Nan,
I think most archived values of "sea_surface_salinity" are practical
salinity, not absolute salinity. How about if we leave
"sea_surface_salinity" as "0.001" (or actually IMHO should be "g/kg")
and deprecate the use of "sea_surface_salinity" in favor of the newer
concepts of "absolute salinity", "preformed_salinity" or "practical
salinity".
>From www.teos-10.org:
"Importantly, while Absolute Salinity (g/kg) is the salinity variable
that is needed in order to calculate density and other seawater
properties, the salinity which should be archived in national data
bases continues to be the measured salinity variable, Practical
Salinity (PSS-78). To avoid confusion while the use of Practical
Salinity in scientific publications is phased out, published values of
salinity should be specifically identified as being either Practical
Salinity with the symbol SP or Absolute Salinity with the symbol SA."
-Rich
On Tue, Jun 2, 2015 at 4:01 PM, Nan Galbraith <ngalbraith at whoi.edu> wrote:
> 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
> right
> 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 *
> *******************************************************
>
>
> _______________________________________________
> CF-metadata mailing list
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>
> _______________________________________________
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> CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
>
>
>
> --
> *******************************************************
> * Nan Galbraith Information Systems Specialist *
> * Upper Ocean Processes Group Mail Stop 29 *
> * Woods Hole Oceanographic Institution *
> * Woods Hole, MA 02543 (508) 289-2444 *
> *******************************************************
>
>
--
Dr. Richard P. Signell (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598
Received on Tue Jun 02 2015 - 14:14:46 BST
