OBSERVATIONS
1.0
There is a large observational dataset of sea ice
optical properties, particularly of sea ice albedos
(Perovich et al. 1986). In this section we present
0.8
an overview of these observations in the context
of illustrating how the optical properties of sea
ice are affected by the physical properties. We
Wavelength = 400 nm
investigate the effects of ice type, surface condi-
0.6
500
tions, ice thickness, ice brine volume, and impu-
rities on albedo, reflectance, transmittance, and
600
extinction coefficient. The simplifying beauty of
optical property observations is that, at least from
700
0.4
400 nm to 750 nm, what you see is what you get.
If the ice looks white, then its albedo will be high
800
and relatively constant with wavelength. Simi-
larly, the spectral albedo of a blue-looking melt
900
pond will have a peak between 400 and 500 nm.
0.2
1000
Albedos
Albedos are sensitive to thickness during the
initial stages of ice growth. Weller (1972) mea-
0
sured total albedos in a freezing lead and found a
5
10
15
20
25
Ice Thickness (cm)
rapid rise in albedo from 0.08 to 0.40 as the ice
Figure 8. Laboratory observations of the increase in
grew from open water to a thickness of 0.30 m,
spectral albedo during initial ice growth (Perovich
followed by a more gradual, asymptotic increase
1979). The ice was grown at an air temperature of
as the ice continued to grow. For ice thicker than
20C.
approximately 0.8 m total, albedo shows little
change with thickness (Maykut 1982).
Spectral changes in albedo during initial ice
bedo did not begin until the ice was 0.05 m thick.
growth are plotted in Figure 8. The ice was grown
During the first 0.05 m of growth, the ice in this
in the laboratory at a constant air temperature of
experiment had not yet begun to cool and brine
20C and had a columnar crystal structure
volumes were quite large. Because of this there
(Perovich 1979). Albedo increased with thickness
was little scattering in the ice. As the ice cooled
at all wavelengths. As the ice grows thicker, op-
and the large brine pockets fragmented into many
portunities for backscattering in the ice are added
smaller pockets, there was more scattering and
and at first albedos rise rapidly. However, the
the albedo increased. This observation illustrates
path length of the backscattered light also in-
again that the optical properties of sea ice are
creases, until finally only a negligible amount of
often more complicated than we would expect.
light penetrates to the bottom of the ice, is
Of course, even for thick ice, sea ice albedos
backscattered, and emerges from the ice surface
vary. From the previous discussion we can see
without being absorbed. At this point increasing
that albedo is sensitive to the ice surface condi-
the ice thickness no longer directly affects the
tions. Spectral albedos for multiyear ice are plot-
albedo and the ice is optically thick. As Figure 8
ted in Figure 9 (taken from Grenfell and Maykut
indicates, this asymptotic ice thickness is smaller
1977). These albedos represent a possible evolu-
at longer wavelengths. At shorter wavelengths
tionary sequence from spring to summer as melt
albedos are still increasing when the ice is 0.25 m
occurs, and the ice cover changes from snow-
thick, while at longer wavelengths (beyond 700
covered ice to bare ice or frozen ponds to melting
nm), the asymptotic nature of the albedo increase
ice to ponded ice. Snow albedos (curve a) are
is evident. This is a direct result of the increase in
large (~0.9) and nearly constant with wavelength
absorption as wavelength increases. This is consis-
in the visible; the snow appears bright and white.
tent with our earlier comment: spectral variations
Scattering coefficients for snow are so large that,
in optical properties are due to absorption. A closer
in the visible, absorption has little impact on the
look at Figure 8 shows that the rapid rise in al-
albedo and there is no wavelength dependence.
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