On 30/09/15 23:33, Jonathan Gregory wrote:
Dear Maarten
ultraviolet_aerosol_index
I've asked several specialists in the field to
come up with a description of this parameter, and they warned: "Here
is my attempt at defining a quantity that is far from clear". The
description does cover what is in the parameter, and the name is
easily found in literature.
OK. In that case I agree we can't make the name self-explanatory. If the
definition in the standard_name table could give an online literature ref
for how it is computed, that would help.
I think that
DOI:10.1029/98JD00900, O. Torres, P. K. Bhartia, J. R. Herman, Z. Ahmad and J.
Gleason, Derivation of aerosol properties from satellite measurements of
backscattered ultraviolet radiation: Theoretical basis, JGR, 103, D14, 1998, pp.
17099-17110.
is quite suitable (and close to the source), although there are more recent articles
that explain the index more clearly.
There is one issue left: the two wavelengths used in the derivation of the 'residue'
(older, even more vague name than ultraviolet_aerosol_index) must somehow be attached
to the variable. We can stipulate that the anciliary_variables link to a variable
with standard_name radiation_wavelength to indicate these two wavelengths. I think
something similar have been done before.
Name: cloud_area_fraction_assuming_fixed_cloud_albedo
Yes, you do change the resulting cloud fraction if you assume a
different cloud albedo. :-)
It is frequently called an unhelpful 'effective cloud fraction'.
many algorithms have taken a different approach, fix the cloud
albedo to a fairly high value (0.8) and fit just two parameters:
cloud_area_fraction_assuming_fixed_cloud_albedo and
air_pressure_at_cloud_optical_centroid. Obviously if you assume a
lower cloud albedo you will end up with a higher cloud fraction,
most likely at a different pressure level.
Thanks for the explanation. I'm feeling uncomfortable about it because it's not
a real cloud area fraction, just an "effective" one to get the right answer.
By contrast, radiative fluxes assuming clear sky, for instance, are really the
output of radiative transfer calculations, with special conditions as input. I
agree that "effective" by itself is not informative, but we do use that word
in effective_radius for cloud particles, where it has the same purpose as in
your case. Would you mind including it here as well?
effective_cloud_area_fraction_assuming_fixed_cloud_albedo
There is an existing standard name with "fixed" in it in this sense. If you
specify the value in a coordinate or scalar coordinate variable, I suggest
that it could have the existing standard_name of cloud_albedo, rather than
defining a new one.
This fitting is also the result of radiative transfer calculations (usually through a
lookup table). I have no objection against the use of 'effective'.
I somehow forget about scalar variables, using attributes for closely related
parameters seems to imply a stronger connection - is this something to address in
CF-2.0? But, yes, it is a standard mechanism.
1 is the only one we're interested in.
There is a clear_sky label that is frequently used, but I haven't
come across the opposite. What do you suggest?
cloud_albedo_assuming_full_cloud_cover
Yes, that would make sense, I think, or it could be assuming_complete_cloud_
cover, or maybe assuming_completely_cloudy_sky, with is more like assuming_
clear_sky - what do you think? I expect that it will be useful in future to
have a phrase for this, which we currently lack, as you say.
I'm not a native speaker, so I don't think I'm the one to pass judgement on matters
of the English language. "assuming_complete_cloud_cover" has my slight preference.
Best,
Maarten Sneep
--
KNMI
T: 030 2206747
E: [email protected]
R: A2.14
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