M.R. DRINKWATER
cated in Fig. 2, including those listed in Table 1. This summary pdf indicates a trimodal
distribution of backscatter aOSAR' with distinct components forming peaks at -11·5, -7.5,
and -2.5 dB. Individual image pdf's indicate that the highest backscatter mode comprises pixels containing glacial ice (i.e., ice shelf). Image 5090-5121 in Fig. 2 was
acquired in the vicinity of R.V. Polarstern as she made a transect along the periphery
of the Riiser-Larsen ice shelf. This scene contains a significant number of ice-shelf pixels, falling in the range -5 to +5 dB: air temperatures were around -15 0 C at the time of
SAR imaging. Similar characteristics are observed in the upper peak in the histogram
of image 5387-5967. This image captures a large iceberg drifting in the outer MIl northeast of the tip of the Antarctic peninsula. Its backscatter values exceed those of the ice
shelf in the previous example, due in large part to its lower latitude and warmer air
temperatures. Glacial ice, therefore, comprises a distinct portion of the upper limb of
the backscatter distribution in the Weddell Sea, and may conveniently be used in winter to find tabular icebergs of significant size, either in the marginal ice zone or within the interior ice pack. Nontabular Antarctic icebergs do not always present a uniform
target area to the SAR, and rotation and drift with respect to the imaging swath often
result in a reduction in contrast between the iceberg and its surrounding background.
The next brightest component distribution is described by the pdf of two contiguous image frames (5387-5949 and 5387-5967) along an orbit crossing the western Weddell Sea MIl (Fig. 2). The peaks in their image histograms overlap almost identically,
but for a small transition from the swell-disturbed marginal ice zone to the outer ice
edge. Mixtures of brash ice and small first-year ice floes together with occasional multiyear ice floes characterized the swell-rocked ice margin at the time of SAR imaging
(Haas et al.1992).Air temperatures and windspeeds at the time of imaging were recorded on board R.V. Polarstern as between -5 and 0 °C, and 5 and 10 m S-1, respectively.
These mixtures have an extremely distinctive signature, with aOSAR values ranging
between -10 and -5 dB, with a mode at -7.3 dB. Rough surface scattering, occurring from
sub-resolution floes, has a distinctively uniform texture and bears a close resemblance
to wave-disturbed northern hemisphere MIl ice signatures observed by airborne SAR
in the Labrador Sea (Drinkwater and Squire 1989; Livingstone and Drinkwater 1991).
The further into the MIlone traverses, the higher the probability of observing large
undeformed first-year ice floes and new ice in leads. These pixels appear as a toe of
darker material in the histogram of image 5387-5949, extending from -12 dB down to
around -19 dB.
Signatures similar to the previous MIl cases (in terms of backscatter amplitude) may
be encountered in the central ice pack, but with the obvious distinction of visible icefloe outlines. Image 5249-4941, shown in Fig. 6 (overlapping the path of R.V.
Polarstern), contains a typical mixture of perennial and seasonal ice, and was acquired
in the outflow path of old ice from the Weddell Sea. The mixture histogram combining
these two distinctive end-members appears as a broad pdf in Fig. 4. Shipborne observations, noted by Haas et al. (1992) as Po lars tern traversed the region encompassed by
this image (less than 24 h after SAR imaging - see Table 1), indicated a closed ice cover (100% concentration) comprising level first-year ice punctuated by distinct ridges
and rougher multiyear ice in fractions up to 30%. First-year ice was extremely rubbled
and ridged, particularly around the perimeter of thick multiyear ice floes. The broad
pdf comprises two overlapping distributions without distinctive peaks, due to the continuum of states of the ice ranging from patches of smooth, relatively undeformed first-
cated in Fig. 2, including those listed in Table 1. This summary pdf indicates a trimodal
distribution of backscatter aOSAR' with distinct components forming peaks at -11·5, -7.5,
and -2.5 dB. Individual image pdf's indicate that the highest backscatter mode comprises pixels containing glacial ice (i.e., ice shelf). Image 5090-5121 in Fig. 2 was
acquired in the vicinity of R.V. Polarstern as she made a transect along the periphery
of the Riiser-Larsen ice shelf. This scene contains a significant number of ice-shelf pixels, falling in the range -5 to +5 dB: air temperatures were around -15 0 C at the time of
SAR imaging. Similar characteristics are observed in the upper peak in the histogram
of image 5387-5967. This image captures a large iceberg drifting in the outer MIl northeast of the tip of the Antarctic peninsula. Its backscatter values exceed those of the ice
shelf in the previous example, due in large part to its lower latitude and warmer air
temperatures. Glacial ice, therefore, comprises a distinct portion of the upper limb of
the backscatter distribution in the Weddell Sea, and may conveniently be used in winter to find tabular icebergs of significant size, either in the marginal ice zone or within the interior ice pack. Nontabular Antarctic icebergs do not always present a uniform
target area to the SAR, and rotation and drift with respect to the imaging swath often
result in a reduction in contrast between the iceberg and its surrounding background.
The next brightest component distribution is described by the pdf of two contiguous image frames (5387-5949 and 5387-5967) along an orbit crossing the western Weddell Sea MIl (Fig. 2). The peaks in their image histograms overlap almost identically,
but for a small transition from the swell-disturbed marginal ice zone to the outer ice
edge. Mixtures of brash ice and small first-year ice floes together with occasional multiyear ice floes characterized the swell-rocked ice margin at the time of SAR imaging
(Haas et al.1992).Air temperatures and windspeeds at the time of imaging were recorded on board R.V. Polarstern as between -5 and 0 °C, and 5 and 10 m S-1, respectively.
These mixtures have an extremely distinctive signature, with aOSAR values ranging
between -10 and -5 dB, with a mode at -7.3 dB. Rough surface scattering, occurring from
sub-resolution floes, has a distinctively uniform texture and bears a close resemblance
to wave-disturbed northern hemisphere MIl ice signatures observed by airborne SAR
in the Labrador Sea (Drinkwater and Squire 1989; Livingstone and Drinkwater 1991).
The further into the MIlone traverses, the higher the probability of observing large
undeformed first-year ice floes and new ice in leads. These pixels appear as a toe of
darker material in the histogram of image 5387-5949, extending from -12 dB down to
around -19 dB.
Signatures similar to the previous MIl cases (in terms of backscatter amplitude) may
be encountered in the central ice pack, but with the obvious distinction of visible icefloe outlines. Image 5249-4941, shown in Fig. 6 (overlapping the path of R.V.
Polarstern), contains a typical mixture of perennial and seasonal ice, and was acquired
in the outflow path of old ice from the Weddell Sea. The mixture histogram combining
these two distinctive end-members appears as a broad pdf in Fig. 4. Shipborne observations, noted by Haas et al. (1992) as Po lars tern traversed the region encompassed by
this image (less than 24 h after SAR imaging - see Table 1), indicated a closed ice cover (100% concentration) comprising level first-year ice punctuated by distinct ridges
and rougher multiyear ice in fractions up to 30%. First-year ice was extremely rubbled
and ridged, particularly around the perimeter of thick multiyear ice floes. The broad
pdf comprises two overlapping distributions without distinctive peaks, due to the continuum of states of the ice ranging from patches of smooth, relatively undeformed first-