[CF-metadata] Is "psu" a valid cf unit?

alison.pamment at stfc.ac.uk alison.pamment at stfc.ac.uk
Tue Jul 18 07:56:04 MDT 2017


Dear Balaji, Roy, Martin

I agree completely with what Roy is saying - that labelling something as psu or dimensionless in the units string should have no bearing whatsoever on the storage precision of the numerical value.

However, regarding the scaling, the CF standard name table currently lists the units of practical salinity as "1". Up to and including version 28 of the standard name table we had the units of sea_water_practical_salinity and change_over_time_in_sea_water_practical_salinity listed as "1e-3" which is still a dimensionless number, but scaled. There are two things to say about this:

(1)    This change was only made after considerable debate on the CF mailing list in 2015, starting with http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2015/058205.html;

(2)    Although the decision was taken to change the scaling in the canonical units to "1", this does not in any way prevent 1e-3 being used in individual data files, if that helps with storage precision, because the two differ only by a scale factor and are in all other aspects equivalent.



Therefore, I don't see the need to have "psu" as a unit in UDunits. It's a matter for the data provider to choose the scaling in his or her data files. Martin is correct that "psu" is not a unit - it refers to a dimensionless number on a calibrated scale.



Best wishes,

Alison

------
Alison Pamment                                                       Tel: +44 1235 778065
Centre for Environmental Data Analysis         Email: alison.pamment at stfc.ac.uk<mailto:J.A.Pamment at rl.ac.uk>
STFC Rutherford Appleton Laboratory
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Harwell Campus, Didcot, OX11 0QX, U.K.


From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Lowry, Roy K.
Sent: 18 July 2017 14:37
To: V. Balaji - NOAA Affiliate
Cc: cf-metadata at cgd.ucar.edu; Juckes, Martin (STFC,RAL,RALSP)
Subject: Re: [CF-metadata] Is "psu" a valid cf unit?


Dear Balaji,



I think there are some crossed wires here. The dimensionless Practical Salinity and a Practical Salinity in PSU are exact numeric equivalents. The only difference is the name that's given to the unit of measure - that's why to make this crystal clear CF includes the scaling factor of 10^-3. So I don't think that this can affect storage precision.



Cheers, Roy.



Please note that I partially retired on 01/11/2015. I am now only working 7.5 hours a week and can only guarantee e-mail response on Wednesdays, my day in the office. All vocabulary queries should be sent to enquiries at bodc.ac.uk<mailto:enquiries at bodc.ac.uk>. Please also use this e-mail if your requirement is urgent.

________________________________
From: V. Balaji - NOAA Affiliate <v.balaji at noaa.gov<mailto:v.balaji at noaa.gov>>
Sent: 18 July 2017 14:12
To: Lowry, Roy K.
Cc: martin.juckes at stfc.ac.uk<mailto:martin.juckes at stfc.ac.uk>; cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>; d.c.hassell at reading.ac.uk<mailto:d.c.hassell at reading.ac.uk>; r.s.hatcher at reading.ac.uk<mailto:r.s.hatcher at reading.ac.uk>
Subject: Re: [CF-metadata] Is "psu" a valid cf unit?

The ocean modeling community is adamant that they will continue to use PSUs for salinity: it's this unit, rather than its SI or dimensionless equivalent, that gives the maximum digits of precision in a floating-point representation of salinity.

It's a valid concern, and CF should perhaps reconsider, with a new discussion giving weight to the views of modelers as well as physical oceanographers.

Thanks,

Lowry, Roy K. writes:

> Hello Martin,
>
>
> This topic has been debated at length in CF. To cut a long story short, the term 'Practical Salinity Unit' was coined when the 1978 Practical Salinity scale was devised. However, the term fell out of favour with the physical oceanographic community whose current recommended practice is that Practical Salinity - a ratio - should be a dimensionless number.  CF followed this recommendation and so PSU is not a part of CF.
>
>
> Have a dig around in the mailing list archive if you want to find out more.
>
>
> Cheers, Roy.
>
>
> Please note that I partially retired on 01/11/2015. I am now only working 7.5 hours a week and can only guarantee e-mail response on Wednesdays, my day in the office. All vocabulary queries should be sent to enquiries at bodc.ac.uk<mailto:enquiries at bodc.ac.uk>. Please also use this e-mail if your requirement is urgent.
>
>
> ________________________________
> From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu<mailto:cf-metadata-bounces at cgd.ucar.edu>> on behalf of martin.juckes at stfc.ac.uk<mailto:martin.juckes at stfc.ac.uk> <martin.juckes at stfc.ac.uk<mailto:martin.juckes at stfc.ac.uk>>
> Sent: 18 July 2017 13:43
> To: cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>; d.c.hassell at reading.ac.uk<mailto:d.c.hassell at reading.ac.uk>; r.s.hatcher at reading.ac.uk<mailto:r.s.hatcher at reading.ac.uk>
> Subject: [CF-metadata] Is "psu" a valid cf unit?
>
> Hello David, all,
>
> Is "psu" a valus CF unit? It is not in Udunits, but it is added in cf-python as a unit alias and also appears to be accpeted by the cf-checker. I can't see any mention of it in the CF Convention document: the latter only lists level, layer, and sigma_level permitted departures from Udunits,
>
> regards,
> Martin
>
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V. Balaji                               Office:  +1-609-452-6516
Head, Modeling Systems Group, GFDL      Mobile:  +1-917-273-9824
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Bio Dr. V. Balaji is affiliated with Princeton University's Cooperative Institute on Climate Sciences. He has headed the Modeling Systems Group at NOAA's Geophysical ...


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