Geoscience Reference
In-Depth Information
reflectance factor (Nolin and Liang,
2000
). Albedo differs from spectral reflectance,
which is the reflectance at a single wavelength or in a narrow spectral band.
5.4.2
Typical Surface Albedos of Major Surface Types
Typical albedos for major surface types in the Arctic are summarized in
Table 5.3
.
There is a wide range of values even within the general categories. In part, the range
is explained by small-scale topography as it influences the geometry of insolation
and heterogeneities in the physical properties of the surface being considered. For
example, the albedo of tundra or boreal forest can differ strongly over small space
scales because of variations in species distribution, leaf area, soil characteristics,
and soil moisture. Most surfaces are forward scattering and scatter differently over
different portions of the solar spectrum. Hence the range also manifests variations
in the spectral nature of the solar flux and the relative magnitude of direct versus
diffuse components. Snow cover, both on land and sea ice, dominates the Arctic for
much of the year and merits special attention. S. Warren (
1982
), Barry (
1996
), and
A. Nolin and S. Liang (
2000
) provide comprehensive reviews. G. Maykut (
1986
)
discusses the spectral reflectance and albedo characteristics of sea ice.
5.4.3
The Albedo of Snow
As outlined by Nolin and Liang (
2000
), snow can be considered as a layered par-
ticulate medium composed of ice spheres in air. Scattering by the spheres is primar-
ily through refraction. Using the refractive indices of ice and an optically equivalent
ice sphere radius, Mie theory (appropriate when the effective particle radius is much
larger than the wavelength of the interacting radiation) can be used to calculate sin-
gle-particle scattering and absorption (i.e., the scattering and absorption of a single
snow grain). In turn, the single-particle scattering properties can be transformed into
multiple scattering properties using radiative transfer approaches such as the delta-
Eddington approximation that characterizes the strong forward scattering of snow
(Mie scattering) in the solar wavelengths (Wiscombe and Warren,
1980
). Although
Table 5.3.
Representative albedos for Arctic surfaces
compiled from numerous sources
Fresh snow
0.70-0.90
Melting Snow
0.50-0.60
Water
0.06-0.10*
Dry Tundra
0.23-0.26
Wet Tundra
0.10-0.20
Multiyear Sea Ice
0.55-0.75
Thick First-Year Sea Ice
0.30-0.60
Meltponds on sea ice
0.15-0.40
* varies widely with solar zenith angle
