ABSTRACT
Radar scattering from the ocean surface exhibits azimuthal dependence due primarily to the directionality of short ocean wind waves, often characterized within anisotropic ocean wave spectral models. Several such anisotropic ocean spectra have recently been investigated in the context of interpreting new satellite L band measurements. As a function of azimuthal dependence, upwind radar-scattering cross sections at L band are generally observed to be larger than cross-wind radar cross sections. However, at lower winds near 3–5 m/sec, a reversal has been observed where the cross-wind radar cross section exceeds upwind. This phenomenon is labelled as a Negative Upwind-Crosswind (NUC) asymmetry. In this chapter, radar scattering at L band by anisotropic ocean spectra are studied using the Numerical Maxwell model of 3 dimensional simulations (NMM3D). In using this numerical method, the ocean surface profiles are generated by ocean spectra, and the electromagnetic surface integral scattering equation is then solved by Method of Moment. Two candidate anisotropic spectra are explored in this chapter: (1) the Elfouhaily Spectrum with and without an imposed nonlinear horseshoe pattern; and (2) the Apel Spectrum with an empirically modified spreading function. The modified angular spreading function is obtained using the advanced integral equation EM scattering method (AIEM) and satellite measurements, and where millimeter and centimeter waves crosswind are allowed to be stronger than that of the upwind/downwind. Simulation results show that the horseshoe pattern does not cause NUC to happen while the new angular spreading function can reproduce NUC.
