A 0.1-m-thick layer of wind-packed Arctic snow
cally has a drained bubbly surface layer with
is sufficient to eliminate any contribution to the
plenty of air bubbles which, while not as strongly
albedo from the underlying ice.
scattering as snow, still contributes considerable
Spectral albedos decrease as the surface
scattering. The result is an overall decrease in
changes from snow-covered ice to cold, bare
albedo of approximately 10% and a slight wave-
multiyear ice (curve b). White multiyear ice typi-
length dependence. As the ice warms and begins
to melt (curve c), albedos continue to decrease
with a more evident wavelength dependence. This
1.0
is due to a decrease in scattering as the ice melts
a
and some of the air voids fill with water. In some
b
areas of the ice cover, water collects on the sur-
face, forming melt ponds (curve d). As the melt
0.8
c
season progresses these ponds can get deeper
(curve e). Albedos of ponded ice are character-
ized by a maximum in the 400500 nm region
d
0.6
and a precipitous decrease between 500 and 800.
The melt ponds look blue. This spectral behavior
is due to the transparency of the water at shorter
e
wavelengths; albedos below 500 nm are deter-
0.4
mined primarily by the scattering properties of
the underlying ice. From 500 to 800 nm the al-
bedo becomes increasingly insensitive to the un-
derlying ice as the absorption in the water be-
0.2
comes the dominant factor. Above 800 nm
absorption in the water is so great that pond albe-
dos are essentially determined by Fresnel reflec-
tion at the surface and are independent of wave-
0
400
500
600
700
800
length. These results indicate that both the
Wavelength (nm)
magnitude and the shape of the spectral albedos
Figure 9. Spectral albedos for a possible evolu-
are extremely sensitive to the amount of liquid
tionary sequence of multiyear ice (Grenfell and
water present in the upper part of the ice.
Maykut 1977): a) snow-covered ice, b) cold bare
When skies are clear, the wavelength interval
ice, c) melting bare ice, d) early-season melt
from 1000 to 2500 nm can contain up to 25% of the
pond and e) mature melt pond.
total incident shortwave energy (Grenfell and
Perovich 1984), so albedos in this re-
1.0
gion can have a significant impact on
a
the heat and mass balance of the ice.
Figure 10 shows a spectral albedo se-
0.8
b
quence that first-year ice might follow
c
as it progresses through a melt cycle
0.6
from (a) ice covered by cold dry snow,
d
(b) to ice covered by melting snow, (c) to
bare ice with a crumbly surface layer,
0.4
and (d) to melting first-year blue ice.
Concentrating on wavelengths beyond
1000 nm, a continual downward trend
0.2
is evident, with albedo reaching a mini-
mum at about 1500 nm. Local maxima
0
are located at 1350, 1900 and 2300 nm
400
800
1200
1600
2000
2400
and correspond to minima in the ab-
Wavelength (nm)
sorption spectrum for ice. In general,
Figure 10. A spectral albedo sequence that first-year ice might
sea ice and snow albedos at longer
follow through a melt cycle (Grenfell and Perovich 1984): a) ice cov-
ered by cold dry snow, to b) ice covered by melting snow, to c) bare
wavelengths are significantly smaller
ice with a crumbly surface layer, to (d) melting first-year blue ice.
than values at visible wavelengths.
9
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