ARCTIC TUNDRA ALBEDO AND ITS ESTIMATION FROM SPECTRAL HEMISPHERIC REFLECTANCE

We present the results of a held experiment in which the nearly comple te bidirectional reflectance distribution function of Alaskan arctic t undra sites early in the growing season is measured by the PARABOLA in strument. The spectral hemispheric reflectances were computed by angul ar integration of these measurements for three wavebands: red (650-670 nm), near-infrared (810-840 nm) and shortwave infrared (1620-1690 nm) . Total albedo was then estimated by weighting the spectral hemispheri c reflectances by the fraction of total solar inadiance in three broad band spectral regions (300-700, 700-1300 and 1300-4000 nm) and represe nting each spectral region by the narrowband PARABOLA measurements. Th ese calculations resulted in albedo estimates with a mean relative err or of 15.7 per cent as compared to pyranometer measured albedo. Since vegetation reflectance varies signifrcantly over each of the three bro adband regions, additional reflectance weighting factors were computed from a combination of high spectral resolution canopy reflectance dat a and corresponding computed spectral solar irradiance. This additiona l reflectance weighting resulted in a reduction in the mean relative e rror to 7.5 per cent relative to pyranometer measured albedo. It is no ted that the three spectral bands of the PARABOLA instrument data repo rted here are similar to those of the spectral wavebands planned for f uture Advanced Very High Resolution Radiometer (AVHRR) sensors on Nati onal Oceanic and Atmospheric Administration (NOAA) satellites. Therefo re the results and techniques presented here may be useful for future global albedo estimation utilizing AVHRR sensors. The analysis present ed here may also be applied to albedo estimation from satellite sensor s with higher spectral resolution and more complete spectral coverage, such as the future orbiting MODIS sensor, in which the errors of spec tral reflectance weighting will be reduced considerably due to a more complete sampling of the reflected spectrum.