Observations of bathymetry-induced ocean roughness modulation in in-situ surface slope measurements and coincident airborne SAR images

Empirical results from a field experiment in the southern North Sea have demonstrated the possibility to detect bathymetry-induced sea surface roughness modulation in the coastal zone using high frequency in-situ slope measurements provided by the Towed Laser Slopemeter. A strong correlation between enhanced/reduced slope power for ocean wavelengths around 50 cm and enhanced/reduced normalized radar cross section (NRCS) at P-band has been established. Areas of smoother and rougher waters were found to occur systematically and consistently up- and down-stream of the sand dune crests, in good agreement with current theory. One case of perfect collocation in both space and time confirmed that the extremes in slope PSD and NRCS are spatially collocated. This dataset allowed the phase shift between the sand dune crests and their radar signature to be estimated at about 50 meters. The occurrence of bathymetry-related modulation was also detected at higher frequencies, although the range of ocean wavelengths affected by the modulation seem to be determined by environmental conditions