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 *
>> *******************************************************
>>
>>
>> _______________________________________________
>> CF-metadata mailing list
>> CF-metadata at cgd.ucar.edu
>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
> _______________________________________________
> CF-metadata mailing list
> CF-metadata at 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
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