Correlation between the specific surface area and the short wave infrared (SWIR) reflectance of snow

The albedo of snow is determined in part by the size and shape of snow crystals, especially in the short wave infrared (SWIR). Many models of snow albedo represent snow crystals by spheres of surface/volume (S/V) ratio equal to that of snow crystals. However, the actual S/V ratio of snow has never been measured simultaneously with the albedo, for a thorough test of models. Using CH4 adsorption at 77 K, we have measured the specific surface area (SSA) of snow samples, i.e. its ratio S/(V · ρ), where ρ is the density of ice, together with the snow spectral albedo using a field radiometer with nadir viewing, at Ny-Ålesund, Svalbard. Tests are performed at 1310, 1629, 1740 and 2260 nm, and we find a good correlation between the SSA and the snow spectral albedo in the SWIR (linear correlation coefficient R2 > 0.98 for the last 3 wavelengths). Snow samples having varied crystals shapes such as rounded crystals in windpacks and hollow faceted crystals in depth hoar were studied and crystal shape did not affect the correlation in a detectable manner. An interest in using SSA rather than crystal size to predict SWIR albedo is that the reflectance of large hollow crystals such as depth hoar or surface hoar will be correctly predicted from their SSA, while considering their large dimensions would underestimate reflectance. We compare these correlations to those predicted by commonly used optical models. The best agreement is found when we compare our data to the modeled hemispheric reflectance, corrected by an adjustable factor that shows a small wavelength dependence. We propose that, once these results have been confirmed by more studies, it may be possible to design a rapid and simple optical method to measure snow SSA in the field. Our results may also allow a more detailed use of remote sensing data to study snow metamorphism, air–snow exchanges of gases, and climate.