Geology Reference
In-Depth Information
300
280
260
240
220
200
180
1973 1974 1975 1976
First-year ice region
Multiyear ice region
160
140
J
F
MA
MJ
J
A
S
OND
Figure 8.16
Seasonal variation of microwave brightness temperature for FY and MY ice in the central Arctic
obtained from ESMR on Nimbus 5 (1973-1976). The sensor operated in 19.53 GHz horizontal polarization
[
Parkinson et al
., 1987].
to equation 7.28 and indirectly by changing ice surface
salinity and consequently its dielectric constant. It might
also be caused by the opening of small leads or polynyas
within the footprint of the sensor (32 × 32 km
2
).
Microwave brightness temperature databases are
needed at the frequencies of the space‐borne microwave
radiometers (e.g., SSM/I, AMSR‐E, and AMSR2). The
frequencies of these radiometers are in the vicinity of 19,
37, and 85-89 GHz. Brightness temperatures at these fre-
quencies have been used to retrieve ice concentration,
snow thickness, and ice surface temperature. A classical
graph that shows typical values of
T
b
at those frequen-
cies and the two polarizations (H and V) from OW, FY
ice and MY ice is presented in
Eppler et al
. [1992]
(Figure 8.17). The data were obtained from observations
over the Arctic ice in January. Each point represents the
mean brightness temperature for a square area encom-
passed by 10 × 10 SSM/I grid map elements (each element
is 25 × 25 km
2
). The figure illustrates two important fea-
tures that are commonly used for ice‐water discrimina-
tion and therefore ice concentration estimates. The first is
that OW emits much less energy in the horizontal polari-
zation than the vertical. This means that the polarization
difference of OW is much higher than that of sea ice from
all frequencies, especially from 19 GHz channel. The sec-
ond feature pertains to the difference in the gradient of
the brightness temperature between OW, FY, and MY ice.
Figure 8.17 shows that the gradient difference [and so is
the gradient ratio as defined in equation (8.11)] between
37 and 19 GHz is positive and relatively high for OW
while it is near zero for FYI and negative for MYI. This
variation is markedly enough to be used in discriminating
350
Open water
First-year ice
Multi-year ice
V-pol
H-pol
300
250
FY ice
MY ice
200
150
OW
100
10
50
100
Frequency (GHz)
Figure 8.17
Polarization and spectral characteristics of open
water, first‐year ice, and Multiyear ice as observed from SSM/I on
17 January 1988. Difference between vertical and horizontal
polarization of open water is more significant than that of ice,
especially at lower frequencies. The short vertical bars repre-
sent ± one standard deviation for 10 × 10 SSM/I points [adapted
from
Eppler et al
., 1992, Figure 4‐18, with permission from AGU].
between these three surface types.
Eppler et al
. [1992] indi-
cated some discrepancies between the data in Figure 8.17
and other data obtained from ground measurements
and airborne sensors. For example, at 37 GHz vertical
polarization, the figure shows that OW is radiometrically
