Change Sea-ice thickness (code 406) definition and units

[jeffrkey]

Initial request

This request is to change the definition and units of the variable Sea-ice thickness (code 406). The current definition does not actually define "thickness". Furthermore, it refers to the thickness of the ice sheet. While sea ice may be thought of as a sheet of ice, the term "ice sheet" almost exclusively refers to land ice, notably the Greenland and Antarctic ice sheets. The proposed definition of sea ice thickness is clearer.

Current name: Sea-ice thickness Proposed name: same Current definition: Thickness of the ice sheet. It is related to sea-ice elevation and ice density. Proposed definition: Vertical distance between the upper sea-ice surface, excluding any snow cover, and the underside of the sea ice. Current units, Measuring and Uncertainy: cm, Horizontal: km, Stability: cm (per decade) Proposed units, Measuring and Uncertainy: m, Horizontal: m, Stability: m (per decade)

Amendment details

The existing variable will be updated as follows:

notation	name	definition
406	Sea-ice thickness	Vertical distance between the upper sea-ice surface, excluding any snow cover, and the underside of the sea ice.

All the units involved exist and no changes are needed.

Requestor(s)

Group: Global Cryosphere Watch (GCW) POCs:

Jeff Key ([email protected]), University of Wisconsin Petra Heil, University of Tasmania and Australian Antarctic Division, [email protected] Rodica Nitu, WMO, [email protected]

Stakeholder(s)

National and international ice services, e.g., US National Ice Center, North American Ice Service, Norwegian Ice Service; scientific community for remote sensing validation, meteorological/climatological and modeling applications

Applications or Systems

• [x] OSCAR/Surface
• [x] OSCAR/requirements
• [ ] Radar/DB
• [ ] OceanOPS
• [ ] WHOS
• [ ] WDQMS
• [ ] GBON Compliance Monitor
• [ ] Other

Expected impact of change

LOW

Collaborators

GCW Sea Ice Best Practices team; GCW Cryosphere and Polar Observations (CRYORA) team Belén Martín Míguez, Ocean Earth System Category (ESAC)

References

To be added.

Comments

No response

Publication(s)

Manual on Codes (WMO-No. 306), Volume I.3, WMO Codes Registry - WMDR

Validation

To be added. There are many studies on the validation of satellite-derived sea ice thickness, some of which use in situ measurements.

[mohntorte]

Stefan Kern: I would perhaps change the order, i.e. "Vertical distance between the upper sea-ice surface excluding any snow and the underside of the sea ice". And we need to pay attention to any amendment of "sea ice freeboard" in terms of how we name the reference levels (see my comment there).

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.02.20 notes: Shuli is assigned

[IARC2000]

Largely agree with the proposed changes, including: "Proposed name: Sea-ice thickness"

With request to make proposed definition even clearer, i.e.: "Proposed definition: Downward vertical distance from the upper sea-ice surface (excluding any snow cover) to the sea-ice base".

Critical points:

• The location of the Zero point needs to be defined, aka the direction of measurement. Sea-ice thickness is measured from the upper surface downward. This contrasts for example with snow, where the Zero point is located at its lower interface and snow thickness is measured with a vertically upward component.
• It is important to introduce the concept of "vertical", i.e., versus perpendicular to a surface.
• All variable names should be unambigous, i.e., spell out "sea ice" rather than referring to "ice" or worse "ice sheet".

[hansl2022]

A suggested minor adjustment to the proposed definition: excluding snow on ice. 'snow cover' typically refers to snow on land.（From an expert I consulted.）

[amilan17]

Code table 1-01-03.csv ObservedVariableOcean Observed variable - measurand (ocean)

with path: Ocean/Cryosphere/Sea ice

[amilan17]

@jeffrkey do you agree with Shuli's suggestion?

notation	name	definition
406	Sea-ice thickness	Vertical distance between the upper sea-ice surface, excluding snow on ice, and the underside of the sea ice.

[jeffrkey]

The proposed definition, changing "snow cover" to "snow on ice", is fine. I don't think it's necessary, but it's fine.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.06.12 notes: @amilan17 update the branch

[joergklausen]

Final version: "Vertical distance between the bottom and the top of sea ice, excluding snow on ice."