Analysis of polarimetric signatures of Arctic lead ice using data from AIRSAR and RADARSAT
Daniel Bäck(1), Ben Holt(2) and Ronald Kwok(2)
(1) Chalmers University of Technology, c/o Leif Eriksson Dept. of Radio and Space Science, 412 96 Göteborg, Sweden
(2) Jet Propulsion Laboratory, 4800 Oak Grove Drive, 91109 Pasadena, United States
Sea-ice acts as an insulator between the comparatively warm sea water and the much colder atmosphere primarily because of its high albedo. Open water leads between ice floes have significantly greater heat transfer than from the ice itself. Any thinning, increased melt, or increase in open water causes further warming and increased melting, a postive feedback. Radar remote sensing has proved to be a very useful tool for mapping the extent of the ice cover but better detection methods for newly opened leads and ice thickness within leads is still needed.
In December 2004 the NASA DC-8 equipped airborne synthetic aperture radar (AIRSAR) acquired fully polarimetric data in the Beaufort Sea. This paper will present the results of the analysis of polarimetric signatures from various sea ice types and in particular different young sea-ice types formed in recently frozen leads. Two weeks of RADARSAT imagery from the same region as the AIRSAR data, was used in order to identify when and where leads were formed and the age of the newly formed ice contained within the leads. By using a known empirical relationship based on freezing degree days, the thickness of a sea-ice layer could be estimated from its age.
In this study, we have identified multiyear and first year ice types using both AIRSAR and Radarsat imagery. In addition, we have identified and examined several stages of new and young ice by using RADARSAT imagery to track back in time to find sea-ice openings: lead ice 1-2 days old, lead ice 2-3 days old, lead ice 9-14 days old, lead ice/first year ice older than 15 days and multi year ice. Furthermore, narrow cracks with unique signatures, hypothesized to be a few hours old, found in several 1-2 day old leads have been included in the analysis. In addition to report the polarimetric signatures of these ice types this study seeks to find which combination of polarimetric parameters that best differentiates the primary ice types as well as the stages of new and young ice within the leads.
The paper presents numerical values of mean and standard deviation of polarimetric parameters such as backscattering coefficients, copolarized ratio and copolarized phase difference for different types and various ages and thicknesses of new and young sea-ice. For the new and young ice, the results suggest to use L-band rather than C-band as an indicator of the age and thickness. We also show that the backscatter tend to increase with the age of ice. This behavior is not observed for C-band; younger ice is observed to give higher backscatter than older thin ice, which is likely due to the development of frost flowers.. The analysis of the phase information shows large variations between the thin ice types and it is hard to draw unambiguous conclusions. However, it could be observed that ice that is thought to be only a few hours old has significantly larger phase differences for C-band than the other ice-types. This ice type also shows the lowest backscatter for C- and L-band, and also the lowest copolar ratio for C-band (VV>>HH). Furthermore, it is the ice type that show negative phase differences for L-band, in correspondence with previous reported results for very young ice. Sea ice older than 15 days appears to give slightly larger phase differences than younger ice for L and P-band. It is also remarked that sea ice 1-2 days old is characteristic for its large variability and negative values of the P-band phase difference.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,