Comparison of two microwave radiobrightness models and validation with field measurements

This paper compares microwave brightness temperature (TB) estimated by two radiobrightness models--a multi-layer coherent radiative transfer (CRT) model and a single-layer Fresnel reflectance model. Two dielectric mixing schemes were used along with the models to calculate permittivity (real part of the dielectric constant). Model TB and permittivity estimates were intercompared and validated against Huntsville ’98 field experiment measurements. Model differences can be attributed to the mixing scheme, the radiobrightness model, or the vertical profile representation. Two sets of simulations were performed to quantify the sources of variation, one using observed soil temperature and moisture profiles as input, and another using uniform profiles. Using uniform profiles, systematic differences in permittivity estimated by the mixing schemes resulted in TB differences as large as 15 K. However, for uniform profiles, differences in TB estimated by the radiobrightness models for a given permittivity value were less than 2 K. For cases using observed profiles, near-surface drying of the profiles resulted in TB values from the CRT model 6-10 K higher than estimates from the Fresnel model, which determines TB based on 0-5 cm mean moisture and temperature. Therefore, the major sources of TB variations were the dielectric mixing scheme and the shape of the near-surface moisture profile. No radiobrightness/mixing scheme combination exhibited superiority across all plots and times.