Global brightness temperature simulations were performed at 0.25 degree resolution both including the atmospheric effect and pixel heterogeneity in wide wave band. For surfaces such as snow, deserts, and vegetation, volumetric scattering was calculated using a two-stream radiative transfer approximation. The reflection and transmission at the surface-air interface and lower boundary were derived by modifying the Frenel equations and QP model to account for cross-polarization. Several models were utilized to compute the optical parameters for the medium. Global Land Data Assimilation Systems (GLDAS) provided time series of the main input variables. These simulations were compared with Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) measurements in January, April, July, and September 2003, including both the spectral and temporal variations. A sensitivity study was also carried out to access the relative contributes of the main parameters (particularly the roughness and soil moisture). Difference between simulated and measured TBs were analyzed, discriminating possible issues either linked to the radiative transfer model or due to land surface parameters .