Dear Jonathan,
thanks for a good and constructive discussion!
Please find my comments inline.
Best regards,
Markus
_______________________________________
Dr. Markus Fiebig
Senior Scientist
Dept. Atmospheric and Climate Research (ATMOS)
Norwegian Institute for Air Research (NILU)
P.O. Box 100
N-2027 Kjeller
Norway
Tel.: +47 6389-8235
Fax : +47 6389-8050
e-mail: Markus.Fiebig at nilu.no
skype: markus.fiebig
-----Original Message-----
From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Jonathan Gregory
Sent: Donnerstag, 17. Oktober 2013 11:05
To: cf-metadata at cgd.ucar.edu
Subject: [CF-metadata] standard names for surface aerosol optical properties
Dear Markus
Thanks for your comments.
> 1)
> "surface_particle_number_concentration_at_stp_in_aerosol (and other similar names). Could this be said more clearly as surface_number_concentration_of_aerosol_in_air_at_stp?
> That would be consistent with existing names e.g.
> number_concentration_of_coarse_mode_ambient_aerosol_in_air"
>
> The standard names I proposed use the term "aerosol" according to its proper textbook definition, i.e. meaning the system of particles and carrier gas. Your wording implies that "aerosol" consists of particles only, which is a common, but colloquial jargon use of the term. I respect the use of "aerosol" in standard names so far, so I worded the proposed names to be backward compatible.
I agree with you about the textbook definition but it appears that existing CF names are not consistent. We have some names with a construction
number_concentration_of_X_aerosol_in_air
and some with
atmosphere_number_content_of_aerosol_particles
The difference between concentration and content is that the first is 3D and the second a vertical integral, so that's not a problem. We could change the first construction in existing names e.g.
number_concentration_of_ambient_aerosol_in_air to
number_concentration_of_X_aerosol_particles_in_air
i.e. insert "particles". Would that be correct? I suppose that in_air is needed because aerosol is not necessarily in air (although the word looks like it should be). It could be any gas. Then for consistency, could you use
number_concentration_of_aerosol_particles_at_stp_in_air
in your new names?
MF: Yes, that would be an option, and probably also a rather consistent and correct one! A few questions to this consensus from my side:
a) I realized that I partly used the "_in_air" qualifier in my proposed standard names and partly not. I don't have any particular preference on using it or not. In the geophysical context, I can't think of any other carrier gas for an aerosol than air, so omitting "_in_air" would probably be ok. On the other hand, we may also keep it in for the sake of precision in wording. Should I consistently use "in_air" in my proposed names or not?
b) In my proposed names for atmospheric aerosol optical properties, I referred only to "aerosol", e.g.
volume_absorption_coefficient_in_air_due_to_dried_aerosol
even though the variable is supposed to refer to the absorption coefficient of the particle phase only. Should I change these names as well, e.g. to
volume_absorption_coefficient_of_dried_aerosol_particles_at_stp_in_air ?
> 2)
> You draw attention to the inclusion of "surface" in the above, but I'm not clear why it's there. Is the measurement actually exactly at the ground? If not, surface should be omitted, and the height indicated by a numerical coordinate, or some other phrase e.g. in_atmosphere_boundary_layer (that one already appears in the stdname table).
>
> The term "surface" is used according to the description given in the
> "Guidelines for Construction of CF Standard Names" at
> http://cf-pcmdi.llnl.gov/documents/cf-standard-names/guidelines
4-5 m above the ground is not really the surface. The constructions surface_ and _at_surface mean the same thing; they're chosen according to which seems to be easier to read in the context concerned. So I'd prefer
> 2) Include in the definition the requirement that the sampling height needs to be included as numerical coordinate.
That's what we do with screen-height temperature, for example. Although it is often called "surface air temperature", its standard name is air_temperature and it has a height coordinate (e.g. 1.5 m or 2.0 m).
MF: Ok, I see the problem. I would in any case swap the "surface_" qualifier with "_at_surface" to avoid confusion, and requiring a height coordinate makes sense anyway. I'm still not quite happy with dropping "_at_surface" altogether. When the standard name is used in a data discovery portal independently of a file, having this information included in the name will be a big help to the user. He will see immediately what's in the file without opening it. You say that "_at_surface" means the exact interface of atmosphere and ground. However, even for models, this definition will depend very much on model resolution. "_at_surface" for a GCM will be very different from "_at_surface" for a microscale boundary layer model. How's that different from my proposed use of "_at_surface? Could I use "_at_surface" AND require stating the sampling height as vertical coordinate?
> 3)
> Clouds do not usually occur at the surface, so "surface" is surprising for CCN.
>
> Not really. The number of cloud condensation nuclei active at a given water vapour supersaturation is independent of the actual existence of a cloud. The instrument measuring this property exposes the aerosol particles to a generated, defined supersaturation, i.e. generates its own "cloud" inside the instrument.
OK, I see.
MF: Solved!
> 4)
> electrical_mobility_particle_diameter. I think the "electrical" here refers to the means of measurement. Usually the CF standard name describes the geophysical quantity itself. Would it be OK to say aerosol_particle_diameter?
>
> The electrical mobility particle diameter is one of many aerosol particle diameters, as opposed to for example the aerodynamic particle diameter (how a particle follows a streamline) or the optical particle diameter (how the particle scatters light). By only saying "aerosol particle diameter", the property is somewhat ill-defined since most aerosol particles aren't spherical. The proposed name avoids this ambiguity.
OK. So it's really part of the definition of a geophysical quantity, not a method of measurement.
MF: Solved!
> 5)
> sizing_relative_humidity. Could "sizing" be omitted? The definition of your standard names can specify what the role of the RH is.
>
> This standard name would be used together with reporting particle size resolved CCN concentrations, i.e. CCN concentrations as function of both, supersaturation and dry particle size. In this context, we need to distinguish between the RH for which the CCN concentration is measured (a few 10ths above 100%), and the RH at which the particle size is selected (usually below or just above 40%). I couldn't find any other way of distinguishing this easily except defining a separate standard name.
I see the need for two distinct standard names. However I wonder if sizing_relative_humidity could be made more self-explanatory somehow, and also whether for the other RH you also need a new and more explicit standard name, in order to make the distinction clearer?
MF: I pondered this one for a good while. I think a separate humidity variable for indicating the humidity of a CCN concentration would add confusion. After all, this "relative_humidity" would be a co-ordinate variable to the CCN concentration or CCN number size distribution, so it's clear immediately what is meant. The "sizing_relative_humidity" would be auxiliary information. I could rename it to "relative_humidity_at aerosol_particle_size_selection", which would be rather self-explaining. Would that be ok?
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Received on Wed Oct 30 2013 - 16:40:21 GMT
