Radar Investigation of the Structure of Wind Waves

Radar data from three experiments are analysed. Scatter characteristics of 50 cm wind-generated waves have been investigated with a C-band radar in a large wind-wave tank. Evidence of wave groups in sea clutter from the west coast of Scotland in the Sound of Sleat is also presented. The spectrum of the waves in the sound is narrow-banded and the waves are young, like the wind-wave spectrum in the laboratory. Clutter measurements, collected on the English south coast at Portland, of more ocean-like waves with broad band spectra also suggest the presence of wave groups. Evidence of the presence of wave groups is demonstrated in range-time images, as well as in the Fourier domain. Some ad hoc processing schemes, the normalised variance and binary threshold techniques, were successfully applied to enhance the appearance of the wave groups. The wind waves change frequency with fetch in the wave tank and the downshifting process is investigated using range-frequency maps of the radar data. The waves appear to change frequency in discrete steps that are associated with wave breaking events. The difference in wave period before and after breaking could be measured, and a wave crest was shown to be lost to compensate for the change in period, as expected. Some downshifting could also be measured in the Sound of Sleat. The ratio of wave group frequency and wave frequency is inaccordance with Benjamin-Fier sideband instability theory, as it is for the data measured at Portland. This revised version was published online in July 2006 with corrections to the Cover Date.