Dear All, As I suspected, the message below indicates a preference by the physical oceanographers involved in TEOS-10 to use 0.001 for 'older style' salinities. This works for me. All we need to do is to prevent semi-intelligent (dumb?) data aggregation systems doing automatic units harmonisation on salinity and creating a 3 order of magnitude error. Salinity interconversions for data aggregation should be always be done using the relevant algorithm.
Sea surface salinity is a real fly in the ointment because it can either be a model output field (units 0.001) or an observation, which could either be 'older' (pre-78) salinity (units 0.001) or Practical Salinity (units 1). Rich Signell's suggestion of discontinuing usage of sea surface salinity is something I argued for (but lost) a decade ago and it isn't an argument I wish to resurrect because of the risk or re-opening the sea surface temperature can of worms. As most sea surface salinity in CF is likely to be model output I would suggest keeping 0.001 as the Canonical unit and recommending that if it's Practical Salinity then label it as Practical Salinity. Cheers, Roy. -----Original Message----- From: Rich Pawlowicz [mailto:rpawlow...@eos.ubc.ca] Sent: 03 June 2015 00:26 To: Lowry, Roy K. Subject: Re: [CF-metadata] Salinity units Roy - I have to confess I am not entirely sure of the ramifications of making changes or how units are modified (by powers of 10, for example) in the CF system. I would agree with g/kg for units on the TEOS-10 Reference Composition Salinity Scale. I am not sure I would use the same unit for old-style titration-salinities; keeping 0.001 might be best for those RIch. On Jun 2, 2015, at 10:06 AM, Lowry, Roy K. <r...@bodc.ac.uk> wrote: > Dear Rich, > > Do you have any comment on the CF community adopting g/kg as the canonical > unit for all forms of salinity other than Practical Salinity? > > Regards, > > Roy Lowry (oceanography domain CF Standards Names Committee member) > ________________________________________ > From: Signell, Richard [rsign...@usgs.gov] > Sent: 02 June 2015 17:27 > To: Alison Pamment > Cc: CF metadata > Subject: Re: [CF-metadata] Salinity units > > Nan, Alison, and Co: > > I agree also. We should use "1" for "practical_salinity", and in the > comments say that to estimate other salinity variables such as > absolute_salinity, a formula must be used (perhaps such as those > provided in the GSW toolbox). > > And one more thing: I see also that in CF Standard Name table 27 we > have "g kg-1" for "absolute_salinity" and "preformed_salinity", but > "0.001" for "sea_surface_salinity" and many other salinities. While > numerically they are the same, with "0.001" it's not clear whether > this is a percentage by mass or volume. Since nobody measures > salinity as "ml liter-1", we should specify "g kg-1" for all > salinities other than "practical_salinity", right? > > This would be consistent with: http://www.teos-10.org/pubs/TEOS-10_Manual.pdf. > Screenshots here (and attached): > http://screencast.com/t/I3COnk3a > http://screencast.com/t/Zbk6uLJP643 > > > -Rich > > On Tue, Jun 2, 2015 at 12:22 PM, <alison.pamm...@stfc.ac.uk> wrote: >> 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 > > > > -- > Dr. Richard P. Signell (508) 457-2229 > USGS, 384 Woods Hole Rd. > Woods Hole, MA 02543-1598 > > 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. > <6-2-2015 12-23-31 PM.png><6-2-2015 12-24-25 PM.png><ATT00002.c> -- Rich Pawlowicz, Associate Professor, Ocean Dynamics Laboratory Dept. of Earth, Ocean, and Atmospheric Sciences, Univ. of British Columbia 6339 Stores Rd., Vancouver, B.C. CANADA V6T 1Z4 email:r...@eos.ubc.ca web:http://www.eos.ubc.ca/~rich ph: (604) 822-1356 fax:(604) 822-6088 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
