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 t o study spectral reflectance characteristics of snow during melt-off. The i nvestigations were performed on snow-covered tundra at Ny-Alesund, Svalbard (79 degrees N). Spectral measurements were acquired with spectroradiometer s covering wavelengths from 350 to 2500 nm. Supporting measurements such as snow thickness, density, content of liquid water, grain size and shape, st ratification of snowpack, as well as cloud observations and air temperature , were monitored throughout the field campaigns. Spectral measurements demo nstrate that the near-infrared albedo is most affected by the ongoing snow metamorphism while the albedo in the visible wavelength range is more stron gly affected by surface pollution. Comparisons of spectral measurements and spectrally integrated measurements emphasize the need for narrow-band to b road-band conversion when applying satellite-derived albedo to surface ener gy-balance calculations. As an example, Landsat TM Band 4 albedo is shown t o 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 (similar to 10%) w ithin two to four weeks and the date when the tundra becomes snow-free vari es from early June to early July. Thus, the changing spectral characteristi cs 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 su rface albedo For an event in which the integrated albedo increased by 7% un der cloudy conditions compared to clear skies without changes of surface pr operties. Finally, the reflectance of snow increases relative to nadir for measurements facing the sun and at azimuths 90 degrees and 180 degrees by 8 , 15, 19, and 26% for viewing angles 15 degrees, 30 degrees, 45 degrees, an d 60 degrees, respectively, due to anisotropic reflection. Copyright (C) 19 99 John Wiley & Sons, Ltd.