Hi,
On 26-11-14 17:52, [email protected] wrote:
Dear Maarten,
Thank you for your proposals. I have now created entries for them in
the standard names editor at
http://cfeditor.ceda.ac.uk/proposals/1?status=active&namefilter=&proposerfilter=Maarten&descfilter=&unitfilter=&yearfilter=&filter+and+display=Filter,
so they certainly will not be lost. (At the moment they appear with
blue headings to indicate that they are new proposals - I wanted to
demonstrate this feature of the editor - but I will shortly change
their status to "Under Discussion"). By proposing your names to the
list they are already "in the process" so now it is a case of
responding to any further comments as they arise. Names are formally
accepted when consensus has been reached on the terms themselves, the
definitions and units. Once accepted, they are automatically included
in the next update of the standard name table.
Thank you.
I have given my detailed comments on the individual proposals and
your additional questions below.
1. toa_incoming_photon_solar_irradiance_per_unit_wavelength
I agree with Jonathan's comment that "solar" is not really necessary
and you have indicated that it is OK to remove it, so we would then
have toa_incoming_photon_irradiance_per_unit_wavelength.
Taking your own definition and adapting the wording slightly for
consistency with existing "toa_incoming" and "irradiance" names I
arrived at the following:
"toa" means top of atmosphere. The TOA incoming photon irradiance is
the photon flux from the sun on a surface perpendicular to the
incoming radiation i.e. the "downwelling" flux. In accordance with
common usage in geophysical disciplines, "flux" implies per unit
area, called "flux density" in physics. A coordinate variable for
radiation wavelength should be given the standard name
radiation_wavelength. A photon flux is specified in terms of numbers
of photons expressed in moles.
Canonical unit: mol m-2 m-1 s-1
This sounds fine to me.
As it stands, this looks fine and could probably be accepted for
inclusion in the standard name table straight away. However, you
included some notes about the coordinate variable and unit which need
some further consideration. I'll address the units issues first, as
these are the easiest to answer!
[...]
It looks as though we would simply need to request an additional
alias of 'photon' for the existing definition of mol/6.02214129e23.
Do you agree?
Yes, this is exactly what I had in mind.
On 12/11/14 Maarten wrote:
Note 4: Other instruments may use other units, depending on the
construction of the instrument. These can not be converted easily
into one another as they require additional knowledge, in
particular knowledge of the wavelength. I'm unsure if UDUnits is
equiped to handle these types of conversion. Additional names may
be needed for using "W m-2 m-1" or "W m-2 m-1 sr-1" respectively
(similar name, but leaving out the 'photon') Another quantity that
is commonly encountered expresses the radiation_wavelength as
wavenumber or frequency, and therefore also alters the
(frequency/energy) interval over which the quantity is integrated.
I'll leave the addition of these quantities to communities that
actually use them, IASI on MetOp comes to mind.
It is accepted CF practice that different units for similar
quantities, where conversion is not simply a matter of scaling,
require different standard names. An existing example would be the
ozone quantities
equivalent_thickness_at_stp_of_atmosphere_ozone_content (canonical
unit of metres) and atmosphere_mole_content_of_ozone (canonical unit
of mol m-2). It is also accepted practice that we do not create new
standard names unless someone actually needs to use them, so I think
you are correct that other communities should propose the additional
names as and when they are required.
Agreed.
On 12/11/14 Maarten wrote:
Note 1: The coordinate variable for radiation wavelength may
require a mapping that is similar to the mapping of geolocation
coordinates for satellite observations, i.e. not a dimension, but a
multi dimensional mapping. This is the case for the OMI instrument,
where a 2D-dectector is used for wavelength (columns of CCD) and a
spatial dimension across the swath (rows of CCD). The mapping is
imperfect, and the wavelength depends on the row-index.
I don't think we have encountered this type of data product before!
The question of how to deal with the coordinate variables should not
necessarily delay agreement of the standard name as coordinates are
more properly in the realm of the main CF Conventions document. If
writing your data will require coordinate variables to be treated in
a new way then it would be necessary to propose a modification to the
conventions as well as proposing the standard name.
I don't think so, but to be sure: adding a reference to a coordinate
variable in a standard name is probably causing a problem somewhere down
the line. It would be fine if instead of coordinate variable, it would
read: "coordinate variable or associated / ancillary / auxiliary
variable". In any case a link that is less strong than a dimension.
Note that I'm not suggesting to alter all standard_names where this may
occur, but it is worth noting that dimensions (and therefore coordinate
variables) are not always obvious.
Certainly the conventions already allow 2D coordinate variables for
use with geospatial grid mappings, but I don't think we have ever had
a situation where one dimension is wavelength and the other a
location. If I understand you correctly, the wavelength of a single
data value also depends on where in the swath the observation was
taken. Is that right? Is this because the instrument is scanning
through a range of wavelengths as the satellite moves along its
track?
Here is the measurement principle:
http://www.knmi.nl/omi/research/instrument/characteristics.php?tag=principle
A CCD is used to record the spectrum, with one dimension for the
wavelengths (CCD columns), and the other for the spatial dimension
across track (CCD rows). Due to the geometry the wavelength registration
differs slightly for each row. There is no scanning, either across track
or through the wavelengths.
If so, I am wondering if it might be possible to get around
this by using a spatial coordinate variable and also a
radiation_wavelength auxiliary coordinate variable on the same
dimension of the data. Would that work?
I think that would work, although we may want to skip the dimension for
the flight direction (along track dimension). Conceptually this sounds
very similar to the situation with a coordinates attribute for satellite
observation swaths.
How many spatial dimensions
are there in your data product?
time (T), ground_pixel (X), scan_line (Y), spectral_channel.
In our product time is a length-1 dimension, with the the offset to the
actual time recorded in an ancillary variable.
ground_pixel is across track, scan_line is the along track dimension.
The spectral_channel is the wavelength direction, but with a (slightly)
different wavelength scale for each ground_pixel. latitude & longitude
are attached via the normal coordinates attribute. Given the orbit (sun
synchronous polar), the axes are as indicated.
I'd welcome input from anyone else
who can help with this issue.
I agree that this is more a CF issue, and I will bring it up in the CF-2
discussions.
2. toa_outgoing_photon_radiance_per_unit_wavelength
Putting together your own definition with existing definitions of
photon_radiance names I arrived at the following:
"toa" means top of atmosphere. Photon radiance is the photon flux in
a particular direction, per unit of solid angle. The direction from
which it is coming must be specified, for instance with a coordinate
of zenith_angle. If the radiation does not depend on direction, a
standard name of isotropic radiance should be chosen instead. A
photon flux is specified in terms of numbers of photons expressed in
moles. In accordance with common usage in geophysical disciplines,
"flux" implies per unit area, called "flux density" in physics. A
coordinate variable for radiation wavelength should be given the
standard name radiation_wavelength.
Canonical unit: mol m-2 m-1 s-1 sr-1
Is this OK?
Yes, with a note that the CF conventions may need to say something about
attaching a viewing geometry (solar zenith angle, instrument zenith
angle, azimuth angles) to a variable that needs this.
My earlier comments on units and coordinate variables apply to this
name also. If necessary, we can start a new thread on the mailing
list to discuss the coordinate issue more thoroughly.
Some discussion here never hurts, but I think adding this to CF-2 will
provide a little more room for a neat solution. Opinions are very
welcome to get it right in CF-2.
Best,
Maarten Sneep
--
KNMI
T: 030 2206747
E: [email protected]
R: A2.14
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