Sea Ice Deformation Mapping by Means of Synthetic Aperture Radar
Zusammenfassung der Projektergebnisse
In a Ph.D.-project, three different concepts were investigated to parameterize the neutral drag coefficient at 10 m height as a function of sea ice surface characteristics. The different methods were compared to the results of airborne measurements over sea ice in the region of Svalbard. Turbulence data (wind speed components, temperature and humidity) were acquired at 1.3 m intervals, and laser profiles were measured with a horizontal resolution of 0.1m and a vertical accuracy of 0.02 m. The former were used to evaluate the atmospheric drag coefficient, the latter to determine different parameters characterizing the surface structure. In the first approach, the drag coefficient is related to parameters characterizing the ice ridging, with the best results using the aspect ratio (mean ridge height divided by mean ridge spacing). The second approach is drag partitioning which compares better with the measurements than the first concept. The third method relates the drag coefficient to the variance computed from the surface roughness spectrum and correlates with the measurement on same levels as the drag partitioning. Pros and cons of the different methods are: • For the spectral variance approach, surface elevation profiles with a horizontal resolution of only a few centimeters are needed. The approach is computationally very efficient. • Using the other two methods (calculation of ridge aspect ratio or drag partitioning) requires an algorithm for ridge detection. The demands on spatial resolution are slightly more relaxed. The former method is computationally simpler than the latter. • Drag partitioning is based on physical principles and can be extended to more complex situations. Information on sea ice surface characteristics can be also derived from imaging radar systems carried on satellites and aircrafts. From the analysis of images acquired by an airborne SAR (synthetic aperture radar) over sea ice around Svalbard, the following results were obtained: • The retrieval of ice surface parameters (areal fraction of ridges, hummocks, and brash ice, sizes of level ice floes) from SAR images depends on the radar frequency and spatial resolution and, to a lesser degree, on polarization and incidence angle. • The retrieval of surface structure is most robust using an L-band SAR at VV- or HH-polarization, incidence angles between 35 and 50 degree, and spatial resolutions of 5 to 10 meters. • Horizontal patterns of ridges and broken ice can be characterized reliably for most ice regimes using L-band SAR images. Hence, parameters such as distance between ridges and orientation of ridge segments can be extracted and used in parameterizations of the atmospheric drag coefficient.
Projektbezogene Publikationen (Auswahl)
- “Use of Remote Sensing Data for Calculation of Atmospheric Drag over Sea Ice”, Koordinierungsworkshop DFG-Schwerpunktprogramm 1158 - Antarktis, Köln, 10. - 11. Okt. 2008
Ropers, M., Hartmann, J., Dierking, W., Lüpkes, C.
- "Parameterization concepts for atmospheric drag over sea ice validated by airborne measurements", International Symposium on sea ice in the physical and biogeochemical system, International Glaciological Society, Tromsø, Norway, 31 May - 4 June 2010
Ropers, M.
