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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>>
>>
>>
>> --
>> Dr. Richard P. Signell (508) 457-2229
>> USGS, 384 Woods Hole Rd.
>> Woods Hole, MA 02543-1598
>
> --
> Rich Pawlowicz, Associate Professor, Ocean Dynamics Laboratory
> Dept. of Earth, Ocean, and Atmospheric Sciences, University of British Columbia
> 6339 Stores Rd., Vancouver BC CANADA V6T 1Z4
> web: http://www.eos.ubc.ca/~rich tel: 604-822-1356
>
>
>
--
Dr. Richard P. Signell (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598
Received on Tue May 26 2015 - 08:48:23 BST
