8 Satellite Microwave Radar Observations of Antarctic Sea Ice
175
bars. The indications are that sea-ice motion in this region can for the most part be
considered in "free drift" (Martinson and Wamser 1990), especially during divergent
conditions when ice interaction forces are negligible. As the low-pressure system in
Fig. 13 moves to the east, winds become more southerly, causing increasing divergence
as high pressure is reestablished. The weak magnitude of the mean coastal
geostrophic current, flowing in the opposite direction in Fig. 13a in the location of
the buoys, appears to have little impact other than moderating the drift velocity. More
deformed coastal ice in SAR drift examples further to the southeast appear to
respond more to basal current stresses due to stronger coastal currents which reach
5 cm S-1 in magnitude.
Episodic bouts of divergence and convergence are observed primarily in response to
storm impulses at the leading and trailing edges of low pressure systems, respectively.
Figure 13b shows the mean synoptic and ice-motion situation prior to the example in
Fig. 13a. In Figure 13b, the passing trough of low pressure causes a rapid adjustment in
the buoy drift, each buoy exhibiting a 90 ° direction change in direction. Geostrophic
winds are observed to change from southerlies to westerlies as a consequence of the
eastward passage of the low-pressure trough, and the along track SAR motion vectors
show a convergence in response to winds towards the Antarctic coast.
During the July drift of the WWGS'92 buoys, the sea-ice drifts at radiosonde balloons launched from R. V. Polarstern close to the location of buoy deployment region
show a mean ageostrophic turning angle of around 44° between the upper-level and
surface winds. The mean angle between the geostrophic wind and the SAR-measured
ice-drift vectors is observed to vary considerably in the range ±20 ° , depending on
the pressure characteristics in the region, as found by Kottmeier and Sellmann
(1996). In general, the sea ice in this region drifts to the left of the surface wind vector with a mean turning angle of around 25 ° , and thus the ice drift vectors for the
most part are closely aligned with the isobars.
8.5.4.3
Sea-Ice Deformation
The advantage which SAR has over large-scale ice motion derived from low resolution satellite images is that the deformational response of the ice may be characterized. Figure 14 illustrates a product derived from the SAR ice-velocity measurements
shown in Fig. 12. Ice deformation is graphically described in terms of the ratio of strainrate invariants EI and En, calculated from the velocity derivatives on the 5 km grid-cell
scale. The angle of change, 8, expresses the style of deformation [8 = tan- 1 (En/E1)],
where EI quantifies positive divergence (or negative convergence) and En the rate of
shearing. The result is a graphical illustration of whether the motion is accompanied
by either predominantly divergence (i.e., 8 "" 0), shear (8 "" 90°), or convergence (8 ""
180°). It may be noted for the field of cyclonic motion recorded in Fig. 12 that a large
proportion of the deformation represents shear-style motion (indicated by dark blue
and pink). Only one single purple 5 km grid cell located to the northwest of the ice
camp (marked by "+") illustrates a situation tending to pure convergence. It clearly
begins as an area with a large fraction of open water. Such sea-ice deformation data
are used in validating ice rheologies presently employed in coupled ice-ocean -atmosphere models (Stern et al. 1995).
175
bars. The indications are that sea-ice motion in this region can for the most part be
considered in "free drift" (Martinson and Wamser 1990), especially during divergent
conditions when ice interaction forces are negligible. As the low-pressure system in
Fig. 13 moves to the east, winds become more southerly, causing increasing divergence
as high pressure is reestablished. The weak magnitude of the mean coastal
geostrophic current, flowing in the opposite direction in Fig. 13a in the location of
the buoys, appears to have little impact other than moderating the drift velocity. More
deformed coastal ice in SAR drift examples further to the southeast appear to
respond more to basal current stresses due to stronger coastal currents which reach
5 cm S-1 in magnitude.
Episodic bouts of divergence and convergence are observed primarily in response to
storm impulses at the leading and trailing edges of low pressure systems, respectively.
Figure 13b shows the mean synoptic and ice-motion situation prior to the example in
Fig. 13a. In Figure 13b, the passing trough of low pressure causes a rapid adjustment in
the buoy drift, each buoy exhibiting a 90 ° direction change in direction. Geostrophic
winds are observed to change from southerlies to westerlies as a consequence of the
eastward passage of the low-pressure trough, and the along track SAR motion vectors
show a convergence in response to winds towards the Antarctic coast.
During the July drift of the WWGS'92 buoys, the sea-ice drifts at radiosonde balloons launched from R. V. Polarstern close to the location of buoy deployment region
show a mean ageostrophic turning angle of around 44° between the upper-level and
surface winds. The mean angle between the geostrophic wind and the SAR-measured
ice-drift vectors is observed to vary considerably in the range ±20 ° , depending on
the pressure characteristics in the region, as found by Kottmeier and Sellmann
(1996). In general, the sea ice in this region drifts to the left of the surface wind vector with a mean turning angle of around 25 ° , and thus the ice drift vectors for the
most part are closely aligned with the isobars.
8.5.4.3
Sea-Ice Deformation
The advantage which SAR has over large-scale ice motion derived from low resolution satellite images is that the deformational response of the ice may be characterized. Figure 14 illustrates a product derived from the SAR ice-velocity measurements
shown in Fig. 12. Ice deformation is graphically described in terms of the ratio of strainrate invariants EI and En, calculated from the velocity derivatives on the 5 km grid-cell
scale. The angle of change, 8, expresses the style of deformation [8 = tan- 1 (En/E1)],
where EI quantifies positive divergence (or negative convergence) and En the rate of
shearing. The result is a graphical illustration of whether the motion is accompanied
by either predominantly divergence (i.e., 8 "" 0), shear (8 "" 90°), or convergence (8 ""
180°). It may be noted for the field of cyclonic motion recorded in Fig. 12 that a large
proportion of the deformation represents shear-style motion (indicated by dark blue
and pink). Only one single purple 5 km grid cell located to the northwest of the ice
camp (marked by "+") illustrates a situation tending to pure convergence. It clearly
begins as an area with a large fraction of open water. Such sea-ice deformation data
are used in validating ice rheologies presently employed in coupled ice-ocean -atmosphere models (Stern et al. 1995).