Standard names containing 'difference'

[ngalbraith]

There are several standard names that describe temperature differences: difference_between_sea_surface_temperature_and_air_temperature, sea_water_sigma_t_difference, and the incredibly succinct sea_water_temperature_difference.

Is there a convention for which term is subtracted from the other? If so, is this documented somewhere, and, in any case, should it be in the definition? I checked the conventions document, the guidelines for constructing standard names, and some of the definitions themselves, and can't find the answer to this.

There are also some names containing the similar concept 'relative', and at least one includes the kind of info I'm looking for; the definition of northward_air_velocity_relative_to_sea_water includes the phrase ' ... calculated as northward_wind minus northward_sea_water_velocity.'

[JonathanGregory]

I agree, it should be documented. I believe that when such names have been defined the understanding has been that difference_between_A_and_B means A minus B.

[JonathanGregory]

The three standard names sea_water_sigma_t_difference, sea_water_sigma_theta_difference and sea_water_temperature_difference were introduced in order to be used as auxiliary coordinate variables for variables with standard names of ocean_mixed_layer_thickness_defined_by_sigma_t, ocean_mixed_layer_thickness_defined_by_sigma_theta
and ocean_mixed_layer_thickness_defined_by_temperature. They are intended to mean the unsigned difference (I suppose) of each quantity from its surface value. Should they be renamed to something more informative? They certainly need better definitions.

[taylor13]

I too have always interpreted difference_between_A_and_B as A minus B, but I think it should say so in the definition. I have nothing useful to say about the others.

[github-actions]

Thank you for your proposal. These terms will be added to the cfeditor (http://cfeditor.ceda.ac.uk/proposals/1) shortly. Your proposal will then be reviewed and commented on by the community and Standard Names moderator.

[efisher008]

Hi @JonathanGregory @taylor13,

I am revisiting this issue as it has been open for a couple of years without a conclusion.

Do you think it would be useful to add some explanatory text in the phrasebank for the phrase difference or difference_between, which could be used to guide writing the description for future standard names with this construction? This would likely also require a review of existing standard names with these phrases to ensure consistent application.

Could there also be the possibility of adding this somewhere in the Guidelines for Construction of Standard Names, or even the CF conventions document, to provide an example for the use of the phrase in standard names?

Best wishes,
Ellie

[JonathanGregory]

Dear Ellie

Yes, it would be useful to add explanatory text for difference_between. Karl and I had the same understanding, which I believe is the intention in standard names. Yes, I agree it belongs in the Guidelines as well. It could go in the Transformations section, where we already have rules for product and ratio.

I think the existing definitions of the standard names sea_water_sigma_t_difference, sea_water_sigma_theta_difference and sea_water_temperature_difference are clear, except that they don't specify whether its a signed or unsigned difference that is used to define the mixed layer thickness. I don't know the answer to that. An observational oceanographer could tell us.

Best wishes

Jonathan

[efisher008]

Dear Jonathan @JonathanGregory,

Thanks for your reply. In that case, might I suggest transferring this to a discussion if more input is needed, particularly from the CF governance panel/conventions committee? Or if there are clear actions which can be identified at this point (i.e. sign convention and direction), perhaps transferring to the cf-conventions/discuss repo (unless you think the vocabularies repo would be the most appropriate/preferable location for it).

Best wishes,
Ellie

[JonathanGregory]

Dear Ellie

I think it would be fine to do these things arising from this issue:

Yes, it would be useful to add explanatory text for difference_between. Karl and I had the same understanding, which I believe is the intention in standard names. Yes, I agree it belongs in the Guidelines as well. It could go in the Transformations section, where we already have rules for product and ratio.

I will ask people who might know about the "difference" question.

Jonathan

[taylor13]

Yes, this sounds good. Thanks.

[JonathanGregory]

Steve Griffies agrees that in the standard names sea_water_sigma_t_difference, sea_water_sigma_theta_difference and sea_water_temperature_difference, the difference should be unsigned i.e. it is | A - B |. I suggest inserting this clarification in their descriptions. As discussed above, all the other difference "between A and B" standard names mean A minus B, and that could be clarified in their descriptions, and also in the guidelines for difference_between_X_and_Y.

[taylor13]

Thanks for getting Steve's expert opinion. Sounds good.

[github-actions]

This issue has had no activity in the last 30 days. Accordingly:

• If you proposed this issue or have contributed to the
  discussion, please reply to any outstanding concerns.
• If there has been little or no discussion, please comment
  on this issue, to assist with reaching a decision.
• If the proposal seems to have come to a consensus, please
  wait for the moderators to take the next steps towards
  acceptance.

Standard name moderators are also reminded to review @feggleton @japamment @efisher008

[taylor13]

Not sure we have a moderator, and I can't volunteer. Maybe that's why this has stalled.

To be sure @JonathanGregory, I guess the sign of the difference mentioned in conjunction with defining a mixed layer thickness (e.g., the sigma_theta_difference) can be negative even though the thickness is by definition positive, so the sign of the proxy used to determine the depth of the mixed layer is irrelevant.

If that's the case, I think we've reached consensus and what's needed is to augment the descriptions of all standard names containing the word "difference". Is that right?

[JonathanGregory]

Yes, I agree with you, Karl @taylor13.

[github-actions]

This issue has had no activity in the last 30 days. Accordingly:

• If you proposed this issue or have contributed to the
  discussion, please reply to any outstanding concerns.
• If there has been little or no discussion, please comment
  on this issue, to assist with reaching a decision.
• If the proposal seems to have come to a consensus, please
  wait for the moderators to take the next steps towards
  acceptance.

Standard name moderators are also reminded to review @feggleton @japamment @efisher008