Direction Dependence of the Fully Polarimetric Wind-induced Ocean Emissivity at L-band: Modeling and Anisotropy Analyses

In this paper,a semi-theoretical model is proposed to numerically investigate the directional properties of L-bandwind-driven ocean emission for full polarizations. By incorporating the well-developed composite model of ocean polarimetric emission, a newly-retuned directional spectrum of short-gravity ocean waves, the advanced Anguelova and Gaiser 2013 model (AG13) for foam emissivity and a novel foam fraction model, the approach can simulate the fully polarimetric brightness temperature (TB) from ocean surface with wind speeds up to 35 m/s. The model is further validated and tuned against the observation data of two L-band radiometer missions, i.e., Soil Moisture Active-Passive (SMAP) mission and Aquarius mission. Comparisons of model simulations to satellite data suggest that the overall accuracy of model could reach 0.2 K. Particularly, the proposed model successfully reproduces the negative and positive upwind-crosswind (UC) asymmetries of L-band ocean surface TB and the transition of the phase signatures versus wind speed for all polarizations. In addition,numerical results also suggest that, at high wind speeds, the anisotropies of L-band ocean surface TB are saturated for vertical- (V-) and horizontal- (H-)polarizations. For the third (3rd) and fourth (4th) Stokes components, the ocean emission anisotropies increase almost linearly from around 10 m/s, and the saturation of anisotropy is only observed for the 3rd Stokes signals at about 24 m/s. The direction-induced L-band TB variations for the third Stokes parameter show less sensitivity to wind speed with the increase of observation angles, while those for the fourth Stokes are on the contrary.