[CF-metadata] Salinity units

Nan Galbraith ngalbraith at whoi.edu
Wed May 27 12:52:26 MDT 2015


Hi Jim -

> So the issue here is that we could have salinity recorded as any one of:
>
>   * arbitrary unitless scale from 0.0 - 1000.0
>   * arbitrary unitless scale from 0.0 - 1.0
>   * parts per thousand
>   * grams per kilogram
>

Limiting the discussion to practical salinity, the answer is ... no.

The 'original' salinity measurements, aka Knudsen salinities, produced by
titration, have units of parts per thousand. Practical salinity, calculated
from conductivity and temperature, do not have units. Absolute salinities,
generated from practical salinity using TEOS-10 software, has units of 
grams
per kilogram.

Funny coincidence, though - all 3 have a range from 0 to 40.

I suppose you could record P.S. as hundredths, so 0 - 4000, but then I think
you'd have to have a scale_factor attribute - or maybe you *could* get away
with using units of .001 ...

Whether we change the canonical units or not, we should probably make it 
clear
that .001 is not to be used as, or interpreted, as a 'scaled version' of 
the udunits
dimensionless unit '1'.

Cheers - Nan

> where by "arbitrary unitless scale" I mean it's a scale that doesn't 
> have a direct linear relationship to parts per thousand or gram per 
> kilogram.
> So, from a UDUNITS standpoint, this is a problem, because it (and 
> ignorant me) happily assumes that units of '0.001', and 'g/kg' are 
> equivalent and differ from '1' by a scale factor of 1000.
>
> Is this right?
>
> Grace and peace,
>
> Jim
>
>
> 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
>>>
>>>
>>>
>>> ------
>>>
>>> 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.
>>>
>>>
>>>
>>>
>>>
>>> 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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