The modulation and coherence functions between atmospheric turbulence at the sea surface and the microwave radar cross section

The concept of the modulation transfer function between the microwave backscatter from meter-sized areas of the sea surface and the long dominant-wave slope is being expanded to include the role of wind fluctuations at the surface. In the experimental results from the SAXON-FPN experiment, X-band radar measurements have been supplemented by simultaneous wind measurements performed by an ultrasonic anemometer positioned 5 meters above the surface. This study is to examine the correlation between the low frequency wind fluctuations and the sizeable low frequency signal in the corresponding radar spectrum that has long been observed but not yet explained. The results of the joint radar and wind data analysis show coherences between the atmospheric turbulence and the RCS fluctuations. In the frequency range where the ocean wave spectrum is dominant (>0.1 Hz) the coherence function for the atmospheric turbulence displays a minor, but still visible role. In the frequency range below 0.01 Hz, the authors find the highest consistent coherences. This finding implies that the 2 cm Bragg waves at a given location appears to be less affected by the local instantaneous wind and stress; it is only correlated with the wind on scales of 10´s of minutes