wmo-im
Change Sea-ice elevation (code 402) to Sea-ice freeboard, name, definition and units
Initial request
This request is to change the name, definition, and units of the variable Sea-ice elevation (code 402). While the elevation of sea ice is measured by laser altimeters (airborne or space-based), it is not directly measured by radar altimeters and it is not measured at all in situ. It is proposed here to instead use Sea-ice freeboard as the variable of interest. It is measured in situ directly, and can be estimate by both laser and radar altimeters. Furthermore, it is more relevant in thermodynamic and dynamic applications.
Current name: Sea-ice elevation Proposed name: Sea-ice freeboard Current definition: Elevation of the surface of the sea-ice sheet above sea level Proposed definition: Elevation of the upper sea-ice surface relative to the sea surface Current units, Measuring and Uncertainy: cm, Horizontal: km, Stability: cm (per decade) Current units, Measuring and Uncertainy: m, Horizontal: m, Stability: m (per decade)
Amendment details
The proposed update of the existing variable is as follows:
| notation | name | definition |
|---|---|---|
| 402 | Sea-ice freeboard | Elevation of the upper sea-ice surface relative to the sea surface |
All units required already exist and no further change is 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
Comments
Supporting comments were received by the community to stay with the originally proposed description.
Publication(s)
Manual on Codes (WMO-No. 306), Volume I.3, WMO Codes Registry - WMDR
To be added
Validation
To be added
A slightly different definition is proposed, in the hope that it may be more understandable than the original wording.
Stefan Kern: There is an amendment of our proposed definition of "Elevation of the upper sea-ice surface relative to the sea surface" towards: "Elevation (aka, vertical distance) of the top of the sea-ice relative to the sea surface"
I am not sure whether "upper sea-ice surface" and "top of the sea ice" go hand in hand. If we accept this amendment we might need to re-think our definition for sea-ice thickness where we also use "upper sea-ice surface". We should be consistent here.
Adding "(aka, vertical distance)" sounds reasonable at the first place. However, a sea-ice freeboard can be negative. I am not entirely sure whether it is similarly common to speak about a "negative distance" compared to a "negative elevation". I am inclined to say that elevation is more clear (or clear enough) and should perhaps be mixed with a vertical distance. I note that we use "vertical distance" for sea-ice thickness, a quantity that is equal or larger than zero but never negative.
Not sure whether it would make sense to list me as one of the requestors?
Dear @jeffrkey, dear @mohntorte, I would vote for the definition proposed originally for this quantity:
Elevation of the upper sea-ice surface relative to the sea surface
Fully endorsing: The proposed name: Sea-ice freeboard and the proposed definition: Elevation of the upper sea-ice surface relative to the sea surface
Supporting comments were received by the community to stay with the originally proposed description.
This will go in the ObservedVariableOcean table with the following path to be used in OSCAR/Surface
Ocean/Cryosphere/Sea ice
https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.06.12 notes:
@jeffrkey please review branch