182
M.R. DRINKWATER
8.6.2.1
Summer and Winter SAR Backscatter Characteristics
If contrasts in midwinter and midsummer conditions are first considered, then a collection of all statistics of calibrated backscatter images may be made on the basis of the
ice growth or melt seasons. Since the Weddell Sea ice cover is largely composed of seasonal ice, the residual ice cover at the end of summer typifies the various states of perennial ice, all ice surviving the summer melt being defined as multiyear ice. All the
grouped pixel values are combined into either summer and winter pdf's in Fig. 17 to
describe the differences between the summer and winter perennial, or the annual ice
signatures. The summer pdf shows a mode around -6 dB, with a small tail to higher values comprising highly deformed ice and icebergs. Ice concentrations typically exceed
95% in the perennial ice pack and the contribution from wind-roughened open water
is negligible (Drinkwater and Lytle 1997).At the lower end of the summer pdf is a longer
tail extending to -15 dB and beyond. This includes level, undeformed multiyear ice and
snow-covered first-year ice thick enough to survive the summer melt.
In winter, the pdf becomes bimodal with the appearance of seasonal ice. As time progresses, advection of an increasing fraction of multiyear ice northwards out of the basin
reduces the probability of multiyear ice, thereby producing the imbalanced bimodal
midwinter distribution shown in Fig. 17. While a secondary peak remains, with a mode
at around -7 dB, ilie main peak now occurs at around -12.5 dB and comprises more
prevalent forms oflevel, first-year ice. The low end of the pdf, at values of -15 dB and
below, indicates the least deformed first-year ice. A winter tail extending down to the
noise floor of the SAR (approximately -25 dB) includes very small proportions of new
and young ice forms, and recent results from (Morris and Jeffries 1997) in the Bellingshausen Sea indicate iliat typical un deformed new ice appears with a mean backscatter value of -23± 0.5 dB in small areas of coastal polynyas and flaw leads.
8.6.2.2
Time-Series Observations of Backscatter
Early observations of the temporally changing C-band backscatter response within
fixed regions have been made in Drinkwater et al. (1993b). However, temporal monitoring of the backscatter signature of moving floes has not previously been attempted.
Studies focusing specifically on multiyear ice in summer and winter SAR images show
that its backscatter coefficient changes seasonally, and the mode of the multiyear ice
pdf in Fig. 17 is observed to shift slightly from around -6 dB to -7 dB between warm and
cold seasons (i.e., around 25% change in image scattering intensity). Surface measurements made at ISW (Lytle and Ackley 1996) illustrate that perennial ice retains a deep
snowcover on its surface. A large proportion of the ISW camp floe and surrounding ice
floes experienced summer flooding at the base of the snow.
Figure 18 illustrates the temporal change of the SAR backscatter signature of tracked
perennial ice floes in the vicinity of ISW as they cooled during the summer-autumn
transition period.As the cold wave (i.e.,o·C isotherm) propagates downwards through
the snow, areas of flooded or saturated basal snow (slush) cool and refreeze. In response
to this process, C-band microwave backscatter values shown in Fig. 18 decrease (after
Drinkwater and Lytle 1997). The period of cooling air temperatures at ISW (Fig. 18a) is
M.R. DRINKWATER
8.6.2.1
Summer and Winter SAR Backscatter Characteristics
If contrasts in midwinter and midsummer conditions are first considered, then a collection of all statistics of calibrated backscatter images may be made on the basis of the
ice growth or melt seasons. Since the Weddell Sea ice cover is largely composed of seasonal ice, the residual ice cover at the end of summer typifies the various states of perennial ice, all ice surviving the summer melt being defined as multiyear ice. All the
grouped pixel values are combined into either summer and winter pdf's in Fig. 17 to
describe the differences between the summer and winter perennial, or the annual ice
signatures. The summer pdf shows a mode around -6 dB, with a small tail to higher values comprising highly deformed ice and icebergs. Ice concentrations typically exceed
95% in the perennial ice pack and the contribution from wind-roughened open water
is negligible (Drinkwater and Lytle 1997).At the lower end of the summer pdf is a longer
tail extending to -15 dB and beyond. This includes level, undeformed multiyear ice and
snow-covered first-year ice thick enough to survive the summer melt.
In winter, the pdf becomes bimodal with the appearance of seasonal ice. As time progresses, advection of an increasing fraction of multiyear ice northwards out of the basin
reduces the probability of multiyear ice, thereby producing the imbalanced bimodal
midwinter distribution shown in Fig. 17. While a secondary peak remains, with a mode
at around -7 dB, ilie main peak now occurs at around -12.5 dB and comprises more
prevalent forms oflevel, first-year ice. The low end of the pdf, at values of -15 dB and
below, indicates the least deformed first-year ice. A winter tail extending down to the
noise floor of the SAR (approximately -25 dB) includes very small proportions of new
and young ice forms, and recent results from (Morris and Jeffries 1997) in the Bellingshausen Sea indicate iliat typical un deformed new ice appears with a mean backscatter value of -23± 0.5 dB in small areas of coastal polynyas and flaw leads.
8.6.2.2
Time-Series Observations of Backscatter
Early observations of the temporally changing C-band backscatter response within
fixed regions have been made in Drinkwater et al. (1993b). However, temporal monitoring of the backscatter signature of moving floes has not previously been attempted.
Studies focusing specifically on multiyear ice in summer and winter SAR images show
that its backscatter coefficient changes seasonally, and the mode of the multiyear ice
pdf in Fig. 17 is observed to shift slightly from around -6 dB to -7 dB between warm and
cold seasons (i.e., around 25% change in image scattering intensity). Surface measurements made at ISW (Lytle and Ackley 1996) illustrate that perennial ice retains a deep
snowcover on its surface. A large proportion of the ISW camp floe and surrounding ice
floes experienced summer flooding at the base of the snow.
Figure 18 illustrates the temporal change of the SAR backscatter signature of tracked
perennial ice floes in the vicinity of ISW as they cooled during the summer-autumn
transition period.As the cold wave (i.e.,o·C isotherm) propagates downwards through
the snow, areas of flooded or saturated basal snow (slush) cool and refreeze. In response
to this process, C-band microwave backscatter values shown in Fig. 18 decrease (after
Drinkwater and Lytle 1997). The period of cooling air temperatures at ISW (Fig. 18a) is