CS2SMOS¶

CryoSat-2/SMOS Merged Sea Ice Thickness (ESA SMOS-CryoSat L4 Sea Ice Thickness)

Discontinued

At a glance¶

Note on period: Starts 2010-11-15 according to the ESA catalogue. Coverage is limited to the Northern Hemisphere October–April (winter) period; summers are empty. The last v206 season is winter 2024/25, and three weeks of L4 data (4–23 February 2025) are missing because the JRA-3Q reanalysis input was unavailable.

Overview¶

A weekly Arctic sea ice thickness data record from merged CryoSat-2 and SMOS satellite data. The CryoSat-2 radar altimeter is most accurate over thick ice, while the SMOS L-band radiometer is most sensitive to thin ice (below about 1 m); the two are combined by optimal interpolation. Compared with CryoSat-2 monthly fields alone, the product offers better temporal resolution and gap-free coverage, and it has been widely used for sea ice prediction initialisation, model evaluation and shipping-route studies. It is produced by AWI and distributed by ESA as SMOS-CryoSat L4 Sea Ice Thickness.

What changes with the successor¶

CS2SMOS v206 → CS3SMOS v300¶

Transition: 2025-10

The second major revision of AWI's merged sea ice thickness product: Sentinel-3A/B altimetry was added and the processing system rebuilt in Python. Resolution improved from 25 km to 12.5 km, austral winter fields were added, and the uncertainty model and file content changed. It replaced v206 from the 2025/26 winter (operational from 2025-10-23).

Sources: AWI CS3SMOS/CS2SMOS product page (version history) · AWI CS2SMOS-PDS announcements (v300 start on 2025-10-23) · ESA SMOS-CryoSat L4 Sea Ice Thickness catalogue (v206) · ESA CS2SMOS Product Description Document (PDD v1.6, v206)

Compare the two side by side

Available variables¶

Variables as listed in the v206 Product Description Document (PDD v1.6, Table 3). Figure captions in the same document use some other names, so check an actual file once. The successor CS3SMOS (v300) has a new file format and different variable names.

instantaneous mean accumulated maximum minimum static — instantaneous: value at the time stamp · mean/accumulated/maximum/minimum: over the indicated interval · static: does not change with time

Merged thickness (main product)¶

Thickness by input sensor¶

Auxiliary fields¶

The full variable list is in the official documentation.

How to get the data¶

• ESA SMOS Online Dissemination Service — L4_SIT_Open collection — HTTPS / FTPS (ftps://smos-diss.eo.esa.int/)
  • Log in with a free ESA EO Sign In account to download. The full v206 record from November 2010 is here.
• ESA Earth Online catalogue — SMOS-CryoSat L4 Sea Ice Thickness — Web documentation (DOI, PDD, Read-me-first note, ATBD)
• AWI FTP — sea ice product root — Anonymous FTP No login needed
  • No account needed. According to AWI quicklook paths, CS3SMOS is in the cryosat2_smos/v300 folder; check the FTP listing for the exact subfolders.
• AWI CS2SMOS-PDS wiki (announcements and documents) — Web documentation
  • The first place where production starts, interruptions and data gaps are announced.
• C3S Climate Data Store — Sea ice thickness gridded data for the Arctic derived from satellite observations — Web download / CDS API
  • A separately processed satellite-based Arctic sea ice thickness dataset provided by C3S; not the same product as CS2SMOS. Requires an ECMWF account.

Example code¶

Assumes a v206 weekly file downloaded from the ESA server, e.g. W_XX-ESA,SMOS_CS2,NH_25KM_EASE2_20190304_20190310_o_v206_01_l4sit.nc

import xarray as xr

ds = xr.open_dataset("W_XX-ESA,SMOS_CS2,NH_25KM_EASE2_20190304_20190310_o_v206_01_l4sit.nc")

sit = ds["analysis_sea_ice_thickness"].squeeze()        # m
unc = ds["analysis_sea_ice_thickness_unc"].squeeze()    # m

# Compare SMOS and CryoSat-2 contributions over thin ice (< 1 m)
thin = sit < 1.0
print("SMOS  :", float(ds["smos_sea_ice_thickness"].squeeze().where(thin).mean()))
print("CS2   :", float(ds["cryosat_sea_ice_thickness"].squeeze().where(thin).mean()))
print("Merged:", float(sit.where(thin).mean()))

Practical tips¶

• Each file is a 7-day mean. A new file appears every day, but files on consecutive days share six days of data, so do not treat them as an independent daily series and ignore the autocorrelation. Using only non-overlapping weekly files (Monday–Sunday) is the safe choice.
• Operational ('o') files and reprocessed ('r') files can differ. Where both exist for the same period, use the reprocessed version.
• SMOS thickness is known to be underestimated where ice concentration is below 100%, and SMOS data are frequently missing over the Barents, Kara and Laptev Seas because of radio-frequency interference (RFI). Merged thickness there relies essentially on altimetry, so interpret it with care.
• v206 and its successor CS3SMOS (v300) differ in resolution, input satellites and uncertainty model and should not be treated as one record. To extend beyond the 2025/26 winter, move the whole analysis to v300 where possible; if you combine them, state the switch date explicitly.

Notices and change history¶

References for your paper¶

Dataset citation¶

ESA SMOS-CryoSat L4 Sea Ice Thickness, Version 206 — DOI 10.57780/sm1-4f787c3

@misc{esa_smos_cryosat_l4_sit_v206,
  author    = {{European Space Agency}},
  title     = {{SMOS-CryoSat} {L4} Sea Ice Thickness, Version 206},
  year      = {2023},
  publisher = {European Space Agency},
  doi       = {10.57780/sm1-4f787c3}
}

Reference papers¶

CS2SMOS method paper (citation requested by AWI and ESA) — DOI 10.5194/tc-11-1607-2017

@article{ricker2017cs2smos,
  author  = {Ricker, Robert and Hendricks, Stefan and Kaleschke, Lars and Tian-Kunze, Xiangshan and King, Jennifer and Haas, Christian},
  title   = {A weekly {Arctic} sea-ice thickness data record from merged {CryoSat-2} and {SMOS} satellite data},
  journal = {The Cryosphere},
  volume  = {11},
  number  = {4},
  pages   = {1607--1623},
  year    = {2017},
  doi     = {10.5194/tc-11-1607-2017}
}

SMOS thin sea ice thickness algorithm paper (optional) — DOI 10.5194/tc-8-997-2014

Acknowledgements¶

Along with the paper citation, AWI asks users to acknowledge: "The production of the merged CryoSat-SMOS sea ice thickness data was funded by the ESA project SMOS & CryoSat-2 Sea Ice Data Product Processing and Dissemination Service, and data from DATE to DATE were obtained from AWI." (replace AWI with ESA if you downloaded from ESA). Check the AWI CS2SMOS wiki and the Read-me-first note for the current wording.

Information on the datasets used in this study was compiled with the help of the Korean Climate Data Navigator (KCDN) (Sim, 2026; https://doi.org/10.5281/zenodo.23300923).

Links¶

• ESA Read-me-first note (v206; known issues and validation)
• ESA Algorithm Theoretical Basis Document (ATBD)
• AWI CS3SMOS/CS2SMOS product page (version history)
• ESA SMOS mission page
• This site — SMOS thin sea ice thickness (a CS2SMOS input)