Electromagnetic scattering from large steady breaking waves

A submerged hydrofoil generated large steady breaking waves of 0.3 m and 0. 4 m height in a circulating water channel. We measured water fraction in th e breakers with conductivity probes. We observed the radar cross-section of the breakers at X-band with a pulsed step-frequency instrumentation radar with high spatial resolution in the downstream direction. The normalized ra dar cross-section increases with increasing elevation angle of observation for both vertical and horizontal polarization. This variation is consistent with a simple interpretation of the breaking wave as a diffuse (Lambertian ) surface. However, the observed sizes and shapes of fluid elements in the breakers clearly show that construction of a theory for electromagnetic sca ttering from first principles will be challenging. We also obtained the vel ocity spectrum of the scattering features within the breakers. This spectru m indicates that slower moving small liquid elements rather than the faster moving large disturbances are responsible for most of the electromagnetic scattering.