Hello All,
After some hunting, I found a copy of a letter from Frank Millero (1993 - "What is PSU") (
http://www.marbef.org/wiki/Salinity_sensors -- box on right) which is adamant that PSU is not a unit and the "Joint Panel on Oceanographic Tables and Standards" has made this clear in their definition of the international equation of state.
There is perhaps a parallel here with "Decibel", which is accepted by cf-python and rejected by Udunits. There has been discussion on unidata pages, the outcome of which, I think, is that Decibel is a reference to a methodolgy used to arrive at a non-dimensional number, not a unit of measure.
The CF Standard Name list now has distinct standard names for salinities defined by different algorithms (e.g. sea_water_practical_salinity, sea_water_cox_salinity).
One compromise might be to add some recognised standard methodological keywords and define a way of placing them in NetCDF files,
regards,
Martin
________________________________
From: Lowry, Roy K. [rkl at bodc.ac.uk]
Sent: 18 July 2017 14:36
To: V. Balaji - NOAA Affiliate
Cc: Juckes, Martin (STFC,RAL,RALSP); cf-metadata at cgd.ucar.edu; d.c.hassell at reading.ac.uk; r.s.hatcher at reading.ac.uk
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. Please also use this e-mail if your requirement is urgent.
________________________________
From: V. Balaji - NOAA Affiliate <v.balaji at noaa.gov>
Sent: 18 July 2017 14:12
To: Lowry, Roy K.
Cc: martin.juckes at stfc.ac.uk; cf-metadata at cgd.ucar.edu; d.c.hassell at reading.ac.uk; 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. Please also use this e-mail if your requirement is urgent.
>
>
> ________________________________
> From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of martin.juckes at stfc.ac.uk <martin.juckes at stfc.ac.uk>
> Sent: 18 July 2017 13:43
> To: cf-metadata at cgd.ucar.edu; d.c.hassell at reading.ac.uk; 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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Received on Tue Jul 18 2017 - 07:50:17 BST
