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

The temporal coherence between atmospheric turbulence 5 meters above the mean sea surface and the small scale roughness, was measured by four different microwave radar wavelengths during the SAXON-FPN experiment. Continuous radar cross section measurements (one second averages) and spatially coincident, simultaneous sonic anemometer measurements of the 3 wind components provide the first known coherence function for frequencies from .0005 to .5 Hz. Unexpectedly, the wind fluctuations above .01 Hz do not induce correlated surface roughness variations, as observed by L,S,X and Ka-band radars. Only below .01 Hz, and only for X-band and Ka-band roughness scales do the authors measure coherences above 0.5. These results indicate that the short waves are only correlated with the wind on scales of 10´s of minutes. Therefore the authors cannot assume that these short sea waves receive their energy directly from the wind, and they must identify a wave generating mechanism for these centimeter waves based on hydrodynamic interactions.