Geology Reference
In-Depth Information
(a)
(b)
0.25
240
0.25
240
Ice salinity = 10 ppt
Ice salinity = 20 ppt
220
220
200
200
0.20
0.20
180
180
160
160
0.15
0.15
Brine volume
Brine volume
140
140
120
Brine salinity
120
Brine salinity
0.10
0.10
100
100
80
80
0.50
60
0.50
60
40
40
20
20
0.00
0.00
-
30
-
26
-
22
-
18
-
14
-
10
-
6
-
2
-
30
-
26
-
22
-
18
-
14
-
10
-
6
-
2
Temperature (°C)
Temperature (°C)
Figure 3.18
Variation of brine volume and brine salinity with bulk ice temperatures; calculations were made for
ice salinities of (a) 10‰ and (b) 20‰.
nonlinear variation of volume fractions of ice and brine
at temperatures above −10 °C. These two parameters are
complementary because the air volume fraction is very
small and not affected by ice temperature. Below −10 °C
the brine volume fraction decreases linearly with
decreasing temperature (the opposite is observed for the
ice volume fraction).
Variation of brine salinity [equation (3.23) and (3.24)]
and brine volume fraction [equation (2.36)] with tempera-
ture are plotted in Figure 3.18. A peak in brine salinity is
noticeable at the precipitation temperature of sodium
chloride (around −22.9 °C). Below this temperature, a con-
siderable mass of salt freezes and brine salinity decreases
accordingly. This effect reverses the trend of the increase
of brine salinity as temperature decreases (a consequence
of brine pockets shrinkage). Another slight discontinuity
in the brine salinity curve occurs at about −8.2 °C. This
is the precipitation temperature of sodium sulfate
(Na
2
SO
4
· 10H
2
O), which constitutes only 4‰ of impurities
in seawater [
Assur,
1958]. Unlike brine volume, brine salin-
ity is independent of ice salinity, as shown by the identical
curves of brine salinity in both parts of the figure.
essentially a solid material that hosts brine in liquid
phase, heat transfer within it is largely accomplished by
conduction and partly by convection.
The key thermal processes in sea ice are: (1) the heat
exchange at the ice surface (between air and ice), (2) the
heat conduction in the snow and ice (in winter, it is
usually from the bottom up), (3) the heat exchange at the
ice‐water interface, and (4) the latent heat, loss or gain
due to phase change within sea ice as temperature changes
(solidification and melting of ice crystals as well as
precipitation of solid salts in brine pockets). Thermal
parameters relevant to these processes include conductivity,
specific heat, and latent heat of fusion. The ratio of the
thermal conductivity to specific heat is known as thermal
diffusivity. These parameters depend on ice temperature,
which determines the ice composition under phase equi-
librium. Together with the corresponding thermal prop-
erties of the snow, the parameters can be used to estimate
the heat exchange between the ocean and the air through
the ice cover and therefore determine the ice growth or
decay conditions.
Models of thermodynamic growth of sea ice are pre-
sented in the original work of
Untersteiner
[1964],
Maykut
and Untersteiner
[1971],
Maykut
[1978], and
Semtner
[1976]. Advanced models that are more suitable for climate
research have been developed later by
Ebert and Curry
[1993] and
Flato and Brown
[1996]. A notable energy con-
serving thermodynamic model is presented in
Bitz and
Lipscomb
[1999]. This model accounts not only for the
heat capacity of ice but also for the energy required for
melting. As mentioned in section 2.3.3.2 as the ice cools,
water in the brine pockets freezes and consequently the
salinity of brine pockets increases to maintain thermal
equilibrium. Inversely, as the ice warms, the salinity of
brine pockets decreases by melting ice along the walls of
the brine pockets. Therefore, the heat capacity of sea ice
includes both the energy required to raise the pure ice
3.5. Thermal properTies
Because sea ice is a composite material where solid, liq-
uid, and gas phases coexist, its thermal behavior is also
complex. Change of phase between solid and liquid takes
place continually in response to variation in temperature
in order to maintain the thermal equilibrium inside the
sea ice composition. This is manifested in the melting of
pure ice crystals at the brine pockets' wall as the ice
temperature increases and vice versa. It is also evident in
the precipitation of dissolution of salts in brine when the
temperature crosses the eutectic point of any given salt.
The change of phase of sea ice constituents gives rise to a
large change of its thermal properties. Since sea ice is
