the two cases are smaller than differences within
instruments mounted on aircraft or satellites typi-
the two.
cally have narrow fields of view and measure
reflected radiance, rather than irradiance. To con-
Reflectance
vert these observations of radiance to an estimate
When considering light reflected from sea ice,
of irradiance, information on the bidirectional re-
the albedo is the parameter of prime climatologi-
flectance distribution function (BRDF) is needed.
cal importance. However, optical remote sensing
Under cloudy conditions the incident radiation
field is diffuse, so the light reflected from the
1.0
surface is also diffuse. Reflected radiance is es-
sentially the same in any direction and the re-
Fast Ice
flected irradiance is easily computed. When it is
w/30 cm Snow
sunny, though, the incident radiation consists of
0.8
a diffuse sky component plus a very strong solar
direct beam, and the incident radiance field is
strongly anisotropic. For these incident conditions
the angular distribution of reflected radiance can
be complex. Angular reflectances (reflected radi-
0.6
ance normalized to a white reference standard)
measured under sunny skies for snow-covered
ice and bare blue ice are plotted in Figure 13.
Young, Gray Ice
Snow-covered ice reflectances are fairly constant
0.4
w/2 cm Snow
with angle, except for a 30% increase at the angle
of reflection of the solar beam. The peak in reflec-
Young,
tance at the angle of reflection is even more pro-
Gray Ice
nounced in the bare blue ice case, with an in-
0.2
crease to nearly twice the value of the albedo. At
Nilas 3-4 cm
other angles, blue ice reflectances are equal to or
slightly less than the albedo. The differences be-
tween R and α show the importance of the BRDF
Open Water
in determining albedos from observations of re-
0
1000
600
800
400
flected radiance. The presence of any systematic
Wavelength (nm)
topographic features, such as sastrugi, will fur-
Figure 12. Spectral albedos of Antarctic sea ice (from
ther complicate the BRDF.
Allison et al. 1993).
2.0
Zenith = 30
Zenith = 60
1.5
Figure 13. Bidirectional
Snow
reflectance distribution
1.0
function at 450 nm for
snow-covered ice and bare
blue ice (from Perovich
Blue Ice
α
1994). R0 is the normal-
Ro
0.5
ized reflected radiance at
nadir and α is the albedo.
The measurements were
made at a solar zenith
angle of 60 under clear
0
180 180 120 60
0
60
120
180
180 120 60
0
60
120
skies.
Azimuth
Azimuth
11
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