Geology Reference
In-Depth Information
10
1
Salinity by weight
Fy 1. 0‰
Vant et al. [1978]
Vant et al. [1974]
Hallikainen [1983]
Hallikainen et al. [1983]
5
-
20°C
2
My 1.3‰
-
5°C
10
0
-
20°C
-
2°C
-
20°C
Fy/columnar
4.6‰
-
10°C
5
-
0.5°C
-
5°C
-
5°C
2
-
10°C
-
5°C
-
2°C
10
-
1
-
20°C
-
10°C
-
1°C
-
5°C
5
Fy/frazil
4.4‰
2
Fy0.8‰
10
-
2
1
2
3
4
5
6
7
8
9
10
11
0
Frequency GHz
Figure 8.41
Variation of penetration depth in FY and MY ice with microwave frequencies obtained for different
ice temperatures and salinity. Data at 10 GHz from FY frazil and columnar ice are also shown. Data are compiled
from the shown studies [
Hallikainen and Winebrenner
, 1992, Figure 3‐7, with permission from AGU].
takes values around 0.07 m for frazil (4.4‰ salinity) and
0.16 m for columnar ice (4.6‰ salinity). Although salinity
is nearly the same in these two cases, the penetration depth
in frazil ice is almost half its value in frazil ice. Frazil ice has
crystals much smaller in size than the columnar ice crystals
and therefore has more brine pockets. This causes the bulk
salinity of frazil ice to be distributed into the brine pockets
within the interstitial of the smaller ice crystals in frazil ice.
As more scattering occurs in this situation, the losses of the
penetrated signal are expected to be higher, and conse-
quently the penetration depth will be smaller. At the same
temperature (of −10°C) and for salinity 1.3‰, the penetra-
tion depth in MY ice is 0.4 m.
Due to the rapid desalination processes during the
early growth stage of sea ice (the first few centimeters
thick),
δ
p
varies significantly with ice thickness. This was
captured in calculations presented in
Onstott
[1991] using
a model developed by
Kovacs et al.
[1986] to calculate the
dielectric constant profile in sea ice as a function of brine
volume. Results are presented in Table 8.12 for a few
microwave frequencies. For ice less than 2 cm, the pene-
tration depth of the C‐band signal exceeds the given
thickness. At 16.6 GHz
δ
p
is limited to the top few mil-
limeters (this is the frequency used in the Ku band of
scatterometer systems and it is not very far from the oper-
ational 18-19 GHz used in passive microwave sensors).
Snow over sea ice makes a variable contribution to the
penetration depth. While fresh dry snow is transparent at
microwave frequencies, wet or metamorphosed snow
causes signal loss through absorption and scattering and
hence decreases the penetration depth significantly.
Theoretical calculations presented in Figure 8.40 show
that the penetration depth in dry snow varies between
Table 8.12
Calculated penetration depths of microwave
signal in sea ice with thickness range from 1 to 8 cm.
Ice
Thickness
(cm)
Penetration Depth in Sea Ice (cm)
5.25 GHz
9.6 GHz
13.6 GHz
16.6 GHz
1
>1
0.8
0.5
0.4
2
2.0
1.0
0.7
0.5
4
3.3
1.6
1.0
0.8
8
3.7
1.6
1.0
0.8
Source
:
Onstott
[1991].
0.6 m at 100 GHz and 6 m at the longer wave of 1 GHz
frequency. However, once the snow temperature rises to
values near the melting point, snow wetness increases and
the dielectric loss becomes considerably higher
[
Hallikainen and Winebrenner
, 1992]. This decreases the
penetration depth of the EM wave significantly. Variation
of
δ
p
with volumetric water contents (VWC) in the snow
for a range of microwave frequencies is presented in
Hallikainen and Winebrenner
[1992] and reproduced here
in Figure 8.42. The calculations were made for frequen-
cies between 1 and 37 GHz based on experimental data
presented in
Hallikainen et al
. [1986],
Mätzler
[1985], and
Tiuri et al
. [1984].
δ
p
decreases by one order of magnitude
as the VWC increases from 1% to 12%. This is mainly the
result of an increasing dielectric loss. A large rate of
decrease is found at the transition from dry to slightly wet
snow. For example, at 37 GHz the dry snow has a pene-
tration depth around 1.1 m (Figure 8.40), but this value
decreases to approximately 3.5 cm when VWC reaches
2%. At 18 GHz the penetration depth is roughly 10 cm at
