Correlation of hydrodynamic features with LGA radar backscatter from breaking waves

Backscatter characteristics of 1-4-m-long, mechanically generated breaking waves have been investigated with a C-band frequency modulated continuous wave (FMCW) radar (up to 3.77-cm range resolution) in the large wind-wave tank at the Ocean Engineering Laboratory, University of California, Santa Barbara. The grazing angle was 6 degrees, Wave breaking was caused to occur in the test section due to wave group selfmodulation, just as it has been observed in the ocean. The central purpose of these experiments was to determine the hydrodynamic sources of the low grazing angle sea spike and its structure. Using typical, strong 2.3-m-long breakers, four phases of the breaking process with distinct hydrodynamic characteristics were identified visually and correlated with synchronous radar data. These breaking waves yielded a horizontal copolarization (NH) radar cross section (RCS) of up to 10 m(2) and concurrent copolarization ratios (HH/vertical copolarization[VV]) exceeding 40 dB. Synchronous high-speed video images showed that these peak values appeared just after a plunging jet developed at the breaker crest, well before it hit the front face of the wave. Focusing the electromagnetic energy on the jet by the parabolic front face of the breaking wave is suggested as a mechanism that yields both high HH returns and high HH/VV ratios. Statistics of HH peak RCS for the complete range of wavelengths tested show the dependence of radar backscatter from energetic breaking waves on their wavelength and implies a scaling law such that the optimum return is obtained when lambda(wave) = 40 lambda(radar). Under fixed conditions of lambda(wave) and lambda(radar), large deviations in HH RCS have been found and have been shown to be dependent on the strength of the breaker.