Abstract
In order to improve the understanding of the dynamical deformation processes of sea ice in the seasonal ice zone (SIZ), measures to detect deformed ice were developed and validated using satellite L-band synthetic aperture radar (ScanSAR) images for the southern Sea of Okhotsk. To approach, sea ice was categorized into three ice types, typical of the sea ice in this region: nilas (thin level), pancake ice (thin rough), and deformed ice (thick rough), and then the measures to classify into these categories were developed using ALOS/Phased Array type L-band Synthetic Aperture Radar (PALSAR) as a function of backscatter coefficients at HH polarization ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma _{\text {HH}}^{0}$ </tex-math></inline-formula> ) and incidence angle ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta _{i}$ </tex-math></inline-formula> ), based on the field observations. Comparative analysis confirmed that PALSAR can detect deformed ice more efficiently than RADARSAT-2 (C-band SAR). The temporal evolution of the area, judged as deformed ice from these measures, shows significant variability with both time and space, and deformed ice regions appear in relatively linear alignments with a width of a few tens of kilometers in the inner ice pack region, consistent with ice drift convergence. To confirm the results, PALSAR-2 images at HH and HV polarizations were examined as a function of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta _{i}$ </tex-math></inline-formula> , based on the four-year field observations in the same area. The results revealed that <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma _{\text {HH}}^{0}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma _{\text{HV}}^{0}$ </tex-math></inline-formula> are both subject to floe sizes as well as deformed ice, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma _{\text{HV}}^{0}$ </tex-math></inline-formula> is more sensitive. This indicates that care should be taken when applying these measures to the ice areas where significantly small floes are dominant like the marginal ice zone.
Highlights
S EA ice plays an important role in shaping the polar climate
We focused on the sea ice in the southern Sea of Okhotsk (43◦ to 50◦ N, 142◦ to 148◦ E, Fig. 1), using three satellites synthetic aperture radar (SAR): Phased Array type L-band Synthetic Aperture Radar (PALSAR), RADARSAT-2, and PALSAR-2, TOYOTA et al.: MEASURING DEFORMED SEA ICE IN SIZS USING L-BAND SAR IMAGES
The results show that sea ice area started to extend around the Gulf of Patience off Sakhalin [Fig. 1(a)] with deformed ice occupying about half of the total ice area on January 7, and spread rapidly to cover the wide area off Sakhalin with a decrease in deformed ice regions by January 31, probably because deformed ice was disintegrated by the divergent motion caused by prevailing easterly winds
Summary
Associated with the significant reduction in sea ice extent in the Arctic Ocean for several decades, the Arcticwide warming trend is twice as fast as the surrounding regions [1]. It is quite important to reproduce the sea ice extent and thickness in the climate models to predict the future climate in the Arctic region. It was reported that none of the Intergovernmental Panel on Climate Change (IPCC) climate models could reproduce the observed rapid thinning trend of mean ice thickness in the Arctic Ocean. Takenobu Toyota is with the Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan Noriaki Kimura is with Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa 277-8564, Japan.
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