Harmonize the description of several standard names for various types of precipitation

[larsbarring]

Some time ago, when looking at cf-convention/vocabularies#67, I discovered inconsistent and/or rather too brief description(s) of several standard names for various types precipitation. Hence I think that it would be useful to make a concerted effort to harmonise these.

problem	standard name	description	canonical unit
too brief	convective_rainfall_amount	"Amount" means mass per unit area.	kg m-2
too brief	convective_rainfall_flux	In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics.	kg m-2 s-1
too brief	convective_rainfall_rate	No help available	m s-1
too brief	rainfall_amount	"Amount" means mass per unit area.	kg m-2
too brief	rainfall_flux	In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics.	kg m-2 s-1
too brief	rainfall_rate	No help available	m s-1
term mixup	stratiform_graupel_ flux Alias: large_scale_graupel_ flux	In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. Stratiform precipitation, whether liquid or frozen, is precipitation that formed in stratiform cloud. Graupel consists of heavily rimed snow particles, often called snow pellets; often indistinguishable from very small soft hail except when the size convention that hail must have a diameter greater than 5 mm is adopted. Reference: American Meteorological Society Glossary http://glossary.ametsoc.org/wiki/Graupel. There are also separate standard names for hail. Standard names for "graupel_and_hail" should be used to describe data produced by models that do not distinguish between hail and graupel.	kg m-2 s-1
term mixup (#159)*()**	stratiform_rainfall_amount Alias: large_scale_rainfall_amount	Stratiform precipitation, whether liquid or frozen, is precipitation that formed in stratiform cloud. "Amount" means mass per unit area.	kg m-2
term mixup	stratiform_rainfall_flux Alias: large_scale_rainfall_flux	In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. Stratiform precipitation, whether liquid or frozen, is precipitation that formed in stratiform cloud.	kg m-2 s-1
term mixup	stratiform_rainfall_rate Alias: large_scale_rainfall_rate	Stratiform precipitation, whether liquid or frozen, is precipitation that formed in stratiform cloud.	m s-1
term mixup	stratiform_snowfall_amVocabulariesount Alias: large_scale_snowfall_amount	Stratiform precipitation, whether liquid or frozen, is precipitation that formed in stratiform cloud. "Amount" means mass per unit area.	kg m-2
term mixup	stratiform_snowfall_flux Alias: large_scale_snowfall_flux	In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. Stratiform precipitation, whether liquid or frozen, is precipitation that formed in stratiform cloud.	kg m-2 s-1

This table only include standard names where I think that there is room for improvements and/or clarifications. Related to these 12 ones there are 43 other standard names from which useful boilerplate pieces of text can be extracted, or otherwise used as inspiration. Thus I believe the work involved is not overwhelming to improve the description of these 12 standard names.

In addition to these standard names in need of better descriptions, there is one standard name that I believe is wrong or at least inconsistent:

standard name	description	canonical unit
convective_precipitation_rate	"Precipitation rate" means the depth or thickness of the layer formed by precipitation per unit time. Convective precipitation is that produced by the convection schemes in an atmosphere model. "Precipitation" in the earth's atmosphere means precipitation of water in all phases.	m s-1	convective_precipitation_rate is the sum of convective_rainfall_rate and what (and the three non-existing convectivesnow| hail | _graupel_fall_rate)? This would be inconsistent because of what the height represents. This quantity is better represented by the already existing lwe_convective_precipitation_rate

The problem with this is that convective_precipitation_rate (supposedly, and in line with other standard names) is the sum of convective_rainfall_rate and what (and the three non-existing convective_snow_| _hail_ | _graupel_fall_rate)? This would be inconsistent because of what the height represents.

Thus I suggest that convective_precipitation_rate is deprecated in favour of lwe_convective_precipitation_rate.

The full table of the standard names is attached as an .xls spreadsheet file: Precip_Standard_Names_gh.xlsx

*() EDIT:** The issue reference is not correct, either it was wrong in the first place, or it somehow got mangled when moving to the new repo. Probably the correct issue reference should now be #67.

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[larsbarring]

ping @japamment and @ethanrd for inclusion as a hackaton theme in the schedule for the CF2023 workshop.

[efisher008]

Hello @ethanrd, @japamment,

Was there more offline discussion on this topic in the 2023 CF workshop in October?

Best wishes, Ellie

[larsbarring]

Yes, thanks for reminding me. I will update with some notes from our breakout conversation.

[efisher008]

Hello @larsbarring,

Yes, thanks for reminding me. I will update with some notes from our breakout conversation.

Would you be able to update the post with notes from the conversation at the CF workshop, please? It looks like the last edit to the original post was October and it would be useful to have any more discussion recorded. Thank you!

Best wishes, Ellie

[efisher008]

Dear Lars @larsbarring,

I am revisiting this issue while looking through the currently open issues in the new cf-conventions/vocabularies repo. I realise this was created last year and unfortunately never progressed beyond the CF workshop 2023. Do you still feel it's worth pursuing? Perhaps we could revive the conversation in time for the CF workshop 2024 this September.

Best regards, Ellie

[larsbarring]

Dear Ellie @efisher008,

I have not been thinking about this for quite some time. But in general I think that all the different variants of precipitation standard names -- due to the difference in physical quantities and their units, in combination with the various types of hydrometeors -- should be carefully crafted and explained/defined to help users unambiguously find the correct standard name for their data.

But since back then I come to understand that much of what is in the first table is related to the term database of the cf-editor. As I have no insight into this I am not sure that I can add much, except help out with comments and suggestions if someone would be willing and have time to take this on.

Regarding convective_precipitation_rate, which i believe should be deprecated, it might be handled as a separate issue.

Cheers, Lars

[efisher008]

Hi Lars @larsbarring,

Thank you for your reply. I agree that it would be valuable to clarify the description, and in some cases, restructure, these precipitation standard names to ensure there is a) no overlap in meaning b), no ambiguity in their interpretation, and c) ease of findability for the correct name to use.

I have commented in #67 that the subject of that issue (clarifying the description of the term rain/rainfall in various standard names, especially that of stratiform_rainfall_rate) could be transferred to the prospective changes/improvements being proposed in this issue. We shall see what the decision is there.

To be clear, when you refer to the "term database of the cf-editor", are you talking about the CF phrasebank, where terms used in standard name descriptions are defined for future use? It could be worth updating some descriptions to make terms clearer/less ambiguous if this is desired.

Best wishes, Ellie

[larsbarring]

Yes, the CF phrasebank it is.

[taylor13]

I am not sure this is backward-compatible, but I think "rain" should be reserved for liquid water only and "frozen water" should refer to all non-liquid droplets (snow, hail, etc.). "precipitation" would be the sum of falling rain+frozen water.

We could also consider distinguishing between "rainfall" and "rain", with the former reserved to references to the rain at the surface, but I don't know what the comparable terms would be for precipitation and frozen water.

[larsbarring]

Yes, I agree with you first point, which fits nicely with #59. Regarding your second point, isn't there both "snowfall" and "hailfall" that could be used in analogy with "rainfall"? Or do the latter two more clearly (than "rainfall") relate to the process of falling rather than accumulation on the ground. I never heard of "graupelfall", although I imagine that we could create the term if ever needed. EDIT: There are already standard names containing hail_fall and graupel_fall (see the table attached to the first post) so that is already taken care of.

[JonathanGregory]

I agree with Lars that convective_precipitation_rate ought to be deprecated, because of the mixture of phase. lwe_convective_precipitation_rate is much to be preferred.

I agree with Karl that rain is liquid water and frozen_water is all forms of solid water. I devised some of these standard names, a long time ago, and tried to be consistent in using rainfall for rain when thinking of it as arriving at the surface, with rain for thinking of it as constituent of the air. There is a similar distinction between snowfall and snow, and also a distinction between snowfall and surface_snow. The latter is useful because "snow amount" can refer to either (falling or lying).

[taylor13]

I'm happy with the convention that "??fall" refers to "??" arriving at the surface. In the case of "precipitation", is that already implied to be precipitation arriving at the surface, or would we need (for consistency) a standard name "precipitation_fall". If precipitation already means surface precipitation, what is the name for precipitation passing through some level in the atmosphere?

[JonathanGregory]

We haven't made a similar distinction for precipitation. For example, mass_fraction_of_precipitation_in_air refers to particles of condensed water of any form that are in the air, and on the way down, I suppose, while mass_fraction_of_solid_precipitation_falling_onto_surface_snow refers to precipitation arriving at the ground. I think we don't strictly need the distinction between rain and rainfall, but since English has this distinction, we can try to use those words consistently.

[taylor13]

I've made a relevant comment here: https://github.com/cf-convention/vocabularies/issues/67#issuecomment-2278882455

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