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° 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_*, 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_* 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