[CF-metadata] New standard_name values for some cloud and aerosol related variables

From: Jonathan Gregory <j.m.gregory>
Date: Wed, 30 Sep 2015 14:50:38 +0100

Dear Maarten

Thanks for your new standard name proposals. I have various comments.

> Name: uv_aerosol_index
>
> Description: UV means ultraviolet radiation. The UV-Aerosol Index is
> a UV colour index that represents the deviation of the outgoing TOA
> radiation in the UV from that of a standard atmosphere, featuring
> Rayleigh scattering and gas absorption, in particular by ozone. No
> cloud droplets or suspended liquid or solid particles (aerosols) are
> present in the standard atmosphere. It is bounded below by a
> Lambertian surface, featuring isotropic reflection, assumed
> independent of wavelength. The UV-AI is computed from the Earth
> reflectances at two UV wavelengths. A positive deviation from the
> standard atmosphere is often, but not exclusively, attributed to the
> absorption of radiation by aerosols, while negative values represent
> increased scattering, not necessarily by aerosols.

>From the definition, I don't properly understand what this quantity is, in
physical terms. Is it possible to describe it more precisely and, if so, would
it be possible to give it a standard_name which conveyed that meaning? This
is not essential. As you know, we have added some other quantities to the
table whose names are opaque, such as atmosphere_stability_total_totals_index.
That quantity is used in practice with that name and is hard to describe as a
general-purpose geophysical quantity. But if a physical description is
possible, it seems preferable to me.

A smaller note: we haven't used "uv" before. It is well-known, of course,
but spelling it out as ultraviolet might be better,

> Name: cloud_optical_centroid_pressure
>
> Description: The cloud optical centriod pressure is a pressure level
> inside the cloud, near the optical thickness center of the cloud.

I think this should be air_pressure, to be precise, and I would suggest
air_pressure_at_cloud_optical_centroid, by analogy with air_pressure_at_
cloud_base and various other air_pressure_at_X names.

> Name: cloud_area_fraction_assuming_fixed_cloud_albedo

> Description: "X_area_fraction" means the fraction of horizontal area
> occupied by X. "X_area" means the horizontal area occupied by X
> within the grid cell or the satellite footprint. Cloud area fraction
> is also called "cloud amount" and "cloud cover". The cloud area
> fraction is for the whole atmosphere column, as seen from the
> surface or the top of the atmosphere. Cloud area fraction assuming
> fixed cloud albedo is the cloud area fraction by assuming the clouds
> in the X_area having the same fixed cloud albedo value (Y). The
> clouds having cloud_area_fraction_assuming_fixed_cloud_albedo and
> cloud albedo Y yield the same reflectance at TOA as the real clouds
> in the X_area. A phrase assuming_condition indicates that the named
> quantity is the value which would obtain if all aspects of the
> system were unaltered except for the assumption of the circumstances
> specified by the condition. The assumed albedo shall be attached to
> the variable either by an attribute 'assumed_cloud_albedo' (if the
> value is independent of geolocation) or through the
> ancillary_variables to a variable with standard_name 'cloud_albedo'.

To me, this name doesn't appear to say what you mean. The name appears to
imply that if you change the albedo of clouds, you change their area. You
mean the cloud fraction of specified albedo which would give the area-average
albedo as the actual clouds do, if I understand correctly. That description
would be a cumbersome standard name. Is this a commonly used quantity which
needs a standard name, and if so, is this what it's normally called?

> Name: cloud_albedo_assuming_cloud_area_fraction_of_1
>
> Description: cloud_albedo_assuming_cloud_area_fraction_of_1 means a
> derived (retrieved) cloud albedo in an area assuming cloud area
> fraction is 1 in this area (the grid cell or satellite footprint). A
> phrase assuming_condition indicates that the named quantity is the
> value which would obtain if all aspects of the system were unaltered
> except for the assumption of the circumstances specified by the
> condition.

Is 1 the only cloud area fraction you're interested in? If so, I think it would
be better to use a phrase in words, meaning the opposite of assuming_clear_sky,
which is used in many standard names to mean cloud area fraction of 0. If you
might want to use arbitrary area fractions, you could use a coordinate variable
of cloud_area_fraction.

> Name: cloud_optical_centroid_pressure_assuming_cloud_area_fraction_of_1

Earlier comments apply.

Best wishes

Jonathan
Received on Wed Sep 30 2015 - 07:50:38 BST