Dear Martin, Karl, et al, I'd like to return to the discussion on the definitions of area_fraction names, as I think we were on the points of agreement. Apologies for the delay in getting back to this.
I think we were pretty much agreed on the following: ' "Area fraction" is the fraction of a grid cell's horizontal area that has some characteristic of interest. It is evaluated as the area of interest divided by the grid cell area. It may be expressed as a fraction, a percentage, or any other dimensionless representation of a fraction. To specify which area is quantified by a variable with standard name area_fraction, provide a coordinate variable or scalar coordinate variable with standard name area_type. Alternatively, if one is defined, use a more specific standard name of X_area_fraction for the fraction of horizontal area occupied by X. ' Karl queried what is meant by "or any other dimensionless representation of a fraction" and whether we need that phrase. Martin pointed out that volume fractions can sometimes be expressed as, for example, 1.e-6 (ppm), even if we don't usually do this for area fractions. Following Martin's comment I've had another look at the existing names - we have 36 area_fraction names and 11 volume_fraction names, none of whose definitions currently explain how the fraction should be expressed. This seems like a good opportunity to clarify both sets of names and standardize the wording of the definitions. I suggest therefore that we update the area_fraction names using the wording agreed above, and the volume_fraction names could be updated similarly. For example, volume_fraction_of_clay_in_soil is currently defined only as ' "Volume fraction" is used in the construction volume_fraction_of_X_in_Y, where X is a material constituent of Y' . This could be updated to: ' "Volume fraction" is the fraction of a grid cell's volume that has some characteristic of interest. It is evaluated as the volume of interest divided by the grid cell volume. The phrase "volume_fraction_of_X_in_soil" refers to the volume of a soil model grid cell. It may be expressed as a fraction, a percentage, or any other dimensionless representation of a fraction.' There is one exception to the general pattern of the volume_fraction names, which I suggest should be updated as follows: volume_fraction_of_condensed_water_in_soil_pores ' "Volume_fraction_of_condensed_water_in_soil_pores" is the ratio of the volume of condensed water in soil pores to the volume of the pores themselves. It may be expressed as a fraction, a percentage, or any other dimensionless representation of a fraction. "Condensed water" means liquid and ice.' There is also an existing volume_fraction name that puzzles me: ocean_volume_fraction, currently defined as ' "X_volume_fraction" means the fraction of volume occupied by X.' Do some models contain grid cells that are partly in the atmosphere and partly in the ocean, and this is the fraction of grid cell volume that is beneath the sea surface? Or does it mean the fraction of the water in an ocean column that is contained within a particular grid cell? Or something else? Does anyone know what this name is used for? Best wishes, Alison ----------------------------------------------------------------------------------------------------------- Alison Pamment Tel: +44 1235 778065 NCAS/Centre for Environmental Data Analysis Email: [email protected] STFC Rutherford Appleton Laboratory R25, 2.22 Harwell Oxford, Didcot, OX11 0QX, U.K. -----Original Message----- From: CF-metadata <[email protected]> On Behalf Of Martin Juckes - UKRI STFC Sent: 15 February 2019 09:21 To: Taylor, Karl E. <[email protected]>; [email protected] Subject: Re: [CF-metadata] Putting the units in a CF standard name: area_fraction Hello Karl, "other dimensionless representations" are common in volume fractions, e.g. 1.e-6 (ppm). This is not usually used for area fractions, but it is allowed. regards, Martin ________________________________ From: CF-metadata <[email protected]> on behalf of Taylor, Karl E. <[email protected]> Sent: 12 February 2019 06:08:31 To: [email protected] Subject: Re: [CF-metadata] Putting the units in a CF standard name: area_fraction Hi Alison, Looks good to me. Perhaps Martin can weigh in on whether or not the phrase "or any other dimensionless representation of a fraction" is needed. Are there any such entities? best regards, Karl On 2/11/19 11:14 AM, Alison Pamment - UKRI STFC wrote: > Dear Karl, > > I like that definition - it gives a clear explanation of the purpose of the > name as well as the acceptable ways of expressing the fraction. > > We should also retain the existing text about the use of area_type or more > specific X_area_fraction names to specify *which* area is being quantified. > So then we'd have: > ' "Area fraction" is the fraction of a grid cell's horizontal area that has > some characteristic of interest. It is evaluated as the area of interest > divided by the grid cell area. It may be expressed as a fraction, a > percentage, or any other dimensionless representation of a fraction. To > specify which area is quantified by a variable with standard name > area_fraction, provide a coordinate variable or scalar coordinate variable > with standard name area_type. Alternatively, if one is defined, use a more > specific standard name of X_area_fraction for the fraction of horizontal area > occupied by X. ' > > (Out of curiosity I tried entering k% into UDunits. Not too surprisingly it > responded with "Don't recognize " k%" "). > > Best wishes, > Alison > > ------ > Alison Pamment Tel: +44 1235 778065 > NCAS/Centre for Environmental Data Archival Email: > [email protected] > STFC Rutherford Appleton Laboratory > R25, 2.22 > Harwell Oxford, Didcot, OX11 0QX, U.K. > > > -----Original Message----- > From: CF-metadata <[email protected]> On Behalf Of Taylor, > Karl E. > Sent: 07 February 2019 17:24 > To: [email protected] > Subject: Re: [CF-metadata] Putting the units in a CF standard name: > area_fraction > > HI Martin and all, > > I agree that the best option is to modify the text. In that regard, I > stumbled over the word "proportional" ... proportional to what? Also, only > udunits experts will recognize that "1" has a specific meaning when appearing > as a unit, so "conforms to 1" might be unclear. Would something like the > following be better? > > "Area Fraction" is the fraction of a grid cell's horizontal area that has > some characteristic of interest. It is evaluated as the area of interest > divided by the grid cell area. It may be expressed as a fraction, a > percentage, or any other dimensionless representation of a fraction." > > By the way, off hand I can't think of "other dimensionless representations of > a fraction" Is kilo-percent (k%) legal? > > regards, > Karl > > On 2/7/19 8:57 AM, Martin Juckes - UKRI STFC wrote: >> Dear Jonathan, >> >> Thanks, that justification will be helpful in replying to people. >> >> To summarise, the proposal (now backed by Jonathan and John -- after >> dropping the idea of changing the standard name) is that the current text >> '"Area fraction" means the fraction of horizontal area.' in the description >> of the standard name "area_fraction" should be replaced with the following: >> "Area Fraction" is a dimensionless number representing a relative or >> proportional area. It may be expressed as a fraction, percentage or any >> other unit that conforms to "1". It is evaluated as the area of interest >> divided by the grid cell area, scaled for the units chosen. >> >> regards, >> Martin >> >> ________________________________ >> From: CF-metadata <[email protected]> on behalf of >> Jonathan Gregory <[email protected]> >> Sent: 06 February 2019 21:23 >> To: [email protected] >> Subject: [CF-metadata] Putting the units in a CF standard name: >> area_fraction >> >> Dear Martin >> >> I would say yes, that the use of "fraction" in area_fraction is for >> consistency with all the other uses of "fraction" in standard names >> (mass, mole, time and volume). In addition I would say that "cover" >> would be a confusing word to use, because "land cover" often means >> "land surface type". Finally, I would say to experts who are offended >> that in this case, as in plenty of others where CF has not quite >> followed familiar terminology in the domain, there is no implication >> that anyone thinks they are "wrong" in their terminology. It's just >> that CF is used across a wide range of disciplines and as far as possible >> all of it has to be consistent and intelligible to everyone. >> >> Best wishes >> >> Jonathan >> >> >> ----- Forwarded message from Martin Juckes - UKRI STFC >> <[email protected]> ----- >> >>> Date: Wed, 6 Feb 2019 12:16:06 +0000 >>> From: Martin Juckes - UKRI STFC <[email protected]> >>> To: John Graybeal <[email protected]>, Jim Biard >>> <[email protected]> >>> Cc: "[email protected]" <[email protected]> >>> Subject: Re: [CF-metadata] Putting the units in a CF standard name: >>> area_fraction >>> >>> Hello John, others, >>> >>> >>> Thanks for those comments. I can see the value of maintaining consistency >>> and being careful about changing things which have worked well for a long >>> time, but I would rather not go back to the people who find the existing >>> terminology confusing (these are people who have specifically commented on >>> the standard name area_fraction) and tell them that we are not changing it >>> because it has always been like that. I'd rather have a more positive >>> message that might encourage them to appreciate the value of CF. >>> >>> >>> I'm not sure if this is true, but it looks to me as though the formulation >>> "area_fraction" owes something to "volume_fraction", "mass_fraction" and >>> "mole_fraction", all of which follow wide spread usage in the atmospheric >>> and oceanographic science communities. People who use mass and volume >>> fractions appear to be accustomed to having these expressed as percentages >>> outside CF, so it is no surprise to find this done in CF. For >>> "area_fraction" we have a slightly different situation: the term doesn't >>> arise from expressions used in the land surface science communities, rather >>> it is a semantic structure being imposed on them. Does anyone now if this >>> interpretation is correct (i.e. that we use "area_fraction" rather than >>> something which might be more familiar for land surface scientists such as >>> "area_cover" in order to maintain consistency with mass, volume and mole >>> fractions)? >>> >>> >>> regards, >>> >>> Martin >>> >>> >>> >>> ________________________________ >>> From: CF-metadata <[email protected]> on behalf of >>> John Graybeal <[email protected]> >>> Sent: 01 February 2019 07:12 >>> To: Jim Biard >>> Cc: [email protected] >>> Subject: Re: [CF-metadata] Putting the units in a CF standard name: >>> area_fraction >>> >>> Martin, >>> >>> I like your definition. >>> >>> While there is a case for renaming the standard name, it's long-time use, >>> validity, and the fact only sophisticated data managers use standard names >>> (and most data users just look primarily at variable names) says to me we >>> should keep the existing standard names with fraction. >>> >>> John >>> >>> On Jan 31, 2019, at 08:07, Jim Biard >>> <[email protected]<mailto:[email protected]>> wrote: >>> >>> >>> Hi. >>> >>> I understand that concern, but it has always been true that the units for a >>> quantity identified by a standard name only has to be convertible using >>> UDUNITS from the canonical units specified in the definition for that >>> standard name. So percent is, by definition, valid for a quantity with >>> units of '1'. As you can see below: >>> >>>> udunits2 >>> You have: 1 >>> You want: percent >>> 1 = 100 percent >>> x/percent = 100*(x/) >>> >>> I guess I don't see the need for guidance here. >>> >>> Grace and peace, >>> >>> Jim >>> >>> On 1/31/19 10:51 AM, Martin Juckes - UKRI STFC wrote: >>> >>> Dear Jonathan, >>> >>> >>> we could certainly take that approach, though the definitions are not >>> always accessible to people looking at the standard name, so they do not >>> compensate for ambiguity in the name itself. >>> >>> >>> The current text '"Area fraction" means the fraction of horizontal >>> area.' could be replaced with >>> >>> >>> "Area Fraction" is a dimensionless number representing a relative or >>> proportional area. It may be expressed as a fraction, percentage or any >>> other unit that conforms to "1". It is evaluated as the area of interest >>> divided by the grid cell area, scaled for the units chosen. >>> >>> >>> I still feel that there is a case for changing the name to, for >>> example, "relative_area" in order to reduce confusion caused by >>> people who assume that a fraction is a quantity that does not have >>> units, >>> >>> >>> regards, >>> >>> Martin >>> >>> >>> >>> >>> ________________________________ >>> From: CF-metadata >>> <[email protected]><mailto:[email protected] >>> a r.edu> on behalf of Jonathan Gregory >>> <[email protected]><mailto:[email protected]> >>> Sent: 31 January 2019 13:20:24 >>> To: [email protected]<mailto:[email protected]> >>> Subject: [CF-metadata] Putting the units in a CF standard name: >>> area_fraction >>> >>> Dear Martin >>> >>> I'd rather we retained "fraction" in the standard name, because it's >>> always been there, it's used in other contexts in a consistent way, >>> and there isn't anything actually incorrect with it, as you say. >>> Could we instead add a note to the definitions pointing out that percent is >>> acceptable as a unit for them? >>> >>> Best wishes >>> >>> Jonathan >>> >>> ----- Forwarded message from Martin Juckes - UKRI STFC >>> <[email protected]><mailto:[email protected]> ----- >>> >>> >>> >>> Date: Wed, 30 Jan 2019 22:40:12 +0000 >>> From: Martin Juckes - UKRI STFC >>> <[email protected]><mailto:[email protected]> >>> To: Steven Emmerson <[email protected]><mailto:[email protected]> >>> Cc: "CF-metadata >>> ([email protected]<mailto:[email protected]>)" >>> <[email protected]><mailto:[email protected]> >>> Subject: Re: [CF-metadata] Putting the units in a CF standard name: >>> area_fraction >>> >>> Hi Steve, >>> >>> >>> The issue is more that CF allows more freedom in the choice of units than >>> many people expect from a "fraction". >>> >>> >>> A second problem, I think the problem is that I didn't explain the issue >>> clearly. In the CMIP data request we are specifying that variables with >>> standard name "area_fraction" should be given as percentages. This is >>> allowed by the CF convention: an "area_fraction" can be 0.5 or 50%. The >>> reason that percentages are being used is because "area_fraction" is being >>> used like the proportion of land covered in grass, and people are used to >>> having these as percentages rather than fractions. It is all perfectly >>> correct as far as the convention goes, but people often interpret the use >>> of "area_fraction" for a percentage as an error. >>> >>> >>> Given that we have the framework of allowing flexibility in the choice of >>> units, I feel it would be better to avoid having the term "fraction" in the >>> standard name, given that it is often interpreted as implying a specific >>> choice for the units. >>> >>> >>> regards, >>> >>> Martin >>> >>> >>> ________________________________ >>> From: Steven Emmerson <[email protected]><mailto:[email protected]> >>> Sent: 30 January 2019 21:37 >>> To: Juckes, Martin (STFC,RAL,RALSP) >>> Cc: CF-metadata >>> ([email protected]<mailto:[email protected]>) >>> Subject: Re: [CF-metadata] Putting the units in a CF standard name: >>> area_fraction >>> >>> On Wed, Jan 30, 2019 at 12:54 PM Martin Juckes - UKRI STFC >>> <[email protected]<mailto:[email protected]><mailto:[email protected]><mailto:[email protected]>> >>> wrote: >>> >>> I'm afraid I don't understand your comment. When I search for "fraction" in >>> the NIST document I find it defined as being a ratio, which is inconsistent >>> with the current CF usage. The CF standard name concept "area_fraction" is >>> not what NIST or others understand as a "fraction". I'm suggesting a change >>> to remove this inconsistency. >>> >>> Unless we're talking past one another, I'll have to disagree. The NIST >>> unit for "mass fraction" is "1" -- even though it's a ratio. A fraction can >>> be represented many ways. "1:2", "1/2", and "0.5" all represent the same >>> fraction, for example. >>> >>> Does the CF convention require a particular representation for a fraction? >>> >>> Regards, >>> Steve Emmerson >>> _______________________________________________ >>> CF-metadata mailing list >>> [email protected]<mailto:[email protected]> >>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >>> >>> >>> ----- End forwarded message ----- >>> _______________________________________________ >>> CF-metadata mailing list >>> [email protected]<mailto:[email protected]> >>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >>> _______________________________________________ >>> CF-metadata mailing list >>> [email protected]<mailto:[email protected]> >>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >>> >>> >>> -- >>> [CICS-NC] <http://www.cicsnc.org/> Visit us on >>> Facebook <http://www.facebook.com/cicsnc> Jim Biard >>> Research Scholar >>> Cooperative Institute for Climate and Satellites NC >>> <http://cicsnc.org/> North Carolina State University >>> <http://ncsu.edu/> NOAA National Centers for Environmental >>> Information <http://ncdc.noaa.gov/> formerly NOAA's National >>> Climatic Data Center >>> 151 Patton Ave, Asheville, NC 28801 >>> e: [email protected]<mailto:[email protected]> >>> o: +1 828 271 4900 >>> >>> Connect with us on Facebook for >>> climate<https://www.facebook.com/NOAANCEIclimate> and ocean and >>> geophysics<https://www.facebook.com/NOAANCEIoceangeo> information, and >>> follow us on Twitter at >>> @NOAANCEIclimate<https://twitter.com/NOAANCEIclimate> and >>> @NOAANCEIocngeo<https://twitter.com/NOAANCEIocngeo>. >>> >>> _______________________________________________ >>> CF-metadata mailing list >>> [email protected]<mailto:[email protected]> >>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >>> _______________________________________________ >>> CF-metadata mailing list >>> [email protected] >>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >> ----- End forwarded message ----- >> _______________________________________________ >> CF-metadata mailing list >> [email protected] >> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >> _______________________________________________ >> CF-metadata mailing list >> [email protected] >> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
