[CF-metadata] mixing ratio

From: Philip J. Cameronsmith1 <cameronsmith1>
Date: Thu, 6 Nov 2008 11:37:59 -0800 (PST)

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
------------------------------------------------------------------------
Received on Thu Nov 06 2008 - 12:37:59 GMT