Microwave radar observations of nearshore ocean dynamics

The relationship between microwave imaging radar measurements of the nearshore ocean region and nearshore dynamics is studied. Normalized radar cross section and Doppler velocity are estimated from radar measurements of nearshore waves at near-grazing angles. Radar scattering is classified using joint histograms of radar cross section and Doppler velocities. Scattering in these distributions is investigated through comparisons with theoretical wave predictions, video particle image velocimetry (PIV), and in situ acoustic Doppler velocimeter (ADV) measurements. This analysis shows that shoaling and breaking waves measured through radar grating lobes significantly affect Doppler velocities near the edges of the images and also scattering from the back faces of waves. Doppler velocities from breaking waves are found to agree well with wave phase velocity predictions in the surf zone (the rms difference is 0.14 m/s), and large radar cross section features are correlated with breaking waves and the motion of surf-zone bores in video imagery. Differences in inter-bore velocities are expected, since the measurements are not collocated. However; Doppler inter-bore velocities are found to correlate well with fluid velocities (the correlation coefficient is 0.65), but are offset by around 0.8 m/s. This offset may be due to a combination of the Bragg wave phase velocities, radar sensitivity to short wavelength waves in the nearshore which is limited by the spatial resolution of the radar, and inherent biasing of Doppler velocities towards velocities of large NRCS scattering.; In low-wind conditions, radar measurements of the nearshore show patches of increased backscatter. Animation of sequences of the images shows movement of these patches. A feature-tracking algorithm based on PIV is presented to quantify the velocities of the observed features. The nature of the patches is also investigated through comparison with video images. It is concluded that the patches of backscatter are due to low-grazing angle scattering from streaks of foam, however contributions from mechanically-generated surface roughness are not ruled out. Comparisons between surface velocities derived from the feature-tracking algorithm and subsurface velocities measured by in situ current meters show best agreement for sensors beyond the surf zone. It is concluded that the estimated velocity fields are related to nearshore flows such as wave-induced cell circulations and longshore currents.