On 30/09/15 15:50, Jonathan Gregory wrote:
> 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.
It isn't a physical quantity (unit = "1"). It indicates a deviation of the
top-of-atmosphere reflectance from the reflectance of a pure Rayleigh atmosphere
including absorbing trace gases (notably Ozone). Such a deviation can be caused by
absorbing aerosols, but the value has many complicating contributions, including the
altitude of the aerosols, the single scattering albedo, presence of clouds, and
probably more. This parameter is however one of a few that can identify some
important classes of aerosols in the presence of clouds, for instance biomass burning
aerosols and volcanic ash.
The parameter has been derived from TOMS up to the present on various instruments, at
first to identify ozone retrievals that were affected by aerosols. 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.
> A smaller note: we haven't used "uv" before. It is well-known, of course,
> but spelling it out as ultraviolet might be better,
That is fine with me, so: ultraviolet_aerosol_index
>> 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.
Ok: air_pressure_at_cloud_optical_centroid.
>> 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?
Yes, you do change the resulting cloud fraction if you assume a different cloud
albedo. :-)
It is frequently called an unhelpful 'effective cloud fraction'.
Background: UV-VIS-NIR space-borne spectrometers (GOME, GOME2, Sciamachy, OMI,
TROPOMI) have observation areas that are large, especially when compared to typical
dedicated cloud instruments (MODIS, VIIRS, Seviri). Think 300x40 km for GOME down to
7x7km for TROPOMI. These instruments are geared towards trace gas retrievals, but
need a matching cloud parametrization to correct the trace gas retrievals for the
presence of clouds. We can't assume a homogeneous scene (like many of the cloud
specific instruments implicitly do). While it is possible to retrieve three cloud
parameters (cloud albedo, cloud fraction and cloud pressure) using a very simplified
cloud model, there are instrumental limitations that make this unstable in practice
(mainly the combination of NIR spectra with a UV signal is tricky, as col-location is
a limiting factor). Instead 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.
The albedo we assume is (much) higher than 'typical' clouds, and the resulting cloud
fraction is much lower than a geometric cloud fraction. But the top of atmosphere
radiance of the _whole_ pixel is accounted for. So it is pixel averaged, not cloud
area averaged. That leads I think to the name I proposed.
>> 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.
No, 1 is the only one we're interested in. To refer back to the cloud model, if we
have a scene with a high surface albedo (i.e. snow or ice) we cannot distinguish
between cloud and surface. In this case we use a different cloud model: assume the
whole scene is cloud-covered, and retrieve both cloud albedo and cloud pressure. For
scenes where we could potentially use the other cloud model, this model is more
sensitive to very light cloud cover, which becomes visible as a scene pressure (see
below) that deviates from the true surface air pressure.
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
>> Name: cloud_optical_centroid_pressure_assuming_cloud_area_fraction_of_1
>
> Earlier comments apply.
Which combined leads me to:
air_pressure_at_cloud_optical_centroid_assuming_full_cloud_cover
Best,
Maarten Sneep
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KNMI
T: 030 2206747
E: maarten.sneep at knmi.nl
R: A2.14
Received on Wed Sep 30 2015 - 09:16:15 BST
