Dear All, I wholeheartedly support the use of '1' for Practical Salinity.
I can see the pragmatic sense in Rich's suggestion, but fear that specialist physical oceanographers might object as they see g/kg as a 'special' unit for absolute salinity. I'll check with the chair of TEOS-10 and report back. Cheers, Roy. ________________________________________ From: alison.pamm...@stfc.ac.uk [alison.pamm...@stfc.ac.uk] Sent: 02 June 2015 17:22 To: ngalbra...@whoi.edu; cf-metadata@cgd.ucar.edu Subject: Re: [CF-metadata] Salinity units Dear Nan, All, Reading through what you and Rich saying i.e. that the values are (and probably always have been) in the range 0-40, then I agree that changing the canonical unit is not likely to cause problems with existing data, which was my main concern. In fact, I think using "1" does make more sense than 1 e-3 if practical salinity is a number on a relative scale rather than having any direct relation to concentration. So I now support this change. I will need to put some more thought into the definitions - I'm a little pushed for time today but will have another think about this tomorrow. Best wishes, Alison ------ Alison Pamment Tel: +44 1235 778065 NCAS/Centre for Environmental Data Archival Email: alison.pamm...@stfc.ac.uk STFC Rutherford Appleton Laboratory R25, 2.22 Harwell Oxford, Didcot, OX11 0QX, U.K. > -----Original Message----- > From: CF-metadata [mailto:cf-metadata-boun...@cgd.ucar.edu] On Behalf > Of Nan Galbraith > Sent: 02 June 2015 15:47 > To: cf-metadata@cgd.ucar.edu > Subject: Re: [CF-metadata] Salinity units > > Hi All - > > Can we move on this question? > > I think the real problems with the unit .001, as mentioned by various > people > elsewhere in this thread, are that it invites users to treat it as a > scale factor, > or to mistake it for g/kg. > > Would it be possible to change canonical units to '1' and to add to the > definitions > a statement that the 'units' field can't be used to convey a scale > factor, that > scaling of data has to be done in another field (is it scale_factor?) > for dimensionless > variables? I think this would protect existing datasets from > misinterpretation - > basically making '.001' equivalent to '1' for these data variable. > > It seems unlikely that there are existing CF datasets that were written > using '1' > as a practical salinity unit, intending it to convey that data should be > divided by > 1000 to return to its non-dimensional state. If there are such datasets > out there, > they presumably have the standard name table version included > somewhere in > their metadata - but it would be very surprising if the unit has been > used this way. > > I don't see any down side to making this change, since it moves CF closer to > the widely-accepted view on PS units, without endangering any existing > data sets. > > OceanSITES is champing at the bit to have this resolved, so I'd hate to > see us > drop the discussion at this point. > > Cheers - and thanks - > Nan > > >> On 5/27/15 1:56 PM, Signell, Richard wrote: > >>> For all these salinity datasets, the actual data values for salinity > >>> are in the range of 0-40, not 0.0-0.040. And I don't think people > >>> want that to change. So the problem is users understanding the > >>> difference between values of 0-40 that *are not* supposed to be used > >>> as "g/kg" and values of 0-40 that *are* supposed to be used as "g/kg". > >>> > >>> On Wed, May 27, 2015 at 1:11 PM,<alison.pamm...@stfc.ac.uk> wrote: > >>>> Dear Nan, All, > >>>> > >>>> Certainly this topic has come up several times and we never seem to > quite > >>>> get to a solution that suits everybody. > >>>> > >>>> I don’t know why 1e-3 was originally chosen for use in the standard > name > >>>> table, but even if you go back to version 1 it is in there, which means > >>>> it > >>>> was agreed prior to 2006 when the CF website at PCMDI was set up. > >>>> > >>>> The last time the question of salinity units was aired in detail was > during > >>>> the TEOS-10 discussions in 2011. Unfortunately, the mailing list archive > >>>> seems to be unavailable at the moment, but I can vouch for the fact > that the > >>>> current definitions of the salinity names came from the very detailed > >>>> discussions that we had at that time. In particular, we added the > following > >>>> wording to the definition of sea_water_practical_salinity: ‘Practical > >>>> Salinity, S_P, is defined on the Practical Salinity Scale of 1978 > >>>> (PSS-78) > >>>> and is calculated from the electrical conductivity of sea water (as well > as > >>>> temperature and pressure). Officially S_P is dimensionless so that, > while > >>>> convenient, and while it is common practice, it is not officially > sanctioned > >>>> to say S_P = 35 psu. Often authors use PSS-78, as in S_P = 35 PSS-78. If > >>>> salinity was measured using remote sensing techniques and not > conductivity, > >>>> then it is recommended that additional metadata > (calibration/validation > >>>> information) be described in the variable comment attribute.’ > >>>> > >>>> Once upon a time (back in 2009) there was a discussion about allowing > CF to > >>>> use ‘psu’ as a unit in its own right, but I think the TEOS-10 discussion > >>>> made it clear that ‘psu’ is not really a unit at all, so that idea was > >>>> dropped and we continued to use 1e-3. > >>>> > >>>> We should remember that the canonical unit of ‘1e-3’ doesn’t prevent > anyone > >>>> using ‘1’ in their files if they prefer it, and vice versa. As Jim has > >>>> already pointed out, UDunits can certainly cope with that. So in one > sense, > >>>> it doesn’t really matter to CF which we choose as the canonical unit as > long > >>>> as we can agree and, most importantly, make the definition really > really > >>>> clear so that consumers of the files know how to interpret the data. > >>>> > >>>> In previous discussions there has never been unanimous agreement > about > >>>> whether it is better to use ‘1e-3’ or ‘1’. My niggling concern about > >>>> changing the unit after all these years is whether it will lead to > >>>> misinterpretation of existing data files. Is that going to be a problem? > We > >>>> have in the past changed the canonical units of standard names, but > only to > >>>> correct outright errors, rather than to change the interpretation of a > name. > >>>> How big a problem is it for the oceanographic community if we don’t > change > >>>> the unit? > >>>> > >>>> If we do decide to go with ‘1’ as the canonical unit, is there a > >>>> reference, > >>>> such as TEOS-10, which we can use to support our decision? It would be > >>>> useful to include it in the definition and hopefully reduce the need to > keep > >>>> revisiting this same question. > >>>> > >>>> Either way, I think we can improve further on the definition to help > people > >>>> better understand the data. > >>>> > >>>> Best wishes, > >>>> > >>>> Alison > >>>> > >>>> > >>>> > >>>> > >>>> From: CF-metadata [mailto:cf-metadata-boun...@cgd.ucar.edu] On > Behalf Of Nan > >>>> Galbraith > >>>> Sent: 27 May 2015 15:45 > >>>> To: Rich Pawlowicz > >>>> Cc:cf-metadata@cgd.ucar.edu > >>>> > >>>> > >>>> Subject: Re: [CF-metadata] Salinity units > >>>> > >>>> Hi all - > >>>> > >>>> The '.001' units for P.S. doesn't mean that stored values of practical > >>>> salinity differs from A.S. by 'a factor of around a 1000', as far as I > >>>> know. If that's the logical inference, then this unit is really a > >>>> problem, > >>>> and maybe we should do something about it. > >>>> > >>>> I wish my CF email archive went back a little further, because there's > >>>> nothing (since 2004) that I can find that explains the rationale for > >>>> this unit. It certainly *looks* like a compromise between a unit for a > >>>> non-dimensional variable and PPT ... When this was originally under > >>>> discussion, way back when, I'll bet someone argued that it would > eventually > >>>> be a big problem. I'd really love to see that email thread! > >>>> > >>>> Cheers - > >>>> Nan > >>>> > >>>> > >>>> On 5/26/15 11:52 AM, Rich Pawlowicz wrote: > >>>> > >>>> > >>>> > >>>> I’m not sure what the best answer is either, but I think the “correct” > way > >>>> is > >>>> to have people deal with Practical Salinity in some special fashion in > >>>> their workflow, because it *is* defined in a weird way that is generally > >>>> incompatible with the general idea of ‘quantities with units’) - getting > >>>> a salinity definition that is aligned with the way all other quantities > >>>> in the world are defined was one of the motivating factors behind > TEOS-10! > >>>> > >>>> So, essentially people would have to make their own choice about > >>>> what to do with ‘practical salinity’ for whatever they are doing. > >>>> > >>>> I will point out, though, that having two kinds of data that differ > >>>> numerically by a factor of around a 1000 is a good way of getting > >>>> them to realize that they really are not exactly compatible - you > >>>> wouldn’t *want* Practical Salinity and Absolute salinity on the same > >>>> plot (“look - salinity increased by 0.16 g/kg everywhere in 2010!”) > >>>> > >>>> But I understand that one might want to make this as painless as > >>>> possible. > >>>> > >>>> > >>>> On May 26, 2015, at 8:48 AM, Signell, Richard<rsign...@usgs.gov> > wrote: > >>>> > >>>> > >>>> Rich, > >>>> Thanks for this. Yes, I guess my concern is that folks will do a > >>>> catalog search for *salinity* variables, and with a few spot checks, > >>>> see that they are have data values in the range of 29-36 or so, and > >>>> then go ahead and run a workflow that converts all units using the > >>>> units attribute. And if "practical salinity" has units of "1" and > >>>> "absolute salinity" has units of "g/kg" = "0.001", then the data might > >>>> not appear on that fixed y-axis plot with [29 36]. But I don't > >>>> have a good alternative. I guess we have to rely that people will > >>>> realize from the standard_names that for comparison, you need to > >>>> estimate absolute salinity from practical salinity using tools like > >>>> GSW toolbox. > >>>> > >>>> -Rich > >>>> > >>>> On Fri, May 22, 2015 at 3:59 PM, Rich > Pawlowicz<rpawlow...@eos.ubc.ca> > >>>> wrote: > >>>> > >>>> > >>>> Ummm…I’m not entirely what you are asking, but > >>>> > >>>> a) PSS-78 Practical Salinity is a dimensionless number. It was defined > >>>> such that "the numerical values of practical salinity would be similar to > >>>> the > >>>> numerical values of previous salinity data, when expressed in ‰”, but > >>>> it isn’t in fact ppt or anything, and you shouldn’t be multiplying it up > >>>> or > >>>> down by factors of 1000. > >>>> > >>>> b) "Previous salinity data”, (Cox or Knudsen salinity) which > >>>> was obtained from titrations, does in fact represent a > >>>> mass fraction of something (because you are titrating > >>>> with a mass of silver). This was denoted by the ppt ‘unit'. > >>>> > >>>> c) TEOS-10 Absolute Salinity is also a mass fraction (of dissolved solute > >>>> on the Reference Composition Salinity Scale). However, nowadays the > >>>> SI brochure suggests that different quantities should be distinguished > >>>> by their symbols, not their units. So, there isn’t actually a > recommended > >>>> unit for Absolute Salinity. You can write > >>>> > >>>> S_A = 35 g/kg = 0.035 kg/kg = 35000 mg/kg > >>>> > >>>> or, again using SI rules and treating the units as a ‘thing’: > >>>> > >>>> S_A/(g/kg) = 35 > >>>> > >>>> and any of these are valid - the same way lengths can be in > >>>> meters or km or mm or whatever is handy (this is also > >>>> true for preformed salinity). > >>>> > >>>> ‘ppt’ is discouraged as a unit of mass fraction because (for example) it > >>>> could be confused with ‘part per trillion’ > >>>> > >>>> > >>>> Now, the gsw toolbox assumes ‘g/kg’ for its TEOS-10 salinity inputs > >>>> and outputs, but YOU don’t have to do that if you don’t want to. > >>>> > >>>> I admit it is a little magic how we can ESTIMATE Absolute Salinity (with > >>>> units) from Practical Salinity (without units), but keep in mind that > >>>> this > >>>> is only ONE possible way of estimating Absolute Salinity, and in fact it > is > >>>> a method that is metrologically somewhat suspect because of the > >>>> definition of PSS-78. S_A could also be obtained from density > >>>> measurements, for example - and then there is some other > >>>> conversion factor involving different units. > >>>> > >>>> Rich. > >>>> > >>>> > >>>> On May 22, 2015, at 1:01 PM, Signell, Richard<rsign...@usgs.gov> > wrote: > >>>> > >>>> > >>>> Roy, > >>>> > >>>> For sure dimensionless. But "1.0", "0.001" or "g/kg"? > >>>> > >>>> The latest version (27) of the CF Standard Name list > >>>> (http://cfconventions.org/Data/cf-standard-names/27/build/cf- > standard-name-table.html) > >>>> states: > >>>> > >>>> sea_water_salinity: "0.001" > >>>> sea_water_absolute_salinity: "g kg-1" > >>>> sea_water_practical_salinity: "0.001" > >>>> sea_water_preformed_salinity: "g kg-1" > >>>> sea_water_cox_salinity: "0.001" > >>>> > >>>> and units packages, of course, would treat "g kg-1" the same as "0.001". > >>>> > >>>> Yet in the IOC manual on equation of seawater: > >>>> http://www.teos-10.org/pubs/TEOS-10_Manual.pdf > >>>> it states (PDF page 176, printed page 166) that Practical Salinity > >>>> should have units of "1", while "Absolute Salinity" (the argument used > >>>> in the toolbox functions) and "Preformed Salinity" (used in numerical > >>>> ocean models) should have units "g kg-1". > >>>> > >>>> So it appears that TEOS agrees with CF on units for Absolute Salinity > >>>> and Preformed Salinity, but not on Practical Salinity. > >>>> > >>>> And OceanSites (as least here: > >>>> http://www.oceansites.org/docs/OS_PAP-3_201205_P_deepTS.txt) > >>>> is using "sea_water_practical_salinity" with units of "1", so they are > >>>> consistent with the TEOS publication, but not the current CF > >>>> convention (v27). > >>>> > >>>> On the TEOS site, there is software to calculate Absolute Salinity > >>>> from Practical Salinity. So it would seem that the technically > >>>> correct thing to do would be to use the "gsw_SA_from_SP" routine to > >>>> convert OceanSites Practical Salinity (in units of "1") to Absolute > >>>> Salinity (in units of "g/kg") before comparing with the "Preformed > >>>> Salinity" output "g/kg" from ocean models. > >>>> > >>>> I'm pretty confused though, so I'm cc'ing Rich Pawlowicz on this, > >>>> hoping for his input. > >>>> > >>>> Thanks, > >>>> -Rich > >>>> > >>>> > >>>> > >>>> On Fri, May 22, 2015 at 1:49 PM, Lowry, Roy K.<r...@bodc.ac.uk> wrote: > >>>> > >>>> Dimensionless. Please???? > >>>> > >>>> This is the view of physical oceanographers for whom I have the > greatest > >>>> respect. > >>>> > >>>> Cheers, Roy. > >>>> ________________________________________ > >>>> From: Reyna Jenkyns [re...@uvic.ca] > >>>> Sent: 22 May 2015 18:06 > >>>> To:cf-metadata@cgd.ucar.edu; OceanSITES Data Management Team; > Nan Galbraith > >>>> Subject: Re: [CF-metadata] Salinity units > >>>> > >>>> I'm interested in this topic since I didn't realize what had been > discussed > >>>> previously, and now I think we must be non-compliant as well. Is this > >>>> documented formally in the CF documentation? > >>>> > >>>> Reyna Jenkyns | Data Stewardship Team Lead - Digital Infrastructure > >>>> Ocean Networks Canada | T 250 853 3908 | oceannetworks.ca > >>>> University of Victoria PO Box 1700 STN CSC 2300 McKenzie Avenue > Victoria, BC > >>>> V8W 2Y2 > >>>> > >>>> ________________________________________ > >>>> From: CF-metadata<cf-metadata-boun...@cgd.ucar.edu> on behalf of > Nan > >>>> Galbraith<ngalbra...@whoi.edu> > >>>> Sent: Friday, May 22, 2015 10:03 AM > >>>> To:cf-metadata@cgd.ucar.edu; OceanSITES Data Management Team > >>>> Subject: Re: [CF-metadata] Salinity units > >>>> > >>>> Hello all - > >>>> > >>>> It's been a long time, but is anyone interested in re-visiting the > >>>> subject > >>>> of units for practical salinity in CF? > >>>> > >>>> I was recently notified that my salinity data was likely to be > >>>> overlooked by > >>>> some users, because I'd used '1' as the units, not '.001'. Somehow, I'd > >>>> forgotten the (long-ago) discussion on the CF list about salinity units. > >>>> > >>>> Some members of the OceanSITES project are interested in revising > our > >>>> format spec to encourage the use of '1' as an indication that salinity > does > >>>> not have units - but, of course, we'd mostly rather remain CF- > compliant. > >>>> > >>>> Thanks for any feedback on this. > >>>> > >>>> Cheers - Nan > >>>> > >>>> > >>>> On 6/17/09 2:48 AM, Lowry, Roy K wrote: > >>>> > >>>> > >>>> Dear All, > >>>> > >>>> During an exercise with Alison mapping the CF Standard Names to a > >>>> units vocabulary in the BODC vocabulary server I noticed that the > >>>> units for salinity were '1.00E-03', i.e. parts per thousand. My > >>>> understanding in that since the introduction of the Practical > >>>> Salinity Scale that salinity is dimensionless with units of '1'. Is > >>>> there agreement for our changing the units in the Standard Name > >>>> table? > >>>> > >>>> Cheers, Roy. > >>>> > >>>> > >>>> > > > -- > ******************************************************* > * Nan Galbraith Information Systems Specialist * > * Upper Ocean Processes Group Mail Stop 29 * > * Woods Hole Oceanographic Institution * > * Woods Hole, MA 02543 (508) 289-2444 * > ******************************************************* > > > _______________________________________________ > CF-metadata mailing list > CF-metadata@cgd.ucar.edu > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata This message (and any attachments) is for the recipient only. 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