Geology Reference
In-Depth Information
19
37
89
1. 0
1. 0
V
H
0.8
0.8
0.6
V-pol
V
18 GHz
37 GHz
90 GHz
FY ice
MY ice
0.6
0.4
Summer ice
Sea water
H
0.2
10
100
Frequency (GHz)
0.4
0
10
20 30
Ice thickness (mm)
40
50
60
Figure 8.31 Frequency dependence of emissivity of FY ice,
MY ice, OW, and summer ice in the microwave range. The
three frequencies of AMSR‐E are marked by the dashed lines.
Emissivity is calculated for incidence angle 50° [ Spreen et al .,
2008, Figure 1, with permission from AGU].
Figure 8.32 Emissivity as a function of ice thickness for the
vertical polarization of three microwave frequencies: 18, 37,
and 90 GHz. Measurements were obtained from artificial ice
grown in an outdoor tank in CRREL. Data were obtained at a
50° viewing angle [ Grenfell et al . 1988, Figure 1, with permis-
sion from IEEE].
zone presented by Onstott et al ., [1987]. The figure reveals
the overlap between the emissivity of MY ice and FY ice
between 5 and 6 GHz. These frequencies are not usually
used in sea ice applications (AMSR‐E had a channel
measuring radiation at 6.9 GHz but the footprint was
around 69 km). The difference between emissivity of MY
ice in the two polarizations decreases at higher frequen-
cies. Recall that the high‐frequency channel of 85 GHz
has small wavelength around 3.4 mm (Table 7.3). This is
the closest wavelength (among the three operational pas-
sive microwave frequencies around 19, 37, and 85 GHz)
to the typical diameter of air bubbles in hummock ice
(2.58 mm as shown in Table 4.3). This causes more vol-
ume scattering and therefore less emitted energy with less
depolarization. At 85 or 89 GHz MY ice cannot be dis-
criminated from OW based on the difference between
their emissivity (both are radiometrically cold). It can
also be discriminated from FY ice based on the emissivity
from any polarization. Figure  8.31 shows also that the
emissivity difference between winter and summer FY ice
is not significant at all frequencies. The summer data in
the figure were obtained with the exclusion of flooded
surfaces.
Grenfell et al ., [1988] presented a graph that shows the
evolution of the emissivity with sea ice thickness starting
from the new ice formation. The graph is shown in
Figure  8.32 for data from 18, 37, and 90 GHz. The data
were obtained from simulated ice grown in an outdoor tank
in the CRREL in Hanover, New Hampshire. Measurements
on thin ice of a few millimeters or centimeters thick can
only be performed using laboratory‐grown ice. The most
noticeable feature in the figure is the sharp increase of the
emissivity during the first 10 mm growth of ice. The fig-
ure shows also that the rate of increase of emissivity
within this thickness range is higher for higher frequen-
cies. The emissivity peaks around 30 and 10 mm for 18
and 37 GHz, respectively. Eppler et al . [1992] attributed
the peak to the energy emission from the entire skin
depth, which may be submerged below the sea level. The
change in the liquid water triggers an erratic change of
the emitted radiation until interstitial water within the
skin depth has frozen. The nearly linear increase of emis-
sivity with thickness for thin ice may not hold if snow and
freezing rain fall on thin ice.
Microwave spectral emissivity of young ice types
(grease ice and Nilas) is presented in Eppler et al . [1992]
and reproduced in Figure 8.33. Data are compiled from
the same sources as mentioned for Figure 8.30. For grease
ice the emissivity is only slightly higher than that of
OW but shows the same large polarization difference as
depicted in Figure  8.30. Recall that grease ice is not a
completely frozen medium as explained in section 2.1.3).
The remarkable increase of emissivity after that growth
stage is visible in the figure. At any frequency other than
the upper end (>80 GHz) the emissivity can be used as
an indicator of the degree of freezing. The increase of ice
thickness causes gradual depolarization of the microwave
emission for two reasons (note that full depolarization
Search WWH ::




Custom Search