[CF-metadata] mixing ratio

From: Schultz, Martin <m.schultz>
Date: Fri, 7 Nov 2008 08:40:30 +0100

Dear John, Philip and Christiane,

    unless mankind will burn all coal, oil and gas, water vapour is
probably the only constitutent that actually has any noticable impact on
the distinction between "X/(air including X)" and "X/(air not including
X)". To my knowledge, practically *all* models always refer to dry_air
as the denominator. With *all* I mean all models that don't go above
altitudes of ~80 km. In practice, the "dry_air" concept means that a
fixed molecular weight for air (and a constant cv) is used (mw=28.97 g
mole-1 or similar). This doesn't work in or above the mesosphere where
you have mass fractionation and escape of lighter molecules into space.
Once you have to recompute the air mass for every time step and grid box
individually, you can of course just as easily apply the "joghurt"
concept and relate all quantities to "ambient air". For the current and
practical problem of humidity and tracer mass mixing ratios I would
nevertheless strongly advocate to define one common standard name rule.
In my opinion it will be less confusing if users can read the definition
text and find "_in_air relates to dry_air for practical reasons" than if
they are confronted with one term "water_vapour_mixing_ratio" for one
compound and other terms "mass_fraction_of_X_in_air" for everything
else. Perhaps even more important: if you really want to fix the
definition of "air" to mean "ambient air", then all the current chemical
names would have to be changed into "in_dry_air". Of course something
like
"mass_fraction_of_X_in_air_expressed_as_ratio_to_dry_air" may be more
correct, but then we get back into the discussion whether we want
standard names that are usable or whether we define terms that require
an e-book to be read.

    On a more general and "philosophical" point: I am not quite sure if
I always understand the actual target group for the standard names. On
the one hand their purpose is to standardize and harmonize terms for the
community that uses them, on the other hand it is the general
(physically educated) community. Yet, to be honest, I don't believe I
will ever make use of "ocean_tracer_epineutral_biharmonic_diffusivity"
in my scientific lifetime, and I would certainly have to do a bit of
reading before I understand what this means. This might appear different
with the terms used in atmospheric chemistry (because everyone knows
what a joghurt is ;-), but in the end it is actually quite the same in
that the people who want to make use of the standard names will have to
aquire knowledge about their exact meaning and definition. Perhaps this
is a pledge for "keep it simple": I suggest that standard names should
be a reasonable compromise between accuracy and simplicity. We should
definitively avoid jargon and make an effort to come up with defintiions
that are as precise and clear as possible, but it should be recognized
that some prior knowledge might be required before the full meaning of a
term can be understood. This then stresses the need for a
well-maintained set of definitions, and it also re-inforces the idea of
defining "phrases" as general and standard concepts for new standard
names, because then the definition doesn't have to be repeated for each
individual name and can perhaps be more elaborate including for example
a diversion about dry and moist air in the concept of mixing ratios.

Best regards,

Martin



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Received on Fri Nov 07 2008 - 00:40:30 GMT