Geology Reference
In-Depth Information
East Antarctic cruises in the austral spring, covering the
spectral range 0.3-2.8
μ
m. Results are presented in
Table 8.9 for six ice types (MY ice is not included because
it does not often exist in the Antarctic). Open‐water and
grease ice have nearly equal and low albedo. As mentioned
in section 2.1.3 grease ice is not actually a solid material
but rather a “soupy” layer that does not reflect much
light and cannot hold snow. After this early phase of ice
formation, albedo increases rapidly as ice develops into
Nilas and gray ice types. The main point that can be
drawn from Table 8.9 is the significant increase
of surface albedo by adding even a thin layer of snow
(<3 cm) (further discussion on this point can be found in
Allison et al.
[1993] and
Warren et al
. [1997]). Albedo from
thick ice with more 3 cm of snow is nearly twice as high as
its value from bare ice surface. The high broadband albedo
of the snow is mainly generated by the shortwave irradi-
ance [
Allison
, 1993;
Brandt et al
., 1999]. This obviously
causes delay of ice surface ablation and may contribute to
the survival of sea ice in some places [
Ledley
, 1991].
A notable data set of spectral albedo from age‐based ice
types is presented in
Brandt et al
. [2005]. Measurements
were obtained from three ship‐based voyages of the
Australian Research Expedition in the Southern Ocean
in October/November 1988, September/November 1996,
and December 2000. Spectral albedo was measured in
visible and near infrared wavelengths for open‐water,
grease ice, Nilas, gray ice, gray‐white ice, and FY ice with
and without snow cover. Broadband albedo was calcu-
lated from the spectral albedo for clear and cloudy skies.
Measurements were conducted using a radiometer cover-
ing the wavelength region 320-1060 nm. Figure 8.23 shows
results adapted from the aforementioned study. In addi-
tion to the spectral albedo, the broadband solar albedo
α
is given for each surface type. The spectral variation of
albedo is determined by absorption of the solar radiation
rather than by scattering because the scattering elements
(inclusion in sea ice) are much larger than the wavelength
of the visible radiation [
Grenfell
, 1983]. The albedo of 2 cm
Nilas is almost equal to the albedo from water surface.
The later coincides with the calculations from Fresnel
reflection equation (section 7.3.2.). Albedo increases
as ice thickens with a higher rate during the early phase
of ice growth (Nilas/gray ice/ gray‐white ice). The great-
est increase of albedo is observed within the band
Table 8.9
Broadband albedo (0.3-2.8
μ
m) of standard ice
types measured during east Antarctic cruises in the spring.
Thin Snow
<30 mm
Thin Snow
≥30 mm
Ice Type
Snow Free
Open water
0.07
—
—
Grease ice
0.09
—
—
Nilas: <10 cm
0.16
0.42
—
Gray ice:
10-15 cm
0.25
0.52
0.70
Gray‐white ice:
15‐30 cm
0.35
0.62
0.74
FY (thin) ice:
30-70 cm
0.42
0.72
0.77
FY (thick) ice:
>70 cm
0.49
0.81
0.85
Note
: the significant increase of albedo during the develop-
ment of the first 15 cm of thickness. [
Warren et al
., 1997].
1. 0
Deep snow
α
=0.83
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Open water
α
=
0.067
0.0
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Figure 8.23
Spectral albedos of snow‐free and snow‐covered ice types and open water. Broadband solar albedo
α
is also given. The model results are from the delta-Eddington radiative transfer. The curves marked by the
bracket (shown at the bottom right corner) are for snow‐free ice [adapted from
Brandt et al
., 2005].
