I am a bit rusty at this, but in the ocean potential temperature and
salinity are preserved in an adiabatic transition, "potential density" is
not because the eq-of-state is non-linear. So potential density is spoken
of in the ocean, but it gets one into trouble because if parcels are moved
far enough to the reference pressure, the relative potential_density of the
two can switch. So I would think potential_temperature is the better choice
of variable for a modeler, and reference_pressure_for_potential_temperature
is in fact an accurate description of the situation even for
potential_density (salinity does not have a reference pressure).
Benno
On Mon, Dec 13, 2010 at 3:11 PM, Karl Taylor <taylor13 at llnl.gov> wrote:
> Dear Jonathan,
>
> I support this proposal, except I'd suggest defining also
> "reference_pressure_for_potential_density". Hopefully, an ocean modeler
> will weigh in on this, but I think potential density may be directly
> calculated in ocean models, in which case it would be odd to assume that one
> should use the reference pressure for temperature in your definition of
> potential_density.
>
> Best regards,
> Karl
>
> On 12/3/10 1:19 PM, Jonathan Gregory wrote:
>
> Dear Karl
>
> I would suggest that we change the definition of potential temperature so that
> it says *by default* the reference pressure is 1e5 Pa, but that the data var
> could also have a size-one coordinate variable or a scalar coordinate variable
> with the standard_name of reference_pressure_for_potential_temperature, which
> I am proposing as an addition to the standard_name table, that would specify
> the reference pressure. This would be backward-compatible because any existing
> use of sea_water_potential_temperature would be with the default reference
> pressure by definition. I agree that the definition should have said 1e5 Pa,
> not sea level pressure, but I am sure that people will have used it as 1e5 Pa
> and not worried about the difference. If anyone had noticed the definition and
> been concerned, they would have queried it before.
>
> The same issue arises for potential density. Is it OK to use a
> reference_pressure_for_potential_temperature
> to define potential density? I think so. It is the temperature which changes;
> the potential density is computed from the potential temperature.
>
> Cheers
>
> Jonathan
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--
Dr. M. Benno Blumenthal benno at iri.columbia.edu
International Research Institute for climate and society
The Earth Institute at Columbia University
Lamont Campus, Palisades NY 10964-8000 (845) 680-4450
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