Geology Reference
In-Depth Information
Table 8.7
Radiation balance (MJ/m
2
month) of perennial sea ice the central Arctic.
Incoming
Longwave
Outgoing
Longwave
Incoming
Shortwave
Outgoing
Shortwave
Albedo
Shortwave
Net
Radiation
Jan
435
−520
—
—
—
−85
Feb
430
−500
—
—
—
−70
March
430
−505
100
15
0.85
−60
Apr
495
−600
420
340
0.81
−25
May
630
−725
730
585
0.80
50
June
750
−830
785
615
0.78
90
July
800
−835
575
330
0.57
210
Aug
780
−820
370
255
0.69
75
Sep
690
−720
155
120
0.77
5
Oct
580
−610
20
15
0.75
−25
Nov
475
−535
—
—
—
−60
Dec
450
−520
—
—
—
−70
Year
6945
−7720
3155
2345
0.74
35
Adapted from
Maykut
[1986].
Table 8.8
Broadband albedo under clear sky measured from
Arctic sea ice and its snow cover.
measurements. However, due to the dynamic nature of sea
ice, a continuous record of albedo could not be established
using this method (at least during the crucial stage of early
ice growth). Such a record can be established over land
because albedo measurements are usually conducted rou-
tinely in manned weather stations. Measurements of sea ice
albedo are sporadic as they are obtained on opportunity
basis during various field programs. This has led to an
inconsistency of the recorded values between different inves-
tigations, which preclude a straightforward comparison.
De
Abreu
[1996] addressed this point within discussions of
results from a few field programs conducted prior to 1995.
Perovich et al
. [1986] compiled a set of surface‐based
albedo measurements from a number of early studies on
Arctic sea ice under cloudy skies. Table 8.8 shows part of
their results with additional data from
Grenfell and
Maykut
. [1977]. Data are not sorted by ice thickness or
age, although these are the most commonly used criteria
of ice classification. The “white” and “blue” ice types are
defined based on the presence or absence of elements that
scatter light. Pure ice that does not have such scattering
elements is inherently blue because it absorbs all wave-
lengths of the visible spectrum but the blue (the wave-
length of minimum absorption is 0.47
μ
m according to
Grenfell and Perovich
[1981]). Therefore, blue ice acts as a
filter, but it is a weak filter so that only large blocks of ice
(more than 1 m thick) appear bluish to the eyes. Natural
upturned blocks of blue ice are commonly seen in the
Arctic. These are usually FY ice blocks that have under-
gone significant desalination. On the other hand, when
ice contains bubbles that scatter light at all visible wave-
lengths as in the case of glacier ice and to some extent
MY ice, the reflected light increases and the perceived
color changes from blue to white. Table 8.8 shows also
that bare FY ice has albedo around 0.52, which is signifi-
cantly higher than that of the ponded FY ice (typical
Surface Type
Albedo (P)
Albedo (G)
Seawater
0.06
Bare FY ice
0.52
Frozen white ice
0.70
Melting blue ice
0.32
0.25 / 0.39
Melting white ice
0.68
0.56 / 0.76
Ponded FY ice
0.21
0.22 / 0.29
Refrozen melt pond
0.40
New snow
0.87
Dry snow
0.81
Wind packed snow
0.81
Melting old snow
0.77
0.63 / 0.77
Note
: Letters P and G stand for data obtained from
Perovich
et al
. [1986] and
Grenfell et al
. [1977]. The value after the / is
albedo under cloudy sky.
albedo of 0.21). It is interesting to note the difference
between albedo from dry snow (0.87) and melting old
snow (0.77) in the table. In fact, lower albedo can be
reached as snow wetness increases. The water contents
in the melted snow absorb more radiation in the NIR
than the VIS regions. The surfaces shown in Table 8.8
cover the range from the largest to the smallest albedo
on Earth. It is worth noting that the low albedo of OW
shown in the table (0.06) is usually associated with a Sun
elevation of 30° or higher. Albedo increases at lower sun
elevation and in the presence of ocean waves. For example,
Johannessen et al
. [2007] found that the albedo of OW
can reach 0.025 when (the Sun elevation reached 10° over
calm sea, but this decreased to 0.02 in the presence of
moderate waves or lower with larger waves.
In situ measurements of broadband albedo from sev-
eral age‐based ice types in the Antarctic were compiled in
Warren et al
., [1997]. Measurements were obtained during
