Ah...
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
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 *
>>
>> *******************************************************
>>
>>
>>
>>
>>
>>
>> _______________________________________________
>> CF-metadata mailing list
>> CF-metadata at cgd.ucar.edu
>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>
>
>
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Received on Wed May 27 2015 - 12:19:46 BST
