Abstract
Ka-band Radar Doppler velocity is compared to the optical velocity of deep water breaking wind waves observed in field conditions from a stationary oceanographic platform. One typical record is analyzed: wind speed is 11 m/s, dominant wave frequency is 0.24 Hz, radar incidence angle is 53°, and radar-to-wind azimuth is upwind. This 30 minute record includes 42 breaking wave events. The Doppler velocity is estimated as the first moment of instantaneous Doppler spectrum. The optical velocity of breakers is measured using spatially collocated and temporally synchronized video records of the radar footprint. As found the Doppler velocity of breakers is remarkably slower (by a factor of 4) than the “optical” velocity of whitecaps. This suggests that whitecaps (translated with phase velocities of breaking waves) represent a moving source of the enhanced surface roughness providing radar returns. Since this roughness is “embedded” into the water, its Doppler velocity is much smaller than the phase velocity.
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