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The Helsinki University of Technology (HUT) snow emission model is used to calculate the time series of brightness temperature of snow-covered sparsely forested area for the winter 2006-2007. Brightness temperature simulations that apply in situ observed physical parameters as input are compared with the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) observations. Three models for the extinction coefficient of snow and the statistical and physical atmospheric models are compared. Simulation results are presented with full in situ data set and only air temperature and snow depth (SD) as input data. The obtained results indicate that the extinction coefficient model of Hallikainen originally used with the HUT snow emission model is the best suited for the Finnish snow data set used in this paper and also on frequencies which are outside the original range of the extinction coefficient model. The simulation results obtained using only air temperature and SD input data show that the HUT snow model is quite reliable even with a minimal in situ data set. A time series of optimized grain sizes was calculated by minimizing the simulation error. The optimized grain size tended to saturate with large values, and therefore, a new model to calculate an effective grain size was developed. The simulation with the effective grain size as input has lower rms error and higher correlation with AMSR-E data than the simulation with the measured grain size.