8 Satellite Microwave Radar Observations of Antarctic Sea Ice
173
called "ice phase" orbit of ERS-l) were correlated to produce ice motion vectors using
the algorithm described by Kwok et al. (1990). Figure 12 shows the 5-km-gridded icedrift displacement field superimposed on the ERS-l SAR reference image (orbit 3058
frame 5103) acquired at 11:34 GMT on February 15, 1992 at 71.59 oS, 53.04 ow. Spots indicate the gridded starting positions of tracked features, and the scaled vector arrows
indicate the distance traveled in the 17 h period separating the pair of tracked SAR
images. The location of ISW at the time of the image is indicated by the black" +", and
the scaled velocity vector, derived from the corresponding instantaneous GPS locations
of ISW, indicated by an arrow. Ice motion vectors around ISW imply strong cyclonic
motion, and mean and standard deviations of the u and v velocity components are -
lO.43±2.5 and 5.95±2.7 cm S-1, respectively. Partial derivatives of velocity over the entire
tracked portion of the scene indicate a mean divergence of 0.15± 0.63 % d- 1 , a mean vorticity of -0.45± 0.70 % d- 1 , and mean shear of 2.01±1.02 % d- 1 •
In total, 15 pairs of SAR images were processed to ice drift over ISW between February 7 and March 15, 1992. Over this period, the mean sea-ice drift speed around ISW
was 7.45±1.37 cm S-1 on a bearing 323°, while the corresponding mean wind speed was
5.0 m S-1. In summary, there was a net mean divergence of only 0.6% over the period of
satellite observations, indicating that net opening of the ice cover was minimal. Indeed
both opening and closing events were observed, each effectively redistributing the ice
area. Corresponding rotation and shear values were -0.023±0.3Xl0-6 S-1 and
0.65±0.5XlO-6 S-1 respectively (i.e., 0.2±2.5% d- 1 and 5.6±4.3% d- 1 ), indicating shear to
be the primary deformation mechanism.
8.5.4.2
Seasonal Ice Drift
During 1992, several buoys were deployed in the eastern Weddell Sea as part of the
WWGS'92 experiment (Lemke 1994), in a region comprised oflevel, undeformed white
ice. Simultaneous overlapping SAR acquisitions were planned in July 1992 such that the
buoys could be used as validation for the SAR motion retrievals. Under divergent conditions in the central Weddell Sea, and consequently relatively free drift, the majority
of ice-drift vectors observed by SAR in the vicinity of the buoy array follow a pattern
expected from Ekman dynamics. Figure 13 shows 3-day SAR-observed drift vectors
together with the corresponding WWGS'92 buoy drift tracks marked as dotted
Lagrangian trajectories. The superimposed pressure field is optimally interpolated
using a polynomial scheme which combines weighted buoy pressure measurements
together with lOoo-mB ECMWF pressure analysis fields. This technique was originally employed by Kottmeier and Sellmann (1996) but is hybridized here by extending and
constraining the fitting procedure to broaden the geographic limits to include synoptic-scale features and the SAR-tracked domain. Corresponding gridded geostrophic
winds are indicated in Figure 13 together with optimally interpolated climatological
mean geostrophic currents derived from original data supplied by Ross (in preparation) and Kottmeier and Sellmann (1996).
Figure 13a shows a 3-day period with a low pressure system to the southeast and relatively constant surface atmospheric pressure conditions over the buoy array (marked
by diamonds). ERS-l SAR ice tracking from two consecutive parallel descending orbits
captures the east northeast sea ice drift, which aligns itself approximately along the iso-
173
called "ice phase" orbit of ERS-l) were correlated to produce ice motion vectors using
the algorithm described by Kwok et al. (1990). Figure 12 shows the 5-km-gridded icedrift displacement field superimposed on the ERS-l SAR reference image (orbit 3058
frame 5103) acquired at 11:34 GMT on February 15, 1992 at 71.59 oS, 53.04 ow. Spots indicate the gridded starting positions of tracked features, and the scaled vector arrows
indicate the distance traveled in the 17 h period separating the pair of tracked SAR
images. The location of ISW at the time of the image is indicated by the black" +", and
the scaled velocity vector, derived from the corresponding instantaneous GPS locations
of ISW, indicated by an arrow. Ice motion vectors around ISW imply strong cyclonic
motion, and mean and standard deviations of the u and v velocity components are -
lO.43±2.5 and 5.95±2.7 cm S-1, respectively. Partial derivatives of velocity over the entire
tracked portion of the scene indicate a mean divergence of 0.15± 0.63 % d- 1 , a mean vorticity of -0.45± 0.70 % d- 1 , and mean shear of 2.01±1.02 % d- 1 •
In total, 15 pairs of SAR images were processed to ice drift over ISW between February 7 and March 15, 1992. Over this period, the mean sea-ice drift speed around ISW
was 7.45±1.37 cm S-1 on a bearing 323°, while the corresponding mean wind speed was
5.0 m S-1. In summary, there was a net mean divergence of only 0.6% over the period of
satellite observations, indicating that net opening of the ice cover was minimal. Indeed
both opening and closing events were observed, each effectively redistributing the ice
area. Corresponding rotation and shear values were -0.023±0.3Xl0-6 S-1 and
0.65±0.5XlO-6 S-1 respectively (i.e., 0.2±2.5% d- 1 and 5.6±4.3% d- 1 ), indicating shear to
be the primary deformation mechanism.
8.5.4.2
Seasonal Ice Drift
During 1992, several buoys were deployed in the eastern Weddell Sea as part of the
WWGS'92 experiment (Lemke 1994), in a region comprised oflevel, undeformed white
ice. Simultaneous overlapping SAR acquisitions were planned in July 1992 such that the
buoys could be used as validation for the SAR motion retrievals. Under divergent conditions in the central Weddell Sea, and consequently relatively free drift, the majority
of ice-drift vectors observed by SAR in the vicinity of the buoy array follow a pattern
expected from Ekman dynamics. Figure 13 shows 3-day SAR-observed drift vectors
together with the corresponding WWGS'92 buoy drift tracks marked as dotted
Lagrangian trajectories. The superimposed pressure field is optimally interpolated
using a polynomial scheme which combines weighted buoy pressure measurements
together with lOoo-mB ECMWF pressure analysis fields. This technique was originally employed by Kottmeier and Sellmann (1996) but is hybridized here by extending and
constraining the fitting procedure to broaden the geographic limits to include synoptic-scale features and the SAR-tracked domain. Corresponding gridded geostrophic
winds are indicated in Figure 13 together with optimally interpolated climatological
mean geostrophic currents derived from original data supplied by Ross (in preparation) and Kottmeier and Sellmann (1996).
Figure 13a shows a 3-day period with a low pressure system to the southeast and relatively constant surface atmospheric pressure conditions over the buoy array (marked
by diamonds). ERS-l SAR ice tracking from two consecutive parallel descending orbits
captures the east northeast sea ice drift, which aligns itself approximately along the iso-