8 Satellite Microwave Radar Observations of Antarctic Sea Ice
Fig.4. SAR backscatter probability distribution functions
(pdf's) for each individual
scene and for the entire
grouped winter SAR data set
(see Fig. 2 and Table 1 for SAR
scenes) as the stepped thick
black histogram
White Ice Floes
Q)
"
.......
.::.
c:
.Q
U c:
:>
...
c:
.Q
"5
. .g
;;;
0
? :0
0
.0
0
Q:
0.2
5377....50.31
5449....51.39
5.387....5967
5090....5121
5387....5949
- 20
-15
-10
-5
Calibrated Backscatter (dB)
155
o
The mean thin first-year ice or white ice (in the range 30-50 cm thick) signature shown
in Fig. 5a is typical of ice observed in the divergent central Weddell Sea region and is
an average of seven individual site measurements made from July 1 to 14, 1992 in the
location of scene 5377-5031 (Fig. 2). The signature of this relatively smooth, light snowcovered (1-6 cm snowdepth) ice has croShip values between -24 and -26 dB in the ERS
SAR incidence angle range 20· :::; e : : : ; 26·, an intermediate gradient of around 0.49
dB/degree from 20· :::; e:::; 60· incidence, and a neutral cross-polarized backscatter
curve. Large expanses of white ice were scoured by strong winds and had snow depths
of less than 5 cm. Lytle et al. (1996) have shown previously that snow-free young ice
(10-30 cm thick) may result in extremely low backscatter values in an equivalent incidence angle range, thereby increasing as the ice ages (in the absence of deformation)
and roughens. These results nevertheless imply that smooth, level white ice with a bare
ice surface or shallow snow cover alone does not playa dominant role in driving the
backscatter to values shown in the probability distribution functions (pdf's) in Fig. 4,
and suggest alternatively tlIat areas of congealed pancakes or ridged portions of sea ice
drive up measured SAR values in this extensive region.
SmootlI First-Year Ice Floes
The mean signature of smooth, level (undeformed) uniform first-year ice floes shown
in Fig. 5b is constructed from nine individual scatterometer samples, each site of which
exceeded 60 cm ice thickness. A distinguishing feature of this signature is that croShip
exceeds that of the white ice signature at all angles. The surface roughness of such level ice is generally greater than those white ice forms, while also displaying an older and
deeper layered snowcover. ERS SAR backscatter values of equivalent ice floes are
Fig.4. SAR backscatter probability distribution functions
(pdf's) for each individual
scene and for the entire
grouped winter SAR data set
(see Fig. 2 and Table 1 for SAR
scenes) as the stepped thick
black histogram
White Ice Floes
Q)
"
.......
.::.
c:
.Q
U c:
:>
...
c:
.Q
"5
. .g
;;;
0
? :0
0
.0
0
Q:
0.2
5377....50.31
5449....51.39
5.387....5967
5090....5121
5387....5949
- 20
-15
-10
-5
Calibrated Backscatter (dB)
155
o
The mean thin first-year ice or white ice (in the range 30-50 cm thick) signature shown
in Fig. 5a is typical of ice observed in the divergent central Weddell Sea region and is
an average of seven individual site measurements made from July 1 to 14, 1992 in the
location of scene 5377-5031 (Fig. 2). The signature of this relatively smooth, light snowcovered (1-6 cm snowdepth) ice has croShip values between -24 and -26 dB in the ERS
SAR incidence angle range 20· :::; e : : : ; 26·, an intermediate gradient of around 0.49
dB/degree from 20· :::; e:::; 60· incidence, and a neutral cross-polarized backscatter
curve. Large expanses of white ice were scoured by strong winds and had snow depths
of less than 5 cm. Lytle et al. (1996) have shown previously that snow-free young ice
(10-30 cm thick) may result in extremely low backscatter values in an equivalent incidence angle range, thereby increasing as the ice ages (in the absence of deformation)
and roughens. These results nevertheless imply that smooth, level white ice with a bare
ice surface or shallow snow cover alone does not playa dominant role in driving the
backscatter to values shown in the probability distribution functions (pdf's) in Fig. 4,
and suggest alternatively tlIat areas of congealed pancakes or ridged portions of sea ice
drive up measured SAR values in this extensive region.
SmootlI First-Year Ice Floes
The mean signature of smooth, level (undeformed) uniform first-year ice floes shown
in Fig. 5b is constructed from nine individual scatterometer samples, each site of which
exceeded 60 cm ice thickness. A distinguishing feature of this signature is that croShip
exceeds that of the white ice signature at all angles. The surface roughness of such level ice is generally greater than those white ice forms, while also displaying an older and
deeper layered snowcover. ERS SAR backscatter values of equivalent ice floes are