Nan,
So the problem is more about udunits using 0.001 to mean parts per
thousand than about a fundamental incompatibility between quantities?
Would it be wrong to multiply your fractional value by 1000 to get parts
per thousand? (That's what I would assume should be done if your values
were fractions of 1 and I wanted parts per thousand.)
Don't let me take up too much of your time here. This is just me trying
to understand what the particular problem is as part of my ongoing
education.
Jim
On 5/26/15 10:12 AM, Nan Galbraith wrote:
> Hi Jim -
>
> Not too simplistic, but that 'factor of 1000' is the exact problem.
>
> If I'm using '1' as the unit for practical salinity, and the canonical
> unit is
> '.001', the user (maybe via udunits) will assume the data values
> should be
> divided by 1000 before using. That wasn't my intention, and will probably
> mean my data is discarded.
>
> And yes, I think we'd meant this to only be about practical salinity.
>
> Cheers -
> Nan
>
> On 5/26/15 9:56 AM, Jim Biard wrote:
>> Hi.
>>
>> I checked the operation of udunits2, and I found that it is fine with
>> converting between parts per thousand, fraction of whole, and g/kg.
>> From a simple dimensional analysis perspective they sure look like
>> they are all dimensionless and differ from each other only by a
>> factor of 1000 at the most. Is that too simplistic?
>>
>> Grace and peace,
>>
>> Jim
>>
>> On 5/22/15 4:37 PM, Lowry, Roy K. wrote:
>>> Hi Rich,
>>>
>>> I should clarify that my comment was following on from Nan's and referred only to practical salinity, otherwise known as PSS-78. Absolute salinity has dimensions mass/mass and so is not dimensionless.
>>>
>>> Cheers, Roy.
>>> ________________________________________
>>> From: Signell, Richard [rsignell at usgs.gov]
>>> Sent: 22 May 2015 20:01
>>> Subject: Re: [CF-metadata] Salinity units
>>>
>>> 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
>>>> 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
>>>>
>>>>
>>>> ________________________________________
>>>> 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
>>>> 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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Received on Tue May 26 2015 - 08:43:21 BST
