Hi Jonathan,
From an atmospheric chemistry centric position, I personally would still
prefer to use mass_fraction_of_water_vapor_in_dry_air than
humidity_mixing_ratio, but I appreciate that there are many users of CF
who would prefer humidity_mixing_ratio, and that water vapor could be a
special case. If we do choose humidity_mixing_ratio, would it make sense
to create an alias from mass_fraction_of_water_vapor_in_dry_air, so as to
guide chemists to the 'special case'?
As you say, the definition of the denominator is often ambiguous when
concentration and mixing ratio are reported, and that the distinction is
usually not a concern in the climate community (the toxic release
community is a different story). Hence, with the exception of water_vapor
(as discussed above), I agree with your last two paragraphs.
Best wishes,
Philip
On Thu, 6 Nov 2008, Jonathan Gregory wrote:
Dear Philip and Christiane
Thanks for the explanation. I appreciate that the ratio of constituent mass
to dry air mass is a sensible quantity to use in a model; I am not disagreeing
with that at all. The question is what the standard name should be. I was
confused by the proposal and as Philip also says
'water vapor in dry air' initially seems to make no sense.
It makes sense as a quantity to use, but that name is likely to be baffling to
a non-expert user of the dataset, I would say, and so it doesn't seem like an
ideal choice for a standard name.
This quantity is often called humidity mixing ratio, and that is at present
its standard name. We could leave it as that, for the special case of water
vapour. But what about other constituents? As far as I can see from the
web (but you are the experts) mixing_ratio_of_X_in_air is used variously to
mean X/(air including X) or X/(air not including X) and the air might be dry
or ambient. Since the numbers are only slightly different, maybe this vagueness
is tolerable, or even desirable if you want to compare models that make
different choices but still regard the quantities as comparable. If we want
to be precise, I think we need more explicit terms.
The construction mass_fraction_of_X_in_air seems obviously to mean
X/(air including X and ambient moisture). mass_fraction_of_X_in_dry_air
would be fine for any X except water vapour, and would mean
X/(air including X but not water vapour).
Best wishes
Jonathan
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Dr Philip Cameron-Smith Atmospheric, Earth, and Energy Division
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