Mapping the Thickness of Pancake Ice Using Océan
Wave Dispersion in SAR Imagery
P. WÀDHAMS1, F. PARMIGGIANI2, G. DE CAROLIS3 AND M. TaDROSS1
'Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK
2IMGA-CNR, Via Gobetti 101,40129 Bologna, Italy
3ITIS-CNR, P.O. Box 11,75100 Matera, Italy
Abstract
During the early to midwinter period pancake ice is a major component of the
Antarctic sea ice cover, occupying a belt extending 200-300 km from the outer ice
edge with an area of about 6 million km2. Expérience in the Arctic suggests that the
sea ice thickness in this région can be mapped by monitoring the pénétration of
océan waves into it. Spectral analysis of subscenes from ERS-2 synthetic aperture
radar (SAR) images yields the wavelength and direction of the principal spectral
component both outside and inside the ice cover. There is a change of wavelength at
the ice edge associated with a change in the wave dispersion, which can be quantitatively related to the thickness of the ice. The analysis is complex because the true
wave spectrum must be retrieved from the SAR spectrum, which involves an inversion technique requiring a “first-guess” spectrum. The analysis technique is
described, and the wave theory which predicts the change in wavelength.
Expérience in the Chukchi Sea and especially in the Odden ice tongue in the
Greenland Sea is reviewed. Preliminary data from the Ross Sea hâve been analysed,
yielding principal wave components, but the ambient wavelength was too long for
the effect of ice to be détectable. The next application will be to the outer ice edge,
when midwinter data from this région are available at the end of the current season.
1 Introduction: Pancake Ice and Waves
1.1 Antarctic Pancake Ice
In order to study and model the properties of the Antarctic sea ice cover and its
rôle in climate change, an essential parameter to measure is its thickness distribution. Sea ice thickness is a product of thermodynamic and dynamic processes,
and is itself a déterminant of ocean-atmosphere heat flux. In the Arctic most
information on sea ice thickness has corne from upward-looking sonar profiling
carried out by military nuclear submarines. The terms of the Antarctic Treaty do
not allow the use of such Systems iin the Antarctic. Therefore, in attempting to
measure the thickness distribution of different ice types at different seasons, there
has been considérable dependence on drilling lines of closely spaced holes [1],
although the use of moored upward sonar Systems and sledge- or helicoptermounted electromagnetic eddy current Systems is increasing.
Wave Dispersion in SAR Imagery
P. WÀDHAMS1, F. PARMIGGIANI2, G. DE CAROLIS3 AND M. TaDROSS1
'Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK
2IMGA-CNR, Via Gobetti 101,40129 Bologna, Italy
3ITIS-CNR, P.O. Box 11,75100 Matera, Italy
Abstract
During the early to midwinter period pancake ice is a major component of the
Antarctic sea ice cover, occupying a belt extending 200-300 km from the outer ice
edge with an area of about 6 million km2. Expérience in the Arctic suggests that the
sea ice thickness in this région can be mapped by monitoring the pénétration of
océan waves into it. Spectral analysis of subscenes from ERS-2 synthetic aperture
radar (SAR) images yields the wavelength and direction of the principal spectral
component both outside and inside the ice cover. There is a change of wavelength at
the ice edge associated with a change in the wave dispersion, which can be quantitatively related to the thickness of the ice. The analysis is complex because the true
wave spectrum must be retrieved from the SAR spectrum, which involves an inversion technique requiring a “first-guess” spectrum. The analysis technique is
described, and the wave theory which predicts the change in wavelength.
Expérience in the Chukchi Sea and especially in the Odden ice tongue in the
Greenland Sea is reviewed. Preliminary data from the Ross Sea hâve been analysed,
yielding principal wave components, but the ambient wavelength was too long for
the effect of ice to be détectable. The next application will be to the outer ice edge,
when midwinter data from this région are available at the end of the current season.
1 Introduction: Pancake Ice and Waves
1.1 Antarctic Pancake Ice
In order to study and model the properties of the Antarctic sea ice cover and its
rôle in climate change, an essential parameter to measure is its thickness distribution. Sea ice thickness is a product of thermodynamic and dynamic processes,
and is itself a déterminant of ocean-atmosphere heat flux. In the Arctic most
information on sea ice thickness has corne from upward-looking sonar profiling
carried out by military nuclear submarines. The terms of the Antarctic Treaty do
not allow the use of such Systems iin the Antarctic. Therefore, in attempting to
measure the thickness distribution of different ice types at different seasons, there
has been considérable dependence on drilling lines of closely spaced holes [1],
although the use of moored upward sonar Systems and sledge- or helicoptermounted electromagnetic eddy current Systems is increasing.