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. NERC is >>> subject to the Freedom of Information Act 2000 and the contents of this >>> email and any reply you make may be disclosed by NERC unless it is exempt >>> from release under the Act. Any material supplied to NERC may be stored in >>> an electronic records management system. >>> _______________________________________________ >>> 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 > > -- > Rich Pawlowicz, Associate Professor, Ocean Dynamics Laboratory > Dept. of Earth, Ocean, and Atmospheric Sciences, University of British > Columbia > 6339 Stores Rd., Vancouver BC CANADA V6T 1Z4 > web: http://www.eos.ubc.ca/~rich tel: 604-822-1356 > > > -- Dr. Richard P. Signell (508) 457-2229 USGS, 384 Woods Hole Rd. 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