Regional Differences in Sentinel-3 Radar Backscatter over Greenland and its Impact on Ice Sheet Altimetry Applications

Satellite radar altimetry operating at Ku-band has provided crucial observations of ice sheet elevation change for more than three decades, offering a direct means to assess climate impacts. Surface height is derived from radar waveforms using retracking algorithms, while waveform parameters such as the backscatter coefficient and leading-edge characteristics are commonly used to identify surface melt and complex topography. Because elevation estimates are sensitive to these parameters, corrections are required to reduce their influence. We investigate temporal variations in the Ku-band radar backscatter coefficient from Sentinel-3 over two regions of the Greenland Ice Sheet. A northern region, which generally exhibits stable backscatter conditions; however, during the summer of 2023, an anomalous decrease of 16 dB was observed, deviating from its typical behaviour. This unexpected signal motivates a comparative analysis with a southwestern region, where a pronounced and recurring seasonal cycle is observed, characterised by a 16 dB increase in backscatter and a concurrent 26 m decrease in elevation in 2023, robustly linked to summer surface melt. The contrasting responses between the two regions are interpreted in terms of differences in meltwater retention and percolation, which control the balance between surface and volume scattering, as quantified using statistical and geophysical waveform parameters. Incorporating a backscatter-height correlation coefficient into the elevation change algorithm improves agreement with ICESat-2 in the southwestern region. These results highlight the need for region- and process-specific adaptations in retracking strategies to ensure robust elevation retrievals under increasingly variable surface and climate conditions.