[CF-metadata] Salinity units

Nan Galbraith ngalbraith at whoi.edu
Tue Jun 2 14:01:15 MDT 2015

I think  "sea_surface_salinity" is practical salinity at the surface - and
should be dimensionless, or have units of '1'.  There are probably many
other versions of PSAL in the table, I'm just a little pressed for time 
now and can't find them all.

So I think we need to find all the salinities that are not identified as
absolute salinity and give them canonical units of '1'.  The definition
of preformed salinity includes ' Preformed Salinity is Absolute Salinity'
so I guess it keeps its units.

Thanks - Nan

. On 6/2/15 12:27 PM, Signell, Richard wrote:
> 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.pamment at 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.pamment at 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-bounces at cgd.ucar.edu] On Behalf
>>> Of Nan Galbraith
>>> Sent: 02 June 2015 15:47
>>> To: cf-metadata at 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.pamment at 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-bounces at cgd.ucar.edu] On
>>> Behalf Of Nan
>>>>>>> Galbraith
>>>>>>> Sent: 27 May 2015 15:45
>>>>>>> To: Rich Pawlowicz
>>>>>>> Cc:cf-metadata at 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<rsignell at 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<rpawlowicz at 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<rsignell at 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.<rkl at 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 [reyna at uvic.ca]
>>>>>>> Sent: 22 May 2015 18:06
>>>>>>> To:cf-metadata at 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-bounces at cgd.ucar.edu>  on behalf of
>>> Nan
>>>>>>> Galbraith<ngalbraith at whoi.edu>
>>>>>>> Sent: Friday, May 22, 2015 10:03 AM
>>>>>>> To:cf-metadata at 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 *
>>> *******************************************************
>>> _______________________________________________
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>> _______________________________________________
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* Nan Galbraith        Information Systems Specialist *
* Upper Ocean Processes Group            Mail Stop 29 *
* Woods Hole Oceanographic Institution                *
* Woods Hole, MA 02543                 (508) 289-2444 *

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