Geology Reference
In-Depth Information
implemented in the Ocean and Sea Ice Satellite Application
Facility (OSI‐SAF) use monthly sets of tie points to account
for sensor drift and therefore allow for intersensor
comparison.
From the same outdoor ice tank experiment by
Shokr
et al
. [2009], microwave radiation was measured from
seven surfaces that were developed in response to differ-
ent weather conditions: saline water, wet slush, wet bare
ice, refrozen slush, dry sow, wet snow, and dry ice surface.
Figure 8.21 includes plots of brightness temperature
from 19, 37, and 85 GH radiation in horizontal and verti-
cal polarizations, gradient ratios, and polarization ratios.
The data are compiled from measurements on the simu-
lated sea ice while growing up to 22 cm thick. A trend of
increase of brightness temperature that progresses from
wetter to drier surface can be detected, but the variability
and the overlap of
T
b
from different surfaces is not small.
Discrimination between water and slush surfaces is
possible using
T
b
especially from 37 and 19 GHz. The
transition between water and slush is associated with an
increase of
T
b
by about 20 K (or more) from all channels
except 85V where
T
b
is increased by only 10 K. Water
is also separable from all ice surfaces in the gradient
and polarization ratios. As for the gradient ratio, the
narrowest and largest dynamic range is of GR
19
V
37
V
and
GR
85
H
19
H
, respectively. However, the overlap between
surface types for GR
85
H
19
H
is too heavy to allow reliable
surface discrimination. The ratio
GR
85
H
19
H
has its lowest
value for the cases of dry ice surface and dry snow. It
increases for all other surfaces indicted in the figure. This
is in line with a remark made in
Markus and Cavalieri
[2000] that when “surface effects” come into play the
ratio GR
85
H
19
H
increases significantly. The seven surfaces
cannot be reliably distinguished using any polarization
ratio. The 19 GHz offer polarization ratio with largest
dynamic range but, once again, with largest variability.
The variability of each parameter in Figure 8.21 is a result
of natural weather conditions as the ice was growing in
an outdoor facility as mentioned before.
Microwave brightness temperature from ocean surface
is fairly stable. However, it takes different values during
rainfall. Since physical temperature of the water surface
does not change during rain events, then the change in
T
b
must be caused by change of surface emissivity. Again,
since the rain does not change the composition of the
surface, then the change of emissivity must be caused
by emission from the hydrometeors. The modulation of
T
b
during rainfall events over the ocean can be used to
0.35
300
280
260
240
220
200
180
160
140
120
100
Water
Wet slush
Wet bare ice
Refrozen slush
0.30
0.25
Wet snow
0.20
Dry snow
Dry ice srfc.
0.15
0.10
0.05
0.00
19V
37V
85V
19H
37H
85H
-0.05
80
60
-0.10
Wet
slush
Wet
ice srfc.
Refrozen
slush
Wet
snow
Dry ice/
snow.
Water
GR
19
V
/37
V
GR
19
H
/37
H
GR
85
V
/37
V
GR
85
H
/37
H
GR
85
V
/19
V
GR
85
H
/19
H
0.30
Water
Wet slush
Wet bare ice
Refrozen slush
0.25
0.20
Wet snow
Dry snow
Dry ice srfc.
0.15
0.10
0.05
0.00
-0.05
19 GHz
37 GHz
85 GHz
Figure 8.21
Plots of brightness temperature measurements from different surfaces of thin ice grew in an outdoor
tank (from onset of freezing to 22 cm thick) and the corresponding polarization and gradient ratios. Error bars are
standard deviation [
M. Shokr
, unpublished data].
