Spectral reflectance of melting snow in a high Arctic watershed on Svalbard: some implications for optical satellite remote sensing studies

Field campaigns were undertaken in May and June of 1992 and 1997 in order to study spectral reflectance characteristics of snow during melt-off. The investigations were performed on snow-covered tundra at Ny-Alesund, Svalbard (79°N). Spectral measurements were acquired with spectroradiometers covering wavelengths from 350 to 2500 nm. Supporting measurements such as snow thickness, density, content of liquid water, grain size and shape, stratification of snowpack, as well as cloud observations and air temperature, were monitored throughout the field campaigns. Spectral measurements demonstrate that the near-infrared albedo is most affected by the ongoing snow metamorphism while the albedo in the visible wavelength range is more strongly affected by surface pollution. Comparisons of spectral measurements and spectrally integrated measurements emphasize the need for narrow-band to broad-band conversion when applying satellite-derived albedo to surface energy-balance calculations. As an example, Landsat TM Band 4 albedo is shown to produce slightly high albedo values compared to the spectrally integrated albedo (285–2800 nm). Daily albedo measurements from 1981–1997 show that the albedo normally drops from 80% to bare ground levels (∽10%) within two to four weeks and the date when the tundra becomes snow-free varies from early June to early July. Thus, the changing spectral characteristics of snow during melt-off combined with a general rapid decrease in albedo call for cautious use of satellite-derived albedo, especially when used as absolute numbers. Our data also illustrate the effect of cloud cover on surface albedo for an event in which the integrated albedo increased by 7% under cloudy conditions compared to clear skies without changes of surface properties. Finally, the reflectance of snow increases relative to nadir for measurements facing the sun and at azimuths 90° and 180° by 8, 15, 19, and 26% for viewing angles 15°, 30°, 45°, and 60°, respectively, due to anisotropic reflection. Copyright © 1999 John Wiley & Sons, Ltd.