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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Received on Tue Jun 02 2015 - 10:22:55 BST
