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
> <mailto: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 <mailto: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
>>> <mailto: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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