Hi.
I've looked at definitions of terms and such in a few different sources,
and I'd like to suggest these standard names and definitions. I'll
explain reasons after.
solar_irradiance_at_1au (W m-2) defined as:
"Solar" means originating from the Sun. 'Irradiance' means the flux of
radiant energy per unit area normal to the direction of flow of radiant
energy. '1au' means a distance of 1 Astronomical Unit (AU) from the Sun.
The solar irradiance at 1AU is the radiative flux per unit area from the
Sun normal to a vector with a length of 1 AU integrated over all
wavelengths.
solar_irradiance_per_unit_wavelength_at_1au (W m-2 m-1) defined as:
"Solar" means originating from the Sun. 'Irradiance' means the flux of
radiant energy per unit area normal to the direction of flow of radiant
energy. 'Per_unit_wavelength' means '1au' means a distance of 1
Astronomical Unit (AU) from the Sun. The solar irradiance per unit
wavelength at 1AU is the radiative flux per unit area from the Sun
normal to a vector with a length of 1 AU. A coordinate variable for
radiation wavelength should be given the standard name
radiation_wavelength and the bounds of this coordinate variable should
be used to specify the wavelength range(s) to which the data values apply.
Here's why I chose what I did.
* I didn't use 'TOA' because this quantity could easily be measured by
a detector that is not in Earth orbit at all (for example, the
DSCOVR spacecraft could measure this quantity from the L1 point). An
Earth-centric term such as this appears to be inappropriate.
* I used 'solar' because this is radiant energy coming from the Sun.
* I didn't use 'shortwave' because, as Peter stated in a previous
email, their measurements include medium and long wavelengths. Using
this term implies a cutoff that isn't there. Use of 'shortwave' as a
euphemism for 'solar' is discipline-specific and does not
communicate well to other communities.
* I didn't use 'spectral' because it is a less precise term than
'per_unit_wavelength', and this is quite consistent with the
existing body of standard names. Particularly, a measurement taken
per unit wavelength and one taken per unit frequency have different
values and different units, yet both could be considered to be
spectral measurements.
* I used 'irradiance' because we have quite a few terms that use
'radiance'. I don't find any imprecision couched in these terms, and
both are in wide scientific use, so why should we shy away from
them? I particularly don't see any reason why we should use
'radiance' and not 'irradiance'.
* I used 'at_1au' because this normalizing distance is a key part of
the measurements, and it doesn't carry the same level of
Earth-centric implication as using an orbit description does. The
sphere at 1 AU from the Sun is the surface at which this measurement
applies. The measurement could also be taken for a sun in a
different solar system that had no planet at that distance.
It's true that the standard names aren't primarily intended as search
terms. They are more intended for precise definitions of quantities and
how they fit into the 'taxonomy of quantities'. At the same time, I
think we need to be open to allowing valid, non-Earth-centric standard
names. We also need to be careful that we don't make the names
unnecessarily obscure.
Grace and peace,
Jim
On 5/13/15 8:58 AM, Peter Pilewskie wrote:
On 5/13/2015 6:45 AM, Jim Biard wrote:
Hi.
You guys have done a lot of good work, and I don't want to slow
things down, but I would appreciate it if one of the proposers could
explain why the distance from the Sun is significant to this standard
name,
Most users of the data prefer that solar irradiance is referenced to
the standard distance of 1 AU rather than have orbital variations in
the data. In the rare case where someone wants solar irradiance on a
specific day, it is simple to adjust by the inverse of distance
squared. We know this from many years serving the community this data.
This includes climate modelers, radiative transfer specialists, users
interested in calculating reaction rates, renewable energy
specialists, and many others. Users of the data must know whether the
data has been "detrended" for orbital variability. It is about 6%,
more than an order of magnitude larger than solar cycle variability.
and whether or not the word 'shortwave' is appropriate for the data
quantity they are describing.
I covered this in the previous message: "shortwave" covers only part
of the spectral range. TSI covers all wavelengths. 'Shortwave' is
inappropriate.
Peter
Grace and peace,
Jim
On 5/13/15 5:59 AM, [email protected] wrote:
Dear Odele, Judith, Peter et al,
Thank you for your two proposals which have now been added to the list of CF standard names under
discussion:http://cfeditor.ceda.ac.uk/proposals/1?status=active&namefilter=&proposerfilter=Odele&descfilter=&unitfilter=&yearfilter=&filter+and+display=Filter.
TSI:
You proposed toa_total_solar_irradiance (Wm-2) defined as:
' The solar power per unit area integrated over all wavelengths that is
incident at the top of the Earth's atmosphere (TOA), at a standard distance of
one Astronomical Unit (1 AU) from the Sun, in units of Watts per square meter.'
I agree with John Graybeal's comment that the units should not form part of the standard
name definition, but rather should be listed as "canonical units" in the
standard name table. This is standard practice within CF.
Jonathan Gregory has pointed out that we do have the existing standard name
toa_incoming_shortwave_flux (W m-2) defined as:
' "shortwave" means shortwave radiation. "toa" means top of atmosphere. The TOA incoming shortwave flux is
the radiative flux from the sun i.e. the "downwelling" TOA shortwave flux. In accordance with common usage in
geophysical disciplines, "flux" implies per unit area, called "flux density" in physics.'
The definition of the existing name does not mention a standardized distance of
1AU between Earth and sun, and if this consideration is important then I think
you probably do need a separate name for what is essentially a theoretical
quantity.
In climate modelling the term "shortwave" is commonly used to mean solar radiation and indeed this is the convention used in many
existing CF standard names. (It is probably worth adding a sentence to the definition of all shortwave names to make that point clear). I
agree with Jonathan that there is no need to use the word "total" in the name itself because the quantity would be assumed to
represent all "shortwave" wavelengths unless otherwise specified. I also agree with Jonathan regarding the use of
"irradiance" as a term. Many existing longwave and shortwave standard names contain the following text in their definition: 'When
thought of as being incident on a surface, a radiative flux is sometimes called "irradiance",' but the term
"irradiance" does not actually appear in the names themselves.
Taking all these points into consideration, I suggest the following standard
name:
toa_incoming_shortwave_flux_assuming_circular_orbit (Wm-2) defined as:
' "shortwave" means shortwave radiation. "toa" means top of atmosphere. The quantity with standard name
toa_incoming_shortwave_flux_assuming_circular_orbit is the radiative flux from the sun, i.e. the downwelling TOA shortwave flux
assuming that the earth is in a circular orbit around the sun with radius 1 AU. 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. In accordance with common usage in geophysical disciplines, "flux" implies
per unit area, called "flux density" in physics. When thought of as being incident on a surface, a radiative flux is
sometimes called "irradiance".'
I appreciate that the name itself looks rather different from the original
proposal, but I think it is consistent with existing standard names and clearly
distinct from the quantity that does not specify earth-sun distance. I'd
welcome comments on this idea.
SSI:
You proposed toa_solar_spectral_irradiance (W m-2 m-1) defined as:
'The solar power per unit area per unit wavelength that is incident at the top
of the Earth's atmosphere, at a standard distance of one Astronomical Unit (1
AU) from the Sun, in units of Watts per square meter per nanometer (1 nm = 10
^-9 m).'
Again the canonical units should be separated out from the definition and they
should indeed be W m-2 m-1 in the standard name table. This would still allow
you to use W m-2 nm-1 in your data files because the UDunits software which CF
specifies for unit conversion would be able to convert easily between the two.
As Jonathan pointed out, we no longer use the word "spectral" in standard names but rather
"per_unit_wavelength" or "per_unit_wavenumber". (We could also add "per_unit_frequency"
if it is ever needed).
We do have another proposal for a similar standard name:
toa_solar_irradiance_per_unit_wavelength (W m-2 m-1), currently defined as:
' "toa" means top of atmosphere. "solar" indicates contributions from the sun.
Irradiance is the radiant power per unit area incident at a surface.'
(Seehttp://cfeditor.ceda.ac.uk/proposals/1?status=active&namefilter=solar&proposerfilter=Randy&descfilter=&unitfilter=&yearfilter=&filter+and+display=Filter)
but again this proposal makes no mention of a 1AU orbital radius, so I am not
sure that it is really the same quantity that you require.
Taking these points into account and again replacing "solar" with "shortwave"
we arrive at:
toa_incoming_shortwave_flux_per_unit_wavelength_assuming_circular_orbit (W m-2
m-1) defined as:
'"shortwave" means shortwave radiation. "toa" means top of atmosphere. The quantity with standard name
toa_incoming_shortwave_flux_per_unit_wavelength_assuming_circular_orbit is the radiative flux from the sun, i.e. the
downwelling TOA shortwave flux assuming that the earth is in a circular orbit around the sun with radius 1 AU. A coordinate
variable for radiation wavelength should be given the standard name radiation_wavelength and the bounds of this coordinate
variable should be used to specify the wavelength range(s) to which the data values apply. 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. In accordance with common usage in geophysical disciplines,
"flux" implies per unit area, called "flux density" in physics. When thought of as being incident on a
surface, a radiative flux is sometimes called "irradian
ce".'
Again, comments are welcome.
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
NCAS/Centre for Environmental Data Archival Email:[email protected]
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.
-----Original Message-----
From: CF-metadata [mailto:[email protected]] On Behalf
Of Jonathan Gregory
Sent: 06 May 2015 18:27
To:[email protected]
Cc: Odele Coddington;[email protected]; Peter Pilewskie
Subject: Re: [CF-metadata] New standard name requests for TSI and SSI
Dear Odele et al.
Thanks for your proposals.
I'm not sure whether by TOA irradiance you mean the flux incident normal
to
the Earth's surface or parallel to the vector between Sun and Earth. If it is
the former, we already have a standard name for it viz
toa_incoming_shortwave_flux
If it is the latter, does it need "TOA"? In that case it's a quantity in
space, not really do with the Earth itself, just at the right distance.
We don't so far use the word irradiance in standard names (except in the
phrase spherical_irradiance).
I think that "total" should be omitted. It is the usual convention in
standard names that if there is no qualifier the quantity should be
understood
as inclusive. It could be restricted to a range of wavelengths by giving it
a coordinate variable for wavelength.
Finally, we don't use the word "spectral" in standard names. It was replaced
with per_unit_wavelength to be more explicit e.g.
surface_downwelling_radiative_flux_per_unit_wavelength_in_air
Cheers
Jonathan
We are proposing two new standard names for Total Solar Irradiance (TSI)
and Solar Spectral Irradiance (SSI) at the top of the atmosphere. Your
comments on these two proposals would be appreciated.
Regards,
Odele Coddington and Peter Pilewskie (CU-Boulder/LASP) and Judith Lean
(NRL)
standard name:
toa_total_solar_irradiance
definition:
The solar power per unit area integrated over all wavelengths that is
incident at the top of the Earth???s atmosphere (TOA), at a standard
distance
of one Astronomical Unit (1 AU) from the Sun, in units of Watts per square
meter.
canonical units:
W m-2
standard name:
toa_solar_spectral_irradiance
definition:
The solar power per unit area per unit wavelength that is incident at the
top of the Earth???s atmosphere, at a standard distance of one
Astronomical
Unit (1 AU) from the Sun, in units of Watts per square meter per
nanometer
(1 nm = 10 ^???9 m).
canonical units:
W m-2 m-1
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NOAA National Centers for Environmental Information <http://ncdc.noaa.gov/>
/formerly NOAA’s National Climatic Data Center/
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