Geology Reference
In-Depth Information
0.5
20
h_s 2000
h_s 2001
h_i 2000
h_i 2001
0.0
10
Mean daily air temp.
Resolute, 2001
-0.5
0
Ice surface temp.,
d = 10 mm, 2001
-1.0
-10
-1.5
-20
h_s: snow depth
h_i: ice thickness
-2.0
-30
130
150
170
190
210
230
Julian day
May
June
July
Aug
Figure 2.58
Ice and snow thickness along with air and surface temperatures of landfast ice in McDougall Sound, Canadian
Arctic, before and after the onset of ice decay (approximately JD166). Ice surface temperature was measured at a depth of 10 mm
[
Johnston et al
., 2002].
was performed in Mould Bay in the Canadian western
Arctic. The long-term field observations of Mould Bay,
described in Chapter 5, were the major early endeavors
that led to the development of Canada's first remote sens-
ing satellite, Radarsat-1. Figure 5.13a,b present decay in
the FY ice cover from a thickness of 2.17 m on 21 June to
1.27 m on 13 July (of 1982) with the concomitant drastic
changes in the vertical salinity profiles from C to Ɔ shape.
Recently,
Johnston et al
. [2002] conducted short-term
studies on ice decay in McDougall Sound in the Canadian
Arctic from May to July 2000 and from May to June 2001.
Figure 2.58 shows that the onset of ice ablation in mid‐
June coincided with the point at which the snow cover
had melted completely and the ice surface reached
−1.8 °C. Ice ablated from an average thickness of 1.51 to
0.83 m in about 4 weeks in 2000 at a rate of 22 mm/day.
The 2001 measurements, which terminated after about
1 week of ablation, showed that the ice thickness decreased
from about 1.44 m in early May to 1.20 m at end of June.
The temperature profiles of ice before and during its
decay are shown in Figure 2.59. The temperatures were
measured using thermistor string installed on May 7 or
Julian Day 127 (JD127) and removed on July 3 (JD184). As
the season progressed, the temperature at all depths steadily
increased. After June 9 (JD160), more than half of the full
thickness of ice was isothermal at −3 °C. By June 20 (JD170),
the entire ice thickness was isothermal at −2 °C during the
stable morning hours. By the end of the program, July 3
(JD184), the temperature gradient had been inverted; ice in
the surface layers was warmer than the bottom ice.
Brine pockets expand as the ice temperature rises. This
causes an increase of brine volume associated with a
decrease in brine-pocket salinity as brine cells acquire more
water from the melt of the surrounding pure ice crystals.
As the ice temperature increases, the boundaries between
-
10
2
-
20
0
-
30
-
2
-
40
-
4
-
50
-
60
-
6
-
70
-
8
-
80
-
10
-
90
-
12
-
100
130
140
150 60
170 80
Julian day
Figure 2.59
Ice temperature profile before and after the onset
of ice decay (JD166) that starts at the surface. Temperatures
were measured in landfast ice in McDougall Sound, Canadian
Arctic, in 2001. Contours indicate constant temperatures, from
measurements at 06:00 h [
Johnston et al
., 2002].
the grains and the intragranular subgrain boundaries
(Figures 2.24, 2.25 and micrograph 2.26) also get impreg-
nated with liquid-like lattice structure. Moreover, the
mobility of glide and climb of the network of crystalline
defects, such as dislocations inside the subgrains
(Figure 2.27), are also enhanced with the increase in tem-
perature. The increase in temperature, therefore, affects the
