Spectral albedo
|
Spectral albedo |
|
|
||
| Spectral albedos measured at the bare ice site on 12 days between 7 July and 12 August are plotted to the right. The spectral shape is the same for all of these curves. The albedo curve is high (0.7 to 0.8) and flat across the visible, giving a white appearance to the ice. Albedo decreases in the near-infrared. It is interesting to note the tight clustering of the white ice albedos. During this five- week period, the albedo at a particular wavelength never varied by more than 0.1. In sharp contrast to the melt pond sites, the changes that occurred on the bare ice were fluctuations, rather than a systematic downward trend. There was approximately 60-70 cm of surface ablation, yet no systematic change in albedo. For white ice, the albedo is dominated by scattering in a few-centimeter-thick surface scattering layer of granular, decomposing ice. As the melt season progressed, this layer kept renewing itself. We believe that the fluctuations in spectral albedo were due to variations in the thickness of this surface layer. Foggy days, with condensation melting on the surface, tended to thin the scattering layer, while sunny days, with larger fluxes of penetrating solar irradiance, tended to increase the layer thickness. |  |
|
|
There is a pronounced decrease in melt pond spectral albedo during summer. Spectral albedos from 17 July through 14 August for the light and dark ponds are plotted to the right. There was little variability in albedo in the near-infrared at wavelengths beyond 750 nm. At these wavelengths, the absorption in the water is so great that the underlying ice does not contribute to the albedo. The small differences in albedo (~0.05) are due to minor variations in the sky conditions and the water roughness at the pond’s surface. Differences were considerable at visible wavelengths (400 nm – 750 nm). In particular, from 400 nm to 600 nm light pond albedos were 0.2 to 0.25 larger than dark pond values. There was no appreciable difference in the pond water depth between the light and dark portion. As was stated earlier, the large difference in albedo were due to the scattering properties of the underlying ice. The scattering of the underlying ice has the greatest influence on albedo from 400 nm to 600 nm, where the pond water is the most transparent. For both the light and dark portion of the pond there was a steady decrease in albedo between 17 July and 14 August, with a reduction of 0.15 to 0.2. This occurred as melting deepened the ponds and thinned the scattering layer of the underlying ice. |
 |
| The
temporal evolution of areally averaged spectral albedos is similar to the
wavelength-integrated counterparts. Areally-averaged spectral albedos
were computed by averaging measurements along the first 100 m of the
albedo line. The time series of average albedo for selected wavelengths is
plotted below. Decreases in albedo were more rapid and larger in magnitude
at near-infrared wavelengths than at visible wavelengths. The separation
between the curves for different wavelengths reflects the influence of the
melt ponds versus bare and snow-covered ice.
For ponded areas, the spectral gradient between 500 and 1000 nm is
much stronger than for the other ice types, so the separation increases and
the magnitudes decrease as the ponds develop. |
|
Time series of spectral albedo averaged over the first 100 m of the albedo line. |
|
|||||||
|
Wavelength-integrated albedos Spectral albedos Transmittance UV incident
|
|||||||
 |
 |
 |
 |
 |
 |
||
