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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> CF-metadata at cgd.ucar.edu
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>
--
Dr. Richard P. Signell (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598
Received on Wed May 27 2015 - 11:56:48 BST
