A comparison of radiative transfer models for predicting the microwave emission from soils

Two general types of numerical models for predicting microwave emission from soils are compared—coherent and noncoherent. In the former, radiation in the soil is treated coherently, and the boundary conditions on the electric fields across the layer boundaries are used to calculate the radiation intensity. In the latter, the radiation is assumed to be noncoherent, and the intensities of the radiation are considered directly. The results from the two approaches may be different because of the effects of interference, which can cause the transmitted intensity at the surface (i.e., emission) to be sometimes higher and sometimes lower for the coherent case than for the noncoherent case, depending on the relative phases of the reflected fields from the lower layers. This coupling between soil layers in the coherent models leads to greater soil moisture sampling depths observed with this type of model, and is the major difference that is found between the two types of models. In noncoherent models, the transmission at the surface is determined by the dielectric constant at the air/soil interface. The subsequent differences in the results are functions of both the frequency of the radiation being considered and the steepness of the moisture (i.e., dielectric constant) gradient near the surface. The calculations were performed at frequencies of 1.4 and 19.4 GHz and for two sets of soil profiles. Little difference was observed between the models at 19.4 GHz; and only at the lower frequency were differences apparent because of the greater soil moisture sampling depth at this frequency. For those situations which have a nonuniform temperature profile a definition for an effective emissivity is given, that is, the ratio of brightness temperature to the effective radiating temperature, the latter being a weighted average temperature over the emitting depth. Measurements from a tower are presented which show excellent agreement with the calculations from the coherent model.