Landsat TM derived and in situ summer reflectance of glaciers in Svalbard

Article English OPEN
Winther, Jan-Gunnar (1993)

The paper presents satellite-derived and in situ reflectance from the Austre Brøggerbreen and Midre Lovenbreen glaciers located 78°50?N, 11°50?E in the Svalbard archipelago. The satellite data are Landsat-5 TM images recorded on 7 August 1987 and 31 August 1988. In situ measurements of shortwave and spectral reflectance of snow, glacier ice and moraine were carried out in August 1991 and in June 1992. In 1987, Austre Brøggerbreen had a positive net mass balance of +0.22 m of water equivalent while the net mass balance in 1988 was negative (-0.52 m). This is reflected in the atmospherically corrected satellite-derived albedo which shows that the reflective characteristics of the ablation and accumulation zones are more uniform and better separated in 1987 than in 1988. Besides the long-term year to year variations of surface albedo, the glaciers show a considerable short-term as well as spatial variability in reflectance. For example, the satellite-derived TM Band 4 albedo was found to vary between 0.19 and 0.65 at Midre Lovenbreen on 31 August 1988. In situ measurements show a drop in daily mean albedo from 0.88 to 0.13 during 4 days at a fixed location on Austre Brøggerbreen in August 1991 (300rn.a.s.l.). Furthermore, spectral measurements of snow albedo in June 1992 clearly demonstrate that the near-infra-red albedo decreases when the snow metamorphoses, i.e. when the characteristic grain size increases. The effect of cloud cover on the snow albedo is also discussed. The integrated albedo (370–900 nm) increased from 0.812 to 0.869 (7%) when the weather condition changed from clear sky to 100% overcast within 2 hours on 9 June 1992. The bidirectional reflectance of snow is measured by taking spectral scans for viewing angles 0° (nadir), 15°, 30°, 45° and 60° for viewing directions facing the sun and at azimuths 90° and 180° away from the sun. The increase in albedo relative to the nadir albedo is found to be 8, 15, 19 and 26% for viewing angles 15°, 30°, 45° and 60°, respectively. The largest anisotropy is seen for metamorphosed snow in measurements facing the sun. Consequently, it may be necessary to correct for the specular properties of snow if satellite-derived surface reflectance is going to be considered in terms of absolute values.
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