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 <[email protected]> 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 > _______________________________________________ > CF-metadata mailing > [email protected]http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > -- Dr. M. Benno Blumenthal [email protected] 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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