Global retrieval of small-scale roughness over land surfaces at microwave frequency

In various hydrological processes, surface soil moisture is very sensitive to the surface roughness. The surface roughness effect has been incorporated into the calculation of surface reflectivity by modifying the Fresnel reflectivity. The objective of this study is to estimate the small-scale roughness based on the difference of polarization between the rough and specular reflectivities over land at microwave frequencies. A unique approach is presented to estimate the small-scale roughness from the surface reflectivity with the microwave satellite observation. Using an approximate relationship between vertically and horizontally-polarized reflectivities, and rough reflectivity based on the radiative transfer equation and satellite observation, the small-scale roughness is retrieved globally over land surfaces. A realistic forward model to simulate the rough reflectivity is considered for the sensitivity test. Consequently, the small-scale roughness is retrieved within the reasonable range of previous works. This research proposes a unique technique to understand and estimate the roughness parameter over land surface by using the polarization characteristics at microwave frequencies; this research provides information that is extremely relevant to hydrological studies, especially for the global soil moisture retrieval in the SMOS, SMAP, and GPM missions.