Year-Round High-Resolution Sea Ice Freeboard Retrieval Using ICESat-2 ATL03 Photon Data

Abstract

Abstract. Arctic sea ice freeboard is critical for estimating ice thickness and characterizing surface morphology, yet it remains poorly constrained, especially during the melt season due to limitations in conventional altimetry products. The ICESat-2 ATL07 product retrieves surface heights using variable along-track segment lengths (10–200 m) and identifies floes and leads using fixed thresholds based on photon rate, background rate, and width of the photon distribution to support freeboard estimation. This strategy can smooth ice-ridge-related features and reduce reliability over sea-ice surfaces with complex spatial variations or melt-affected conditions. To address these challenges, we present a year-round, high-resolution (5 m) freeboard retrieval method (HRFM) based directly on ICESat-2 ATL03 photon data. A two-stage denoising strategy is implemented to robustly extract signal photons, while a machine-learning classifier, trained using 25 coincident Sentinel-2/ICESat-2 scenes, discriminates between sea ice, thin ice, and leads under both winter and summer surface conditions. Identified lead segments are used to construct local sea-surface references for freeboard estimation. Validation against Airborne Topographic Mapper (ATM) data shows that HRFM reduces the surface-height root-mean-square error (RMSE) for strong beams from 0.12 m (ATL07) to 0.08 m (by 33 %). HRFM better preserves ridge-related heights that are smoothed in ATL07. The classifier attains a precision of 0.96 and a recall of 0.95 for lead detection, supporting reliable freeboard estimation. Weak-beam retrievals also show encouraging validation performance under the same evaluation framework. During the 1-year demonstration period, the spatial patterns of retrieved freeboard are broadly consistent with the ICESat-2 ATL20 product, while monthly mean differences between the two products reach up to 0.04 m. By improving topographic fidelity and lead detection, HRFM provides a framework for high-resolution freeboard retrieval under both winter and summer surface conditions.