| 10 Sep 2026
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.
This is a timely and well-executed study focused on estimating Arctic sea ice freeboard and thickness using the SWOT mission. The topic aligns perfectly with the scope of The Cryosphere, and this work will undoubtedly be of high interest to the polar research community. The methodology is sound, and the manuscript is generally well-structured. I have no major concerns regarding the methodology in this work, However, I do have two major concerns that I would like the authors to address before publication.
1. Validation with Independent In Situ Data:
The comparisons presented in the manuscript rely entirely on other remote sensing products (such as ICESat-2). While this cross-comparison is an acceptable and valuable step for demonstrating consistency, validating the retrievals against ground-truth, in situ observations is essential to demonstrate the product's robustness. If any independent validation data (e.g., from Upward Looking Sonars, airborne campaigns, or buoy data) are available for this period, I strongly encourage the authors to include a comparison. If data are unavailable, the text should explicitly discuss how future field validations will be integrated.
2. Quantitative Impact of Snow Depth Uncertainties:
In the discussion of limitations (around line 405), the authors state that the lack of coincident snow depth observations "may lead to the underestimation or over-adjustment of SWOT total freeboard." While it is good that this limitation is acknowledged, the statement remains quite qualitative. To improve the impact of this section, could the authors provide a more specific or quantitative analysis of how this factor propagates into the final sea ice thickness uncertainties? For example, a sensitivity analysis or a bounding estimate showing the potential range of error (in centimeters or percentages) would greatly clarify how severe this over- or under-adjustment might be for the overall results.