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 > > > > ------ > > 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. > > > > > > 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 > -- Dr. Richard P. Signell (508) 457-2229 USGS, 384 Woods Hole Rd. Woods Hole, MA 02543-1598 _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata