Dear All,
There is just one remaining CMIP5 output quantity for which there is no
existing standard name. The quantity is a 3D field of atmospheric
extinction coefficient at 550 nm due to ambient aerosol. The quantity
has been requested by Michael Shulz and Olivier Boucher. It may be
calculated at each model level as the extinction optical thickness due
to the aerosol contained in that layer alone divided by the thickness of
the model layer. The units are m-1. Some models may do the calculation
the other way round and compute extinction optical thickness by
multiplying the aerosol mass concentration by a mass extinction
coefficient and integrating over the depth of the layer.
The proposed name is
volume_extinction_coefficient_in_air_due_to_ambient_aerosol; m-1.
There is a question regarding the meaning of "extinction" as opposed to
"attenuation" because it has been suggested that their equivalence
should be noted in the explanation of the name. However, it is not
entirely clear whether the two really are equivalent and advice from
radiation experts would be most welcome. In the following I have copied
parts of the preliminary email discussion that have given rise to this
question.
Initially, I suggested that the standard name for this quantity should
be
volume_optical_attenuation_coefficient_in_air_due_to_ambient_aerosol
by analogy with the existing standard names
volume_attenuation_coefficient_of_downwelling_radiative_flux_in_sea_wate
r volume_beam_attenuation_coefficient_of_radiative_flux_in_sea_water.
The explanations of both the existing names contain the sentence
"Attenuation is sometimes called extinction". Also, all the
optical_thickness name explanations refer to path integrals of "volume
scattering/absorption/attenuation" coefficients.
However, Olivier wrote that:
> you're right that "attenuation coefficient" is the terminology in
electromagnetic
> theory (and maybe it is used by the ocean people). However although we
use the term
> attenuation in Beer's law or in expression like "attenuated
backscatter signal" it is
>very rarely used as "attenuation coefficient" in atmospheric optics.
The standard term
> is "extinction coefficient" or "mass/volume extinction coefficient"
and I would prefer
> if we stick to this with a note that it is sometimes called
"attenuation coefficient"
> (even though attenuation is sometimes taken to mean just absorption"
when for
> aerosols/molecules scattering is usually more important than
absorption...).
Michael expressed agreement with Olivier and suggested that we call the
name
volume_extinction_coefficient_in_air_due_to_ambient_aerosol
and include Olivier's note in the explanation.
Karl Taylor is also content to use "extinction" in the name, but has
queried the distinction between "attenuation" and "extinction" because
it is important that the standard name explanation be correct:
> Do these coefficient apply to both direct beam and scattered
radiation? And I think it
> needs to be
> clear how this differs from attenuation. In the case of direct beam
radiation:
>
> 1. I thought attenuation was the fraction absorbed+back-scattered (but
attenuation didn't
> include what was scattered forward, as well the portion of the direct
beam radiation that
> passed through unaffected).
>
> 2. I thought extinction was only the fraction absorbed.
>
Olivier responded to this by saying:
> Extinction is scattering plus absorption. There is no ambiguity on
this.
>
> Attenuation refers to the direct beam so includes forward scattering
as well.
Karl wrote:
> O.K., great, then use of extinction is unambiguous but before noting
its possible
> equivalence with attenuation, we should decide whether it truly is or
if "attenuation is
> sometimes taken to mean just absorption".
Olivier replied:
> For me attenuation means the same as extinction but wikipedia says it
sometimes means only
> absorption. That could be sloppy language when the medium is not
scattering much, just guessing.
Please can anyone advise on whether attenuation and extinction are one
and the same?
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
NCAS/British Atmospheric Data Centre Fax: +44 1235 446314
Rutherford Appleton Laboratory Email: alison.pamment at stfc.ac.uk
Chilton, Didcot, OX11 0QX, U.K.
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Received on Wed Nov 10 2010 - 00:46:23 GMT
