On the determination of surface emissivity from Satellite observations

Terrestrial brightness temperatures measured from satellites have been used to determine the surface emissivity. The results not only depend on surface temperature and on atmospheric properties, but also on the type of surface scattering. For otherwise identical conditions (same emissivity, same nonscattering atmosphere), the radiation above the Lambertian surface is larger than for a specular surface if the incidence angle is smaller than about 55/spl deg/. The opposite is true for larger angles. The effect leads to overestimates of emissivity for observations especially near nadir with the use of algorithms assuming specular reflection. The problem may be solved by the introduction of a specularity parameter to characterize realistic surfaces by a combination of specular and Lambert scattering. A simple solution lies in the use of conically scanning radiometers at a constant incidence angle near 55/spl deg/. Although the topic applies to all ranges of thermal radiation, the present discussion concentrates on the microwave spectrum in the Rayleigh-Jeans approximation.     

Microwave dielectric properties of surface snow

First results from a recently developed surface sensor for measuring the dielectric constant of snow at about 1 GHz are reported together with ground-based measurements of brightness temperatures between 4.9 and 94 GHz. The data are used to derive spectra of complex dielectric constants of wet snow for frequencies between 1 and 100 GHz. The result is simple: Debye relaxation spectra with a constant relaxation frequency of 9 GHz appear in contradiction to the mixing formula of Polder and van Santen. A way of resolving this discrepancy is presented.     

Polarization effects in seaice signatures

Observations of microwave emissivities of multiyear sea ice showed anomalies at horizontal polarization in the frequency range from 5 to 35 GHz during the Norwegian Remote Sensing Experiment (NORSEX) [1] in September and October 1979. The effect can be explained by layers of solid ice present in the dry snow cover throughout the NORSEX area. A special experiment made on a typical multiyear floe confirms this explanation. Since the results also indicate that at 94 GHz the layers do not affect the radiation, a dual-polarized radiometer in the 90-GHz window is a promising sea-ice sensor.