Physically Based Simulations of Foam-Covered Ocean Emission at Microwave Frequencies

An accurate sea foam emissivity model at microwave frequencies is indispensable for applications of microwave data in satellite data assimilation and remote sensing of atmospheric and surface parameters. This study develops an analytic model for estimating sea foam emissivity by integrating the two-stream radiative transfer approximation with a macroscopic parameterization approach. Sea foam is modeled as a vertically inhomogeneous medium, with an exponential vertical profile based on void fraction and foam thickness. The emissivity for vertical and horizontal polarization is derived by combining the interface reflections with the two-stream radiative transfer solutions. The model is validated against the experimental data and shows a high consistency with incoherent methods, thus demonstrating its robustness for remote sensing and oceanographic applications. A sensitivity analysis is also performed for the key model parameters. It is found that within a frequency ranging from 1 to 37 GHz, the L-band is the most sensitive to sea foam thickness and the sensitivity decreases as the frequency increases. The emissivity is saturated for a specific foam thickness. Furthermore, the algorithm developed in this study includes two additional adjustable parameters, namely, the single-scattering reflectance and the asymmetry factor, which enable the simulation to account for the scattering effects of foam, thereby outperforming the traditional incoherent method.