Local Properties of Radar Backscatter at C-band and Off-nadir Angles

Wavelet analysis of time series of C-band radar cross-section s0 at 40 degree incidence angle, of radar Doppler frequency fd and of wind speed u measured simultaneously, has provided some new insight on the mechanism linking wind and waves to the radar backscatter. Several of the characteristics shown by the wavelet coefficient map of the wind and of the radar Doppler frequency, which is related to the gravity wave orbital motion, are related to the wavelet features of s0. The most important results of the analysis concern the correspondence between wave groups and sharp increase of s0, the similarity of the low frequency temporal structure of the wind and of s0 and the lack of interaction between wind gusts and radar cross-section. Furthermore, the investigation of the wavelet phase between s0 and wind has revealed the existence of time intervals of > 20 s during which s0 lags the wind, as well as periods when the opposite occurs. Even if, in average, s0 lags the wind of ~ 2.5 s, this alternance of time lag between the wind and s0 suggests that the generation of the sea surface roughness by the wind occurs like an intermittent, multiple scale, locally coherent process.