Geology Reference
In-Depth Information
discontinuities or discrete scatterers in the ice (e.g., air
bubbles in MY ice) is enhanced and therefore more scat-
tering or absorptions are expected. Penetration depth is
important in modeling the scattering and emission from
snow‐covered sea ice. Usually, the radiating layer of the
emitted microwave is assumed to be equal to the penetra-
tion depth. Therefore, the effective temperature of the
radiating layer can be represented by the temperature at
the penetration depth.
A classical graph showing dependence of δ p on micro-
wave frequency is presented in Ulaby et al. [1986;
Figure  18.52]. Data are calculated for FY ice, MY ice,
and freshwater ice using equation (3.70). A modified ver-
sion of the same graph with the addition of snow data is
presented in Figure  8.40. The calculations were per-
formed using average values of complex permittivity for
each surface type; all valid at −10°C. In the microwave
range 1-20 GHz, this parameter has an upper limit for
the lossless MY ice of (3.0 − j 0.03), and the lower limit of
the lossy FY ice is (3.3 − j 0.25). These two limits were
used to generate the range of variation of penetration
depth for MY and FY ice as shown by the double‐head
arrows in the figure. The average δ p for FY ice at 6 and 19
GHz are 10 and 3 cm, respectively, while the correspond-
ing values for MY are 60 and 11 cm. For the L‐band
(≅1.4 GHz) the penetration depth values shown in the
figure agree with values presented in Heygster et al.
[2009], 2.5 m in MY ice and 0.5 m in FY ice. Freshwater
ice has a much larger penetration depth; near 10 m at 5
GHz and 1 m at 20 GHz. For open water (not shown in
the figure) the penetration depth is only a few millimeters
over the entire frequency range. For dry snow  is chosen
to be (2.0 − j 0.0012). The penetration depth at 1.4 GHz is
an order of magnitude higher than the depth at 90 GHz.
A more detailed graph of penetration depth δ p for FY
and MY ice types, also calculated from equation (3.70)
using input from experimental values of absorption coef-
ficient is presented in Hallikainen and Winebrenner [1992]
for a frequency range between 1 and 10 GHz (Figure 8.41).
This is the range of the operational SAR systems but it is
below the range of passive microwave systems. Experimental
measurements are compiled from studies by Vant et al.
[1974, 1978]; Hallikainen [1983]; and Hallikainen, et al.
[1988]. Observations from the figure can be summarized as
follows. For FY ice, δ p increases as temperature decreases
for a given salinity and as salinity decreases for a given tem-
perature. Salinity of MY ice varies within a narrow range
(between 0‰ and 2‰) so δ p is a function of temperature
only and decreases with temperature. No explanation for
this decrease is offered in Hallikainen and Winebrenner
[1992]. Since MY ice is almost saline free, change in its tem-
perature should leave no impact on its composition. For
the L‐band (frequency between 1 and 2 GHz) δ p for FY ice
at −20°C is 0.45 and 0.9 m for salinity of 10.5‰ and 5.1‰,
respectively. On the other hand, δ p for MY ice with salinity
of 1.3‰ and temperature of −9°C is 3.4 m. For the C‐band
(around 5 GHz frequency) the penetration depth in FY ice
with salinity of 7.5‰ is 0.14 m at an ice temperature of
−5°C and 0.38 at a temperature of −20°C. Figure  8.41
shows comparison of penetration depth between frazil and
columnar FY ice at 10 GHz. At a temperature of −10°C, δ p
Table 8.11 Penetration depth (cm) at different microwave
frequencies for 5 cm thick Nilas, dry snow, FY ice with 13 cm
overlaid snow, and MY ice with 20 cm overlaid snow.
Freq.
(GHz)
FY ice /
13 cm snow
MY ice /
20 cm snow
5 cm Nilas
Dry snow
10 2
1.4
>5.0
-
49.0
160.0
6
5.0
-
7.0
32.0
10 1
10
4.0
-
4.0
22.0
19
2.0
-
2.2
13.0
23/23.8
1.0
-
2.0
11.5
10 0
134.0
1.52
7.76
37/31.4
1.0
-
1.0
9.0
10 - 1
130.0
1.45
7.32
50/50.3
1.0
-
1.0
3.5
96.0
1.28
6.23
10 - 2
89
1.0
-
1.0
0.90
28.0
0.94
4.0
150/157
1.0
-
0.0*
0.0*
10 - 3
13.0
0.75
2.9
10 0
10 1
Frequency (GHz)
10 2
Note : Frequencies following the slash are AMSU frequencies.
Values shown in regular font are from a graph on page 390
in  Wigneron et al . [2006] and in bold font are from Mathew
et al. [2008]. The 0.0* denotes penetration depth limited to
11 cm in the dry snow (i.e., does not reach the ice surface)
Figure 8.40 Penetration depth of microwave signal frequencies
in dry snow, pure ice, MY, and FY ice types. The double‐head
arrows indicate the range of variability of the penetration depth.
 
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