Numerical study of electromagnetic scattering from three-dimensional water waves

Scope and method of study. The purpose of this study was to study the mechanisms of microwave scattering from breaking water waves. The three-dimensional profiles modeling the crests of breaking water waves were synthesized from two-dimensional time history of breaking crests measured in a wave tank. The multilevel fast multipole algorithm (MLFMA) numerical code was implemented with RWG basis functions to provide the reference scattering calculations. A resistive loading was applied to suppress the edge effects in numerical method. The predictions of several analytical scattering models, including the two-scale model and extended geometrical optics/geometrical theory of diffraction, were compared with the reference scattering from breaking waves. A two-dimensional synthesis of the three-dimensional scattering was also considered.; Findings and conclusions. The resistive loading can effectively suppress the edge effects. The MLFMA results show that the co-polarized backscatter from both the spilling and plunging breakers is dominated by the sections which include overturning. The cross-polarized scattering from the post-breaking surface proved comparable with that from the overturning sections. Horizontally co-polarized (HH) backscatter from the complete plunging breaker slightly exceeded that of vertical polarization (VV). A strong super event, where HH is much larger than VV, was found when only the initial overturning of the plunging breaker was considered. The 2-D synthesis of the co-polarized backscatter matched the MLFMA scattering from both the complete and pre-breaking spilling breaker surfaces. However, it was inaccurate when applied to the post-breaking sections. The two-scale model showed a strong dependence on the separation threshold when applied to spilling breaker surface that included the steep/overturning section. It was less sensitive after breaking, and the best results were obtained with a threshold of k/1.2, where k is the electromagnetic wavenumber. EGO was first applied to individual measured 2-D profiles in the plunging breaker wave sequence. It proved able to predict strong super events accurately. EGO was then extended to the 3-D initial-overturning surface derived from the plunging breaker. It predicted the super event after the model parameters were manually adjusted. This preliminary results show that EGO may be useful in modeling the scattering from more realistic 3-D wave profiles.