Inside the sea-spike: low grazing angle radar imaging of laboratory waves repeatedly breaking in wave groups

Backscatter characteristics of 2.3 m waves breaking in wave groups have been investigated with a C-band FMCW radar (7.5 cm range resolution) in the large wind-wave tank of the University of California, Santa Barbara. The purpose of these experiments was to determine the hydrodynamic sources of the sea spike and its structure. This paper covers the experimental subset of mechanically generated breaking waves with the 15% highest radar cross sections (RCS) in horizontal copolarization (HH). Four phases of the breaking process with distinct hydrodynamic characteristics were identified and correlated with synchronous radar data. In addition to RCSs of more than 1 m² the breaking waves displayed concurrent copolarization ratios (HH/VV) exceeding 20 dB. Synchronous high speed video images showed that these peak values appear just after a plunging jet developed at the breaker crest, well before it hit the front face of the wave. Focusing of the electromagnetic energy on the jet by the parabolic front face of the breaking wave is suggested as a mechanism yielding both high HH returns and high HH/VV ratios. The Doppler speed of the HH peaks is 80% of the initial wave speed c₀