Sea ice freeboard extrapolation from ICESat-2 to Sentinel-1

Abstract

The Ice, Cloud and Land Elevation Satellite (ICESat-2) laser altimeter can capture sea ice freeboard along-track at both high vertical and high spatial resolutions. The measurement occurs along three strong and three weak parallel beams. Thus, the across-track direction is only very sparsely covered, and capturing the two-dimensional spatial distribution of freeboard at a high resolution using this instrument alone is not possible. This work shows how, in the early Arctic winter months of October and November, Sentinel-1 synthetic aperture radar (SAR) acquisitions help bridge this gap. Freeboard measurements are shown to be meaningfully extrapolated to a full two-dimensional mapping. To achieve this, it is sufficient to use the cross-polarised (HV) SAR backscatter to sort the pixels by intensity and then map freeboards measured from altimetry in the area via the cumulative distribution functions. With the presented algorithm, the snow and ice freeboard derived from altimetry can be extrapolated to Sentinel-1 SAR scenes, unlocking an additional dimension of Arctic freeboard monitoring at a high spatial resolution, with ice freeboard errors between 6 and 10.5 cm for spatial resolutions between 100 and 400 m.