Laboratory measurements of LGA Doppler spectral components and their physical sources unveiled

A C-band high range resolution (3.77 cm) FMCW radar was used to produce averaged Doppler spectra from water waves. The physical sources of the different spectral components in the Doppler spectra have been identified. Two different 2.3 m mechanical waves were used, a monochromatic wave with steepness 0.165 and a wave repeatedly breaking in wavegroups, with a carrier wave of the same initial steepness. Wind from 2-12 m/s was also added. For monochromatic waves with wind up to 6 m/s, the Doppler spectra in HH and VV can be explained with convected Bragg. At higher wind speeds, the spectral peaks separate, HH being faster than VV. This separation is caused by the appearance of microbreakers, they are compact in range, travel faster than convected Bragg and dominate the HH returns, where Bragg is relatively low. The spectral shapes in HH and VV are quite similar for breaking waves without wind, but VV from convected Bragg dominates at lower frequencies. The spectral peak is in both polarizations at 80% of the phase speed of the wave, and it is caused by the plunging jet from the breaker. As the wind speed grows, a Bragg peak begins to appear in VV, which outgrows the fast peak from the jet at windspeeds above 5 m/s