Radar observations of breaking waves and solitons at low grazing angles

Laboratory experiments are conducted in a wind-wave tank facility to investigate the physics of radar scattering associated with: (1) transient wave packets undergoing various stages of breaking, (2) the effects of propagation of solitons through a pre-existing wind wave sea. The problem is fundamental to the understanding of radar signatures of internal waves, ship wakes and deep water breaking waves. Using a programmable paddle which controls the steepness parameters of the wave-breaking process, five classes of waves with different steepness characteristics are considered including a symmetrical round wave, an asymmetrical steep wave, an incipient breaker, a spilling breaker, and a plunging breaker. Video imaging and resistance wire wave gauges are used to measure the surface displacement and the temporal evolution of the surface. Wave gauges spaced approximately 1.2 m apart are placed along the tank measuring the surface wave height, celerity, and curvature. Each wave gauge corresponds to a specific range bin in the radar's field of view. The radar is a pulsed Doppler dual polarized X-band (9.4 GHz) system with a 3 (ns) pulse width, and the antenna is centered at 80 degrees incidence. Observations are made with the wave packets propagating towards and away from the radar. The backscatter response from the breaking waves exhibit nonlinear behavior such that the magnitude of the return increases with the increase in front face slope and with the onset of breaking, but decreases when the intensity of wave breaking is more vigorous and of plunging type. The differences in modulations for upwave and downwave look directions are examined. For the experiments involving the propagation of solitons through a pre-existing wind wave sea, the wind speeds in the tank are varied between 5-15 ms/sup -1/. Based on the range gated radar time series, the temporal evolution of the surface is characterized into four separate stages. The radar signatures show strong polarization dependence both in the cross-section modulations and Doppler bandwidth characteristics, with stronger modulations in the HH scatter, and upshifts in the Doppler centroids of the HH return with respect to the VV return.< >