[CF-metadata] mixing ratio

[Philip J. Cameronsmith1]

Hi Jonathan,

I agree that 'water vapor in dry air' initially seems to make no sense. 
But it is particularly useful in chemistry transport models that read in 
meteorological data from a file (an off-line model) to use dry air in the 
denominator for all of the species, and this is just the logical extension 
for water vapor.  Such off-line models generally do not implement moist 
processes prognostically (I only know of one exception, and that comes 
with different challenges).  It is easier and more accurate to transport 
species assuming an unchanging airmass.  To really get into dirty details: 
the unchanging airmass may be dry air, or it may implicitly include some 
unchanging water vapor concentration, but the distinction is usually 
unimportant in practice (because this error is generally small compared to 
concentration differences between different models, and observations).

Best wishes,

      Philip


On Thu, 6 Nov 2008, Jonathan Gregory wrote:

> Dear Alison
>
> Ah, now I see. I found that confusing, though. If I read "fraction of A in B"
> I'd assume that A is a subset of B e.g. I assume that mass fraction of fat
> in cream means fat/cream, not fat/(cream-fat), and mole fraction of nitrogen
> in air means nitrogen/air. If I read "mass fraction of fat in fat-free yoghurt"
> I would be confused in the same way as I was about "mass fraction of water
> vapor in dry air".
>
> I agree that your definition is exactly what humidity mixing ratio means.
> Here's a more explicit statement of what it means:
>  mass_ratio_of_water_vapor_to_dry_air_in_air
> But would it be acceptable to stick with humidity_mixing_ratio and regard it
> as an exceptional name? It does seem like a good idea to avoid "mass mixing
> ratio" in general as it is not consistently used regarding the denominator.
>
>> We already have mole_fraction_of_water_vapor_in_air in the table and we
>> could certainly also introduce mass_fraction_of_water_vapor_in_air
>> which, using the same definitions of A and B as before, would mean
>> simply A/B.
>
> Yes. If we need that quantity, it would be the logical name.
>
>> If we use 'ambient air' instead of just 'air' in this case
>> then we ought really to change all the mass|mole_fraction_of_X_in_air
>> names to be consistent.  That would mean creating 104 aliases.
>> Personally, I'm not convinced of the need for this. ...
>> So I would vote for continuing to use 'air' to mean moist/ambient air
>> and 'dry_air' to mean air excluding water vapor.
>
> OK.
>
> Best wishes
>
> Jonathan
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------------------------------------------------------------------------
Dr Philip Cameron-Smith        Atmospheric, Earth, and Energy Division
pjc at llnl.gov                   Lawrence Livermore National Laboratory
+1 925 4236634                 7000 East Avenue, Livermore, CA94550, USA
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