Analysis of Nimbus-7 SMMR 6.6 GHz data over vegetated land surfaces: theory and examples

The scanning multichannel microwave radiometer (SMMR) launched aboard the Nimbus-7 satellite provided global data at five frequencies (6.6, 10.7, 18, and 37 GHz) and two polarizations. The global data over land surfaces from this instrument are available for the period of January 1979 to December of 1985. The 6.6 GHz data set, being the lowest frequency of SMMR is especially important for studying the vegetation canopies and the moisture variations of the underlying soil surface. In this study, a microwave emission model for vegetation canopies has been developed to simulate the 6.6 GHz channel of SMMR. The canopy model consists of three layers of crown, trunk and underlying soil. The emissivity from the canopy is obtained by first computing the bistatic radar cross section of the three layer canopy using the distorted Born approximation and then integrating the radar cross sections over the hemispherical scattering angles according to the conservation of energy. In this formulation, each layer of the canopy is modeled as a random distribution of canonical shape dielectric scatterers (discs and cylinders). The dielectric constant of the scatterers are determined according to the available moisture in various components of the canopy. The model is then verified over homogeneous agricultural canopies using a ground stationed radiometer system. In particular, the properties of the polarization ratio with respect to the available vegetation biomass and soil moisture have been analyzed and it is found that this ratio becomes less sensitive to soil moisture as the vegetation biomass increases. The model has then been modified to take into account the large spatial resolution of the SMMR data by introducing a distribution of gaps in each resolution cell. The model simulations are then used in conjunction with monthly averaged 6.6 GHz SMMR data over two areas of Amazon forest and North-West US to study the effect of the moisture and vegetation changes