Improved Microwave Ocean Emissivity and Reflectivity Models Derived from Two-Scale Roughness Theory

The Geometrical Optics (GO) approach and the FAST Emissivity Model (FASTEM) are widely used to estimate the surface radiative components in atmospheric radiative transfer simulations, but their applications are limited in specific conditions. In this study, a two-scale reflectivity model (TSRM) and a two-scale emissivity model (TSEM) are developed from the two-scale roughness theory. Unlike GO which only computes six non-zero elements in the reflectivity matrix, The TSRM includes 16 elements of Stokes reflectivity matrix which are important for improving radiative transfer simulation accuracy in a scattering atmosphere. It covers the frequency range from L- to W-bands. The dependences of all TSRM elements on zenith angle, wind speed, and frequency are derived and analyzed in details. For a set of downwelling radiances in microwave frequencies, the reflected upwelling brightness temperature (BTs) are calculated from both TSRM and GO and compared for analyzing their discrepancies. The TSRM not only includes the effects of GO but also accounts for the small-scale Bragg scattering effect in an order of several degrees in Kelvins in brightness temperature. Also, the third and fourth components of the Stokes vector can only be produced from the TSRM. For the emitted radiation, BT differences in vertical polarization between a TSEM and FASTEM are generally less than 5 K when the satellite zenith angle is less than 40°, whereas those for the horizontal component can be quite significant, greater than 20 K.