Observing Seawater Flooding on Arctic First-Year Ice during Pre-Melt Using C- and L-Band Synthetic Aperture Radar

Abstract. Seawater flooding from negative freeboard onto sea ice leads to the formation of a saline slush layer at the bottom of the snowpack, affecting snowpack geophysical properties, ice mass balance, remote sensing retrievals, and on-ice travel safety. Synthetic Aperture Radar (SAR) is sensitive to saline slush, though limited information on this dynamic layer exists. This research assesses the sensitivity of C- and L-band frequency SAR to slush occurrence on Arctic first-year ice near Qikiqtarjuaq (Nunavut, Canada) during pre-melt conditions and evaluates the potential of SAR to estimate flooding. SAR parameters backscatter, logarithm ratio, cross- and co-polarization ratios, coherence, and textural features, are evaluated for their ability to distinguish Flooded from Non-flooded classes and results are tested at a regional scale. The loss of coherence in co-polarization L-band SAR, due to the occurrence of a high-dielectric slush layer, is found to be optimal for flooding detection, with the lower frequency being less influenced by the overlying snow. Backscatter, and its temporal change through the logarithm ratio, consistently discriminates classes when the slush is refreezing or has frozen into snow ice. Regional-scale estimation using L-band coherence is demonstrated as achieving an F1-score of 0.81, with higher F1-scores (up to 0.87) obtained when random forest models combine textural information with coherence. These findings highlight the dynamic nature of seawater flooding, and the potential for SAR to be used in related applications and research studies.