From profiles to swaths: First estimates of Arctic sea ice freeboard and thickness from SWOT (2023–2025)

Abstract. We present our first estimates of Arctic sea ice freeboard, sea surface height (SSH) from leads, and sea ice thickness from SWOT wide-swath altimetry, which span two growth seasons between October 2023 and April 2025. An incidence-angle-dependent classification scheme separates returns from leads and ice floes for freeboard retrievals. Open water leads, for freeboard calculations, are characterized by high radar backscatter, and low surface heights. Separation of returns from ice floes and leads yields, as expected, spatially decoupled fields of sea surface height and freeboard. Comparisons of area-averaged ICESat-2 and SWOT estimates of lead SSH anomalies over two Arctic growth seasons show strong agreement (R² = 0.95, slope = 0.91), with mean differences ranging between −8.33 ± 8.0 cm and 2.62 ± 10.1 cm. The area-averaged freeboard estimates from ICESat-2 are consistently higher than those from SWOT, with differences that trend from a low of −0.78 ± 4.22 cm in early fall, to a high of 11.3 ± 5.79 cm in early spring. Regression analysis of the two freeboard records shows consistent seasonal variability, explained largely by the expected growth of the sea ice cover (R² = 0.96). This seasonal behavior of ICESat-2 and SWOT freeboard differences strongly mirrors altimeter-derived snow depths (R = 0.99 and 0.97 for the 2023–2024 and 2024–2025 seasons), suggesting a time-invariant Ka-band penetration depth into the snow cover. Comparisons between ICESat-2 and SWOT sea ice thicknesses—using SWOT total freeboard—reveal close overall agreement, with a mean difference of 0.02 ± 0.17 m for the two seasons.