. The radar was operated at an incidence angle of 30 degrees from nadir and with vertical polarization. Measurements were also made of turbulent stress in air flow above the water surface and the cross-wind and along-wind wave slopes. The friction velocity, u/sub */, ranged from 5 to 100 cm/s. For each wind speed, the azimuthal variation of radar cross section is expressed in terms of the classical three-term Fourier cosine series. As the wind speed increases, the azimuthally averaged radar cross section increases, and the upwind-downwind asymmetry decreases, in general agreement with field observations. However, this Ka-band data set reveals that the cross-wind modulation is not a monotonic function of wind speed, but instead it reaches a peak at u/sub */ of about 25 cm/s. The wave slope data exhibit exhibit a similar feature in the short wave directional development. Measurements from 35- and 95-m fetches indicate that the azimuthal response of the radar is more directly related to wind stress than wind speed. >