In-situ measured spectral directional emissivity of snow and ice in the 8-14 μm atmospheric window

The directional emissivity of snow and ice surfaces in the 8-14 μm thermal infrared (TIR) atmospheric window was determined from spectral radiances obtained by field measurements using a portable Fourier transform infrared spectrometer in conjunction with snow pit work. The dependence of the directional emissivity on the surface snow type (grain size and shape) was examined. We obtained emissivity spectra for five different surface types, i.e., fine dendrite snow, medium granular snow, coarse grain snow, welded sun crust snow, and smooth bare ice. The derived emissivities show a distinct spectral contrast at wavelengths λ = 10.5-12.5 μm which is enhanced with increasing the snow grain size. For example, emissivities at both 10.5 μm and 12.5 μm for the nadir angle were 0.997 and 0.984 for the fine dendrite snow, 0.996 and 0.974 for the medium granular snow, 0.995 and 0.971 for the coarse grain snow, 0.992 and 0.968 for the sun crust, and 0.993 and 0.949 for the bare ice, respectively. In addition, the spectral contrast exhibits a strong angular dependence, particularly for the coarser snow and bare ice, e.g., the emissivity at λ = 12.5 μm for the off-nadir angle of 75° reaches down to 0.927, 0.896, and 0.709 for the coarse grain snow, sun crust, and bare ice cases, respectively. The angular dependent emissivity spectra of the bare ice were quite consistent with the spectra predicted by the Fresnel reflectance theory. The observed results firmly demonstrate that the directional emissivity of snow in the TIR can vary depending upon the surface snow type. The high variability of the spectral emissivity of snow also suggests the possibility to discriminate between snow and ice types from space using the brightness temperature difference in the atmospheric window.