Fwd: [CF-metadata] units in cf standard names

[Keith Dixon]

Jonathan et al.,

   An aside, perhaps...

   I have been told by a physical oceanographer that "PSU" is no longer 
accepted as a valid unit by most in physical oceanography circles.  The 
argument being that, because salinity is measured as "grams per grams", 
many oceanographers prefer a "unitless" description: "35" rather than 
"35 PSU" or "35‰".

   The anti-PSU comment was part of the feedback I got when asking folks 
around here to do QC checks of our CMORized model output for IPCC AR4. I 
can not judge if it is truly representative (i.e., the physical 
oceanographer that sent me the strong anti-PSU comment may be an 
outlier).  Is the physical oceanography community represented on this 
email list?  If so, perhaps they could help clear up the matter of 
salinity units.

-Keith D.
=======================

Jonathan Gregory wrote:
> Dear all
> 
> I don't think it's right to use the units to indicate the quantity, which is
> what kg N would do. COARDS uses special dimensionless units to indicate
> vertical levels in a similar way, but early on in CF we decided to distinguish
> the identification of a quantity from the units it is measured in. The
> quantity being discussed is (for instance) a mass of nitrogen. The units of
> mass are the same for nitrogen, chalk and cheese.
> 
> PSU (practical salinity units) is an interesting case in Alison's list.
> This is really a dimensionless number (a mass fraction), so we could survive
> without a unit for it, but it seems less clear to say the salinity is 35 1e-3
> than to say it is 35 PSU.
> 
>> Clearly with CF1.0 we are stuck with udunits.
>>
>> However, it's worth thinking about the future. I wasn't involved when
>> the decision to be udunit compatible was made. Can anyone summarise the
>> benefits that requiring udunit conformance confer on the CF community?
> 
> The choice of udunits was made by COARDS. We carry on with it for COARDS
> compatibility. Are there any other similarly comprehensive and flexible
> standards for interpreting units strings? I think the main benefit is to
> provide a general syntax, and secondarily some associated software for parsing
> and converting units.
> 
> Cheers
> 
> Jonathan
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