Olsen et al. (2026) A first approach towards dual-hemisphere sea ice reference measurements from multiple data sources repurposed for evaluation and product intercomparison of satellite altimetry

Identification

• Journal: Earth system science data
• Year: 2026
• Date: 2026-04-02
• Authors: Ida Lundtorp Olsen, Henriette Skourup, Heidi Sallila, Stefan Hendricks, Renée Mie Fredensborg Hansen, Stefan Kern, Stephan Paul, Marion Bocquet, Sara Ananda Fleury, Dmitry Divine, Eero Rinne
• DOI: 10.5194/essd-18-2469-2026

Research Groups

• Department of Geodesy and Earth Observation, National Space Institute, Technical University of Denmark (DTU Space)
• National Center for Climate Research (NCKF), Danish Meteorological Institute (DMI)
• Marine Research, Finnish Meteorological Institute (FMI)
• Alfred Wegener Institute (AWI), Helmholtz Centre for Polar and Marine Research
• Department of Civil and Environmental Engineering, Norwegian University of Science and Technology (NTNU)
• Integrated Climate Data Center (ICDC), Center for Earth System Research and Sustainability (CEN), University of Hamburg
• Université de Toulouse, LEGOS (CNES/CNRS/IRD/UT3)
• Norwegian Polar Institute (NPI)
• Arctic Geophysics, University Centre in Svalbard (UNIS)
• Woods Hole Oceanographic Institution (Beaufort Gyre Exploration Program)
• Fisheries and Oceans Canada at the Institute of Ocean Sciences
• North Pole Environmental Observatory
• Polar Science Center
• Russian-German BMBF-funded TRANSDRIFT project
• Norwegian Meteorological Institute (METNO)

Short Summary

This paper introduces the Climate Change Initiative (CCI) Sea Ice Thickness (SIT) Round Robin Data Package (RRDP), a comprehensive collection of dual-hemisphere sea ice reference measurements (freeboard, thickness, draft, snow depth) from various non-satellite sources (1960-2024), repurposed and quality-controlled for evaluating and intercomparing satellite altimetry products (1993-2024).

Objective

• To present a comprehensive collection of dual-hemisphere sea ice reference measurements (CCI SIT RRDP) from multiple data sources, repurposed and processed to match satellite altimetry product resolutions, for the evaluation and intercomparison of satellite-derived sea ice freeboard, thickness, and auxiliary snow depth products.

Study Configuration

• Spatial Scale: Gridded to Equal-Area Scalable Earth Grid version 2 (EASE2); 25 km for Northern Hemisphere (NH), 50 km for Southern Hemisphere (SH).
• Temporal Scale: Monthly averages (30 days) for the polar satellite altimetry era (1993–2024), with some raw data extending back to 1960.

Methodology and Data

• Models used: Not applicable; the study compiles and processes reference measurements for validation of existing satellite-derived products. The satellite products themselves utilize algorithms like the Threshold First Maximum Retracker Algorithm (TFMRA) and waveform fitting, supported by auxiliary data such as DTU21 mean sea surface, Copernicus Climate Change Service (C3S) sea ice concentration, and merged snow depth climatologies.
• Data sources:
  • Reference Measurements (CCI SIT RRDP): Airborne sensors (laser altimeter, snow radar, electromagnetic induction sounding "EM-Bird"), autonomous drifting buoys (Ice Mass Balance (IMB) buoys, Snow and Ice Mass Balance Array (SIMBA), acoustic snow depth buoys), moored and submarine-mounted upward-looking sonars (ULS), visual ship observations (ASSIST, ASPeCt).
  • Satellite Products (for validation): CryoSat-2 (ESA CCI SIT CDR v3.0), Envisat (ESA CCI SIT CDR v3.0), ERS-1/2 (ESA Fundamental Data Records for Altimetry - FDR4ALT).

Main Results

• The CCI SIT RRDP provides a comprehensive, quality-controlled dataset of sea ice freeboard, thickness, draft, and snow depth for both hemispheres, covering the polar satellite altimetry era (1993-2024).
• Reference measurements are processed to match satellite product resolutions (monthly, 25 km NH / 50 km SH) and include uncertainty estimates and quality flags for temporal and spatial representativeness.
• Comparisons with CryoSat-2 and Envisat CCI SIT CDRs show generally good agreement, but highlight limitations of certain reference types.
• Visual ship observations (ASSIST, ASPeCt) are biased low and not suitable for direct satellite altimetry validation in their current gridded form, as ships tend to navigate thinner ice.
• Drifting buoy data, when gridded and time-averaged, show weak linear relationships with satellite products due to localized measurements, but comparability improves when analyzed in a Lagrangian framework or with temporal filtering. MOSAiC SIMBA buoys, part of a distributed network, show higher correlation.
• Airborne and ULS mooring data generally show better agreement and higher correlation with satellite products, especially after applying spatial representativeness filters.
• Auxiliary snow depth products in CCI SIT CDRs tend to exhibit narrower distributions than reference measurements, suggesting they may not adequately capture snow depth variability.
• Uncertainty quantification in the RRDP is a first assessment, with future work needed for individual uncertainty measurements and full error propagation.

Contributions

• Creation and publication of the first comprehensive, dual-hemisphere sea ice reference measurement data package (CCI SIT RRDP) specifically repurposed and processed for satellite altimetry validation.
• Standardization of diverse reference measurements (airborne, buoys, moorings, ships, submarines) to common spatial and temporal scales (monthly, 25/50 km grids) for direct comparison with satellite products.
• Development and application of quality flags (temporal and spatial representativeness) and a unified uncertainty estimation framework for the reference measurements, enhancing their usability and reliability for validation.
• Demonstration of the CCI SIT RRDP's utility through inter-comparison with ESA CCI SIT CDRs (CryoSat-2, Envisat, ERS-1/2), highlighting advantages and limitations of different reference data types and informing best practices for satellite product validation.
• Public release of the data package and processing code, promoting transparency, reproducibility, and adaptability for future research and continuous updates.

Funding

• European Space Agency (Climate Change Initiative (CCI) for sea ice (grant no. 4000126449/19/I-NB))

Citation

@article{Olsen2026first,
  author = {Olsen, Ida Lundtorp and Skourup, Henriette and Sallila, Heidi and Hendricks, Stefan and Hansen, Renée Mie Fredensborg and Kern, Stefan and Paul, Stephan and Bocquet, Marion and Fleury, Sara Ananda and Divine, Dmitry and Rinne, Eero},
  title = {A first approach towards dual-hemisphere sea ice reference measurements from multiple data sources repurposed for evaluation and product intercomparison of satellite altimetry},
  journal = {Earth system science data},
  year = {2026},
  doi = {10.5194/essd-18-2469-2026},
  url = {https://doi.org/10.5194/essd-18-2469-2026}
}