The low-grazing-angle electromagnetic scattering from spilling breaker wate
r waves has been examined using a numerical approach. A moment-method-based
electromagnetic technique was used to find the instantaneous scattering fr
om the crests of breakers generated mechanically in a wave tank in the abse
nce of wind. A high-speed imaging system carried on an instrument carriage
traveling at the phase velocity of the breakers provided continuous measure
ments of the temporal evolution of the crest shape, in turn allowing a cont
inuous calculation of the microwave backscatter from the crest, As the wave
steepens and a bulge forms on the forward face of the crest, horizontal-to
-vertical polarization backscattering ratios (HH/VV) as high as 0 dB are ob
served. Greatly reduced ratios are observed after breaking when no steep fe
atures remain on the wave. A time-dependent Fourier analysis identifies "fa
st" scattering whose Doppler shifts are nearly equal to or greater than the
phase velocity of the breaker, and "slow" scattering whose Doppler shifts
are consistent with the orbital motion of the breaker. Comparison of the Do
ppler shifts with the measured wave profiles shows that the fast scattering
identified is associated with very steep surface features that give specul
ar (or nearly specular) reflection points on the surface, but the magnitude
and HH/VV ratio of the response to a specific steep feature depends upon i
ts electromagnetic size, thus giving a frequency dependence, The slow scatt
ering identified (in the absence of wind) is correlated with the "turbulent
scar" that remains after breaking.