Geology Reference
In-Depth Information
November
November
December
January
February
February
March
April
1977-78
April
1978-79
October
December
January
March
150
Theory
1977-1978
100
Theory
1978-1979
50
1977-1978
(Freeze-up date: 26 oct. 1977)
1978-1979
(Freeze-up date: 22 oct. 1978)
0
0
1
2
Days when temperature was below freezing
3
4
5
Figure 2.14
Growth of ice in Eclipse Sound measured during winters of 1977-1978 and 1978-1979, compared
to calculations. Dates given at the top of the figure indicate the time of the season [
Sinha and Nakawo,
1981].
In the absence of any snow cover, it can further be
reduced to
atmosphere. The high albedo of dry snow causes most
of the received solar radiation to be reflected, thereby
affecting the surface energy balance [
Maykut,
1986].
More importantly, the poor thermal conductivity of the
snow slows down the heat flux from the ice‐water inter-
face to the atmosphere through the ice volume. Depending
on its wetness, the thermal conductivity of snow varies
between 0.1 and 0.4 W/m · K while that of sea ice is roughly
2 W/m · K [
Schwerdtfeger,
1963
; Ono,
1967
; Mellor,
1977
;
Massom et al.,
2001]. In general, the thermal conductivity
of snow is one order of magnitude less than that of sea
ice. This means that 50 mm of snow deposited on sea ice
with thickness of 0.50 m will reduce the ice growth rate
by half. The effectiveness of snow as an insulator depends
also on its compactness (i.e., age). Newly fallen snow is
soft and fluffy, so it is an excellent insulator because of its
high air content. The compacted snow, on the other hand,
is a relatively poor insulator. Table 2.2 describes various
snow conditions in terms of the sea ice thickness that
would have the same insulating effect. It is obvious that a
few millimetres of even loosely packed snow on top of
sea ice will slow down the ice growth rate significantly.
Except for a few field studies, such as the weekly meas-
urements through the entire ice growth seasons of 1977-
78 and 1978-1979 in Eclipse Sound [
Sinha and Nakawo,
1981
; Nakawo and Sinha,
1981], presented in Chapter 3,
and the studies in Mould Bay in 1981-1985 described
in Chapter 5 (section 5.1), the physical properties and
effect of snow over on sea ice have not received as much
attention in the literature as snow over land. The field
2
k
D
h
2
i
(
TTdt
)
(2.18)
i
wa
L
1
i
i
This is the same equation as equation (2.9). The right‐hand
side represents the accumulated freezing degree‐days.
Equation (2.17) was used to calculate ice thickness
versus the number of freezing degree‐days for different
values of snow thickness and (
T
w
−
T
a
). The results are
presented in Figure 2.15. The following values were used
in the equation:
ρ
i
= 917 kg/m
3
,
L
i
= 334.9 × 10
3
J/kg,
k
i
= 2.0 and
k
s
=0.25 J/m · K · s. The point for day 1 in each
graph is generated using the thermal conductivity of sea-
water, 0.6 J/m · K · s, instead of ice. The plots clearly show
the blanketing effect of the presence of snow or higher
air temperatures. Note the pronounced sensitivity of the
temporal growth of ice thickness to the accumulation
of the first 100 mm of snow.
The simple model of ice growth described above does
not take into consideration the complex effects of (1)
uncertainty of thermal conductivity of snow cover, (2)
ocean heat flux, and (3) surface ablation. These factors
are addressed briefly in the following section.
2.2.4.2. Effect of Snow Cover
Snow cover on sea ice impedes ice growth. It acts as a
blanket that affects the thermal diffusivity and optical
transmissivity between the underlying ice sheet and the
