30
P. Wadhams et al.
wave spectrum (first row); (2) the observed SAR spectrum (second row); and (3)
the simulated SAR spectrum (third row), for Windows 3,4,10 and 14, respectively. Windows 3 and 14 are open-sea régions, while Windows 4 and 10 include sea ice
composed of frazil and pancake, respectively. The spectra show striking features
which reveal modifications by the wave Systems during propagation through different media. The wave spectra include two main wave Systems: a broad wind
wave System at high wave numbers (Â. < 100 m) and a wave system composed of
swell. Both Systems are affected by the presence and the type of sea ice, although
major changes are suffered by the wind wave System. The comparison between
observed and simulated SAR spectra demonstrates the quality of the inversion
results for each case. Preliminary results obtained with this methodology were
presented in Wadhams et al. [19].
5 Application to the Antarctic
Our research groups are involved with the Italian National Programme for
Antarctic Research (PNRA). The scientific activities of PNRA are focused on the
Ross Sea, and in particular on the area around Terra Nova Bay where the Italian
base is located. Therefore, we were interested in applying the methodology for ice
thickness détection described in the preceding sections to pancake ice in the
Terra Nova Bay polynya.
Unfortunately, while other parts of the Antarctic continent were covered by
SAR receiving stations, SAR images of the Ross Sea hâve only very recently
become available. A SAR receiving station was installed at McMurdo Station in
the first months of 1995 and became (partially) operative only at the end of the
same year.
The first acquisitions relative to Terra Nova Bay that we were able to obtain
from the European Space Agency (ESA) were three ERS-2 SAR scenes of April 24,
1996. Other scenes of spring 1996 acquired at McMurdo had been recorded incorrectly on tape and could not be decoded at the ESA Processing and Archiving
Facility in Frascati. April 24 is already a very late date for having pancake ice at
75°S and other frames of the same orbit at the sea-ice edge at lower latitude were
not available. Wave data were also not available for the same date since the Italian
base, which is a summer base, was already closed and the wave monitoring buoy
which opérâtes during the period of the campaign near the coast had been
removed.
Thus no wave data could be used for the first guess to initialise the inversion
algorithm described in Section 2.We could only compute the observed spectra for
a set of Windows extracted from each SAR scene. As an example, Fig. 3 shows one
of the three SAR scenes (orbit 5289, frame 5607) just over Terra Nova Bay. One can
observe that the ice was already in an advanced State of formation while a zone of
open water of the polynya still remains (dark area in the centre of the image).
Five Windows, 512 x 512 pixels (see Fig. 3) were extracted and analysed according to the simple procedure described in Wadhams et al. [20]. The observed SAR
spectra for Windows 2 and 5 are shown in Fig. 4 and reveal the presence of a small
wave System with X > 250 m. Very similar results were obtained when the same
P. Wadhams et al.
wave spectrum (first row); (2) the observed SAR spectrum (second row); and (3)
the simulated SAR spectrum (third row), for Windows 3,4,10 and 14, respectively. Windows 3 and 14 are open-sea régions, while Windows 4 and 10 include sea ice
composed of frazil and pancake, respectively. The spectra show striking features
which reveal modifications by the wave Systems during propagation through different media. The wave spectra include two main wave Systems: a broad wind
wave System at high wave numbers (Â. < 100 m) and a wave system composed of
swell. Both Systems are affected by the presence and the type of sea ice, although
major changes are suffered by the wind wave System. The comparison between
observed and simulated SAR spectra demonstrates the quality of the inversion
results for each case. Preliminary results obtained with this methodology were
presented in Wadhams et al. [19].
5 Application to the Antarctic
Our research groups are involved with the Italian National Programme for
Antarctic Research (PNRA). The scientific activities of PNRA are focused on the
Ross Sea, and in particular on the area around Terra Nova Bay where the Italian
base is located. Therefore, we were interested in applying the methodology for ice
thickness détection described in the preceding sections to pancake ice in the
Terra Nova Bay polynya.
Unfortunately, while other parts of the Antarctic continent were covered by
SAR receiving stations, SAR images of the Ross Sea hâve only very recently
become available. A SAR receiving station was installed at McMurdo Station in
the first months of 1995 and became (partially) operative only at the end of the
same year.
The first acquisitions relative to Terra Nova Bay that we were able to obtain
from the European Space Agency (ESA) were three ERS-2 SAR scenes of April 24,
1996. Other scenes of spring 1996 acquired at McMurdo had been recorded incorrectly on tape and could not be decoded at the ESA Processing and Archiving
Facility in Frascati. April 24 is already a very late date for having pancake ice at
75°S and other frames of the same orbit at the sea-ice edge at lower latitude were
not available. Wave data were also not available for the same date since the Italian
base, which is a summer base, was already closed and the wave monitoring buoy
which opérâtes during the period of the campaign near the coast had been
removed.
Thus no wave data could be used for the first guess to initialise the inversion
algorithm described in Section 2.We could only compute the observed spectra for
a set of Windows extracted from each SAR scene. As an example, Fig. 3 shows one
of the three SAR scenes (orbit 5289, frame 5607) just over Terra Nova Bay. One can
observe that the ice was already in an advanced State of formation while a zone of
open water of the polynya still remains (dark area in the centre of the image).
Five Windows, 512 x 512 pixels (see Fig. 3) were extracted and analysed according to the simple procedure described in Wadhams et al. [20]. The observed SAR
spectra for Windows 2 and 5 are shown in Fig. 4 and reveal the presence of a small
wave System with X > 250 m. Very similar results were obtained when the same