Exploiting Altimetry and SAR Synergies: Extrapolating ICESat-2 Freeboard Tracks to Local 2D Maps

Kurzfassung

Satellite altimetry and Synthetic Aperture Radar (SAR) measurements provide both important and to a large degree complementary information about the Arctic sea ice cover. We are currently developing a synergistic approach to expand laser altimetry-derived ice freeboard measurements to SAR scenes. The scope and frequency of SAR acquisitions are complemented by the accuracy and detail of altimetry data products. Whilst both instruments provide valuable data on their own, their synergies have not yet been completely leveraged. However, there is an ongoing effort to relate the measurements for increased insight and possible novel retrieval methods. The ATLAS laser altimeter on ICESat-2 can provide sea ice freeboard measurements along three parallel tracks separated by about 3 km, i.e. there are large gaps between the tracks and the overall measurement coverage of the Arctic Ocean is sparse. Sentinel-1 C-band SAR provides 400 km wide scenes of sea ice backscatter with 40 m resolution in extended wide (EW) mode. The backscatter is related to the ice topography and ice type, but the relationship is complex. As part of an ongoing effort to combine these two data sources, we have constructed a method to extrapolate ICESat-2-derived freeboard measurements to the scale and scope of Sentinel-1 scenes, even when the measurements are up to a day apart. We extrapolate the ICESat-2 measurements on a per-scene basis, so that freeboard maps are constructed for individual Sentinel-1 EW scenes. The extrapolated freeboard is validated against near coincident ICESat-2 freeboard tracks, not used for the extrapolation. At 100 m resolution, these extrapolated freeboard maps show errors of around 10 cm. At 400 m resolution, the errors drop to around 6 cm. Such freeboard extrapolations can fill a knowledge gap of medium to high-resolution, high-frequency Arctic sea ice topography maps and could be converted to sea ice thickness if snow and density information are added. In the future, they might provide relevant information for ice-atmosphere coupling and thermodynamic and dynamic sea ice development. In the planned presentation we aim to showcase the developed method and reach out to the scientific community to discuss the uses of these freeboard maps. With new SAR (NISAR, ROSE-L) and altimetry (SWOT, CRISTAL) missions recently launched and planned for the near future, synergetic uses of these two instrument types are going to stay highly relevant to the scientific community of Polar research for the foreseeable future.