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.pamm...@stfc.ac.uk Chilton, Didcot, OX11 0QX, U.K. -- Scanned by iCritical. _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
