A volume emission model for the radiobrightness of prairie grass

The radiobrightness of snow-free northern prairie at the SSM/I frequencies of 19.35, 37.0, and 85.5 GHz is dominated by absorption and emission by the grass canopy. The authors measured the radiobrightness at SSM/I frequencies of grassland near Sioux Falls, South Dakota, every 15 minutes from October, 1992, through early April, 1993. The apparent H-polarized emissivities for snow-free periods were generally above 0.95. These high emissivities can only be explained by emission from the grass canopy. The vertical distributions of mass and moisture in prairie grass were used to develop a volume emission model for grass over moist soil. The volume emission model was derived from a formally correct version of the radiative transfer equation where radiant intensities were normalized by the refractive index squared. For the 7 cases that were analyzed, scatter darkening was insignificant at 19.35 GHz and significant in only two cases at the higher SSM/I frequencies