Change Sea-ice motion (code 403) definition and units

[jeffrkey]

Initial request

This request is to change the definition and units of the variable Sea-ice motion (code 403). The current definition simply a repeat of the name, which therefore isn't a definition at all. The new definition should reflect the vector nature of velocity, as velocity implies (in science) speed and direction.

Current name: Sea-ice motion Proposed name: Unchanged Current definition: Sea-ice motion Proposed definition: Horizontal displacement of sea ice over a defined time interval, specified as a two-element vector quantity: u and v. Related terms found in the literature are "Sea-ice velocity" and "Sea-ice drift". Current units, Measuring and Uncertainy: km/day, Horizontal: km, Stability: km/day (per decade) Current units, Measuring and Uncertainy: m/s, Horizontal: m, Stability: m/s (per decade)

Amendment details

The proposed update of the existing variable is as follows:

notation	name	definition
403	Sea-ice motion	Horizontal displacement of sea ice over a defined time interval, specified as a two-element vector quantity: u and v. Related terms found in the literature are "Sea-ice velocity" and "Sea-ice drift".

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 and other GCW sea ice experts; 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

Relevant publications will be added

Validation

Yes, validation has been performed. Publications will be added that describe various validation studies.

[IARC2000]

Fully endorse this one. Thank you.

[TomLav]

Dear @jeffrkey ,

While I support giving a more explicit definition and units for sea-ice motion, I am wondering if the proposed definition is not too restrictive for the variety of measurement techniques that can be employed for sea-ice motion.

The proposed definition is :

Horizontal displacement of sea ice over a defined time interval, specified as a two-element vector quantity: u and v

with the Measuring and Uncertainy units: m/s

First we have the cases of sea-ice tethered position loggers (e.g. GPS tags attached to buoys). These do record lat/lon positions at given times. These trajectories of lat/lon/time can indeed be transformed to sea-ice motion velocity vectors, but the observation is generally stored as series of lat/lon/time. Would this type of observation need to be transformed to velocity vectors in order to be accepted as "sea-ice motion (code 403)"?

There are also the cases of fixed Upward Looking Sonars (ULS) that record sea-ice speed (m/s) (via Doppler), but not the direction. These only sense one component of the drift vector, not a two-element vector quantity. Are these accepted in "sea-ice motion (code 403)"? I guess ship-borne, coastal or even space-borne radar doppler-based estimates of sea-ice motion also fall in this category of single-component observations.

Finally, I wonder if the specification of u and v as names for the components implies zonal / meridional components (a widely used naming convention), or if other components would be accepted (e.g. x and y along grid axes, or even speed + direction, in which case the units of direction could not be m/s).

I apologize if the above has already been discussed and concluded upon in previous meetings (to which I might even have been invited to insert-face-palm-emoji). If this is the case we can hopefully record the decisions in the github issue for traceability.

[TomLav]

Questions to @hansl2022 : do you "own" this issue or should all sea-ice related requests be owned by the same person?

I would like to ask how we are supposed to move this (and other) issues forward. Do we have to build a consensus through discussion here? Is there a cut-off date at which the proposal must be concluded? In this particular case, I made a comment and asked questions but did propose an update to the proposed definition. Should I make such a proposal in a later issue?

[hansl2022]

I was unable to log in Github and couldn't reply directly by email. However, I am actively working to push it formard and will provide an update at the earliest opportunity. @TomLav

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.04.17 notes: Shuli suggests that it needs more discussion per comments above; Jeff noted that the name should remain the same (sea-ice motion); Rodica noted that "sea ice drift" is an ECV but "sea ice motion" is in the OSCAR Requirements; Joerg suggests looking at the property list to see if there is something more relevant than "motion", e.g. "velocity" which is more precise;

[TomLav]

To me, sea-ice motion and drift describe the same concept. I would align with GCOS (sea-ice drift) but have a sentence in the definition (as in the current proposal) that this is interchangeable with motion.

One problem (and the root of my comment above) is that both these terms (motion and drift) describe a concept / notion, but not a physical variable. Velocity can be measured, Speed can be measured, Displacement (distance) can be measured, Direction, Rotation, etc... can be measured. Not Drift or Motion (unless they are mapped to other terms).

Thus I see 3 options:

We call this one sea-ice drift (align with GCOS), with an empty units, and add a sentence to state that this is a "collection" of variable, in which all related and derived variable (see list above), with their specific units, are acceptable.

We call it sea-ice drift (align with GCOS), with units m.s-1, but add a sentence to state that other related and derived quantities (see list above), with their specific units, are acceptable.

We call is sea-ice velocity (change wrt GCOS and OSCAR), with units m.s-1, add a sentence to state that other related and derived quantities (see list above), with their specific units, are acceptable.

I do not think we want the 4th option (5+ variables all related to sea-ice motion/drift).

[amilan17]

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

with path: Ocean/Cryosphere/Sea ice

[amilan17]

I would like to ask how we are supposed to move this (and other) issues forward. Do we have to build a consensus through discussion here? Is there a cut-off date at which the proposal must be concluded? In this particular case, I made a comment and asked questions but did propose an update to the proposed definition. Should I make such a proposal in a later issue?

@TomLav We try to build consensus through the comments in the GitHub issues and at the TT-WIGOSMD meetings. We are hoping to have this proposal (and many sea-ice proposals) finalized by mid-June for the WMO approval procedure in July. I hope this helps.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.06.12 notes: The proposal is ready @amilan17 update branch

[jeffrkey]

We just changed the name from Sea-ice motion to Sea-ice velocity.

[amilan17]

@jeffrkey it's a little unclear as to the status of the proposal. Will you confirm or correct the following:

Current definition: Sea-ice motion (same as the current name) Proposed definition: Horizontal displacement of sea ice over a defined time interval, specified as a two-element vector quantity: u and v. Related terms found in the literature are "Sea-ice ~motion~velocity" and "Sea-ice drift".

[rodicanitu]

Dear @amilan17 the proposal is that in the initial definition of the issue: to update the name of the variable to sea ice velocity.

It should read "also known as sea ice motion and sea ice drift" this aligns with Joerg's recommendation to use measurable properties, i.e. velocity. the agreement has been to keep "sea ice drift" as is in GCOS as a separate variable, right now. @jeffrkey

please approve the request as made, the conversation that followed, supports this request. it would be a pity to "approve" still motion.

[jeffrkey]

Rodica’s explanation is the same as what mine would have been: change the name to “Sea-ice velocity” and state in the definition that the terms sea-ice motion and sea-ice drift are also used (or it could go in a comment section in OSCAR, I think).

Jeff

From: rodicanitu @.> Date: Friday, July 11, 2025 at 12:14 AM To: wmo-im/wmds @.> Cc: Jeff Key @.>, Mention @.> Subject: Re: [wmo-im/wmds] Table 1-01-03 change Sea-ice motion (code 403) name, definition, and units (Issue #578) [https://avatars.githubusercontent.com/u/32776394?s=20&v=4]rodicanitu left a comment (wmo-im/wmds#578)https://urldefense.com/v3/__https:/github.com/wmo-im/wmds/issues/578*issuecomment-3060570184__;Iw!!Mak6IKo!MOoc6tCDJlyPvaYq-NelT6GsWWr-d-Z2oo8A2s-_Wqu_ZKcnZXwpwTZYdDq1NdvGA4mF1NVN75v1cApfMKMquD0$

Dear @amilan17https://urldefense.com/v3/__https:/github.com/amilan17__;!!Mak6IKo!MOoc6tCDJlyPvaYq-NelT6GsWWr-d-Z2oo8A2s-_Wqu_ZKcnZXwpwTZYdDq1NdvGA4mF1NVN75v1cApfBZgWyM8$ the proposal is that in the initial definition of the issue: to update the name of the variable to sea ice velocity.

It should read "also known as sea ice motion and sea ice drift" this aligns with Joerg's recommendation to use measurable properties, i.e. velocity. the agreement has been to keep "sea ice drift" as is in GCOS as a separate variable, right now. @jeffrkeyhttps://urldefense.com/v3/__https:/github.com/jeffrkey__;!!Mak6IKo!MOoc6tCDJlyPvaYq-NelT6GsWWr-d-Z2oo8A2s-_Wqu_ZKcnZXwpwTZYdDq1NdvGA4mF1NVN75v1cApfCw7BJXA$

please approve the request as made, the conversation that followed, supports this request. it would be a pity to "approve" still motion.

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[amilan17]

@jeffrkey @rodicanitu thank you for the clarification.