The factor b as a Function of Frequency and Canopy Type at H-Polarization.

Abstract

International audienceFor anticipated synergistic approaches of the L-band radiometer on the Soil Moisture and Ocean Salinity (SMOS) mission with higher frequency microwave radiometers such as the Advanced Microwave Scanning Radiometer (AMSR) (C-band), a reanalysis has been performed on the frequency dependence of the linear relationship between vegetation optical depth (tau(o)) and vege- tation water content (W), given by tau(o) = b(.)W. Insight into the frequency dependence of the b-factor is important for the retrieval of surface moisture from dual- or multifrequency microwave brightness temperature observations from space over vegetation-covered regions using model inversion techniques. The b-values presented in the literature are based on different methods and approaches. Therefore, a direct comparison is not straightforward and requires a critical analysis. This paper confirms that when a large frequency domain is considered, the b-factor is inversely proportional to the power of the wavelength b = c/(gimel)(x), which is in line with theoretical considerations. It was found that different canopy types could be separated into different groups, each with a different combination of values for log (c) and x, which characterize the linearized relationship log(b) = log(c) - x (.) log(gimel). A comparison of ratios b(C)/b(L) (with C and L denoting G and L-band, respectively) also resulted in basically the same groups