Dear Philip,
Thank you, this is about what I meant in my email this morning to
Jonathan, but much much much better explained.
Christiane
Philip J. Cameronsmith1 schrieb:
>
> 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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--
======================================================================
Christiane Textor
Laboratoire des Sciences du Climat et de l'Environnement
Unite Mixte de Recherche CEA-CNRS-UVSQ
LSCE/IPSL, CEA Saclay, Orme des Merisiers,
Bat. 701, Piece 3b, Point Courrier 129
F-91191 Gif-sur-Yvette Cedex
FRANCE
mailto: christiane.textor at lsce.ipsl.fr
Tel ++33 1 69 08 34 07 Fax ++33 1 69 08 77 16
GEOmon scientific coordinator http://www.geomon.eu
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Received on Thu Nov 06 2008 - 13:03:20 GMT
