Standard names: *Norm of the horizontal gradient of the chlorophyll concentration*

[abiardeau]

Hello,
I am Aurore BIARDEAU from Mercator Ocean international, working for Copernicus Marine service.
Date : 13-05-2024

We are in the process of implementing a new variable in the Copernicus Marine service and we would be happy to have your feedback on our proposal :

standard_name : mass_concentration_of_chlorophyll_a_in_sea_water_horizontal_gradient

unit : mg / m3 / km

definition (from existing mass_concentration_of_chlorophyll_a_in_sea_water) :
'Mass concentration' means mass per unit volume and is used in the construction mass_concentration_of_X_in_Y, where X is a material constituent of Y. A chemical or biological species denoted by X may be described by a single term such as 'nitrogen' or a phrase such as 'nox_expressed_as_nitrogen'. Chlorophylls are the green pigments found in most plants, algae and cyanobacteria; their presence is essential for photosynthesis to take place. There are several different forms of chlorophyll that occur naturally. All contain a chlorin ring (chemical formula C20H16N4) which gives the green pigment and a side chain whose structure varies. The naturally occurring forms of chlorophyll contain between 35 and 55 carbon atoms. Chlorophyll-a is the most commonly occurring form of natural chlorophyll. The chemical formula of chlorophyll-a is C55H72O5N4Mg.

I can provide more information from my expert colleagues if needed,
Kind regards,
Aurore

[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.

[taylor13]

I wonder why the horizontal gradient is reported rather than the actual mass concentration itself, which provides additional information and from which the gradient could be computed. Is the gradient constrained by measurements other than the concentration? I suppose this goes beyond what CF needs to consider, but I am curious.

[JonathanGregory]

Dear Aurore @abiardeau

Thanks for the proposal. A couple of comments.

• I think that this order of phrases is hard to parse. Do you mean horizontal_gradient_of_mass_concentration_of_chlorophyll_a_in_sea_water?

• What does "horizontal gradient" mean? According to the guidelines, if is the magnitude of a vector gradient, it should be something like magnitude_of_horizontal_gradient. If it's a vector, we need components such as eastward_derivative and northward_derivative. Since your title says Norm of the horizontal gradient, I guess you mean the former. In either case, something should be said in the definition about this.

Best wishes

Jonathan

[efisher008]

Dear Aurore,

Thank you for your proposal, I have added the name to the CF editor. It looks like there is further discussion to be had about the format of the name, so I will wait for your response to the comments from @taylor13 and @JonathanGregory - I do agree with Jonathan's point about the order of phrases, i.e. it would be more comprehensible if horizontal_gradient was written first.

There is also no unit reference within the P06 vocabulary collection I could find relating to your specified units mg / m3 / km (in SI/canonical units: kg / m3 / m).
Would you be able to provide any reference for these units to help understand their dimensions?

Best wishes,
Ellie

[efisher008]

Hi Aurore @abiardeau,

I hope your summer is going well.
Now that your other batch of proposed standard names have been accepted, would you have any time to dedicate to progressing this proposal? It would be great to see it moving towards acceptance.

Best,
Ellie

[abiardeau]

Hello both,

I got some answers to Jonathan questions from the expert I am in contact with :

• I think that this order of phrases is hard to parse. Do you mean horizontal_gradient_of_mass_concentration_of_chlorophyll_a_in_sea_water?

Yes I think we are talking about the scalar magnitude of the horizontal gradient

• What does "horizontal gradient" mean? According to the [guidelines](https://cfconventions.org/Data/cf-standard-names/docs/guidelines.html), if is the magnitude of a vector gradient, it should be something like magnitude_of_horizontal_gradient. If it's a vector, we need components such as eastward_derivative and northward_derivative. Since your title says Norm of the horizontal gradient, I guess you mean the former. In either case, something should be said in the definition about this.

According to the title "Norm of the horizontal gradient" we are not providing the vector quantity but rather a scalar field.

I will ask their opinion for your question about the units as well.

Thanks very much,
Aurore

[JonathanGregory]

Dear Aurore

By "norm", then, do you mean the magnitude_of_horizontal_gradient i.e. |∇ X|, where X is scalar?

Best wishes

Jonathan

[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

[abiardeau]

Dear all,

I just got feedback from the expert I am in contact with about this standard_name.
He confirms that we are talking about magnitude of horizontal gradient, and proposes :

magnitude_of_horizontal_gradient_of_mass_concentration_of_chlorophyll_a_in_sea_water

with this explanation concerning the units :

Regarding the units, we express them that way based on the units of CHL-a, which we provide in mg/m3. The distance dimension (as they are gradients in space) is expressed in km as that is the scale at which the variable has meaning, it is the order of magnitude of the pixel size.

For the exact implementation of the units field, we have proposed “milligram m-3 km-1” following the field provided for the CHL-a variables, it also works for the udunits2 package.

What do you think ?
Aurore

[JonathanGregory]

Dear Aurore @abiardeau

I think that standard name is fine, and correct for what you want. Its canonical unit should be kg m-4, since we always use SI base units for the canonical units, but it's fine to put units="milligram m-3 km-1" in the netCDF file, since they are dimensionally equivalent.

You have: milligram m-3 km-1
You want: kg m-4
    1 milligram m-3 km-1 = 1e-09 (kg m-4)
    x/(kg m-4) = 1e-09*(x/(milligram m-3 km-1))

Best wishes

Jonathan

[abiardeau]

Thanks Jonathan !
We will proceed with this.
Kind regards,
Aurore

[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

[efisher008]

Dear Aurore @abiardeau

It looks as though this name has reached consensus and the units for the term in the editor have been changed to kg m-4. Once the period for further comments/issues has passed, this will be accepted and published in the next standard names update (scheduled for mid-November).

The only question I have is for @JonathanGregory - there is no P06 term for the canonical unit kg m-4. How would you suggest to retrieve the NVS ID from the P06 vocab for this quantity?

Best wishes,
Ellie

[JonathanGregory]

Dear Ellie

There is no P06 term for the canonical unit kg m-4. How would you suggest to retrieve the NVS ID from the P06 vocab for this quantity?

I'm sorry, I don't know. I don't think that CF should be constrained by the capabilities of the NERC vocabulary server, though. Such questions are being discussed in https://github.com/orgs/cf-convention/discussions/366.

Best wishes

Jonathan

[feggleton]

@efisher008 if the unit isn't in the P06 vocabulary you can request for it to be added by Gwen. If it is valid and should be in the units vocab.

[efisher008]

Thank you Fran @feggleton. I'll get in touch with Gwen about the new P06 unit.