HI Nan and all,
I would suggest that for CF, the term albedo should be limited to its
stricter traditional definition of applying to the full spectrum of
solar radiation (in which case we would not suggest that a coordinate
variable be used to specify a radiation wavelength or frequency). If
you want to indicate the the fraction reflected radiation for a subset
of angles or wavelengths, there are in physics better terms to use (see
http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Reflectivity.html )
Here are some standard names that we might want to include in CF, which
would cover various cases described at the above URL:
reflectivity (a function of wavelength and angle)
hemispherical_reflectivity (a function of wavelength band)
hemispherical_spectral_reflectivity (a function of wavelength)
total_hemispherical_reflectivity (for our purposes this would be
redundant with albedo, I think)
I suggest "hemispherical_reflectivity" would be a good choice when
specifying a wavelength range.
best regards,
Karl
On 6/21/13 10:38 AM, Nan Galbraith wrote:
Hi Karl -
I've only used it the way you describe, but the definition that was
originally
proposed said 'A coordinate variable can be used to specify the
radiation wavelength or frequency.'
I checked some on line definitions, and that seems to be correct. E.g.
wikipedia
says 'When quoted unqualified, it usually refers to some appropriate
average
across the spectrum of visible light.' I didn't find any definitions
that ruled
out the use of a specific frequency range.
My change was just to clarify that when a frequency is not specified,
we mean
the 'appropriate average'.
Nan
On 6/19/13 5:18 PM, Karl Taylor wrote:
Dear Nan,
I'm not positive about this, but I thought albedo always referred to
the total fraction reflected (integrating over all shortwave
wavelengths and all angles). If you want to limit it to only a
portion of the shortwave spectrum, I think a different term
(reflectivity? reflectance?) should be used.
Karl
On 6/19/13 8:32 AM, Nan Galbraith wrote:
Can we also update the definition for surface_albedo now? It's
currently ' The surface
called "surface" means the lower boundary of the atmosphere.' which
doesn't quite
capture the concept.
We use a (constant, inferred) albedo in our surface flux
calculations, and I'm
planning to include it as a variable when I start publishing those
files in CF.
I'd like to see one detail added to the definitions below, that the
outgoing energy in
the ratio is reflected - not that I can think of another source of
outgoing energy off
the top of my head ...
For example, the ESR glossary says: 'Albedo is the fraction of solar
energy (shortwave
radiation) reflected from the earth back into space. It is a measure
of the reflectivity
of the earth's surface' - so we could use something like
'Albedo is the ratio of outgoing reflected energy to incoming energy
(irradiance).'
Also, should we add the default frequency, for cases where no coordinate
is supplied?
'A coordinate variable can be used to specify the radiation wavelength
or frequency; the default frequency is anappropriateaverage across
the spectrum of visible light.'
(Note, the 'appropriate average' term is used in several descriptions)
Thanks -
Nan
On 5/18/13 10:42 AM, Jonathan Gregory wrote:
Dear Ted
That looks fine to me. It'd be interesting to know if anyone else has comments.
Thanks
Jonathan
----- Forwarded message from Ted Kennelly<[email protected]> -----
Date: Mon, 13 May 2013 18:01:03 -0400
From: Ted Kennelly<[email protected]>
To: Jonathan Gregory<[email protected]>
Subject: Re: [CF-metadata] New reflectance standard names
Below are the updated descriptions for the two proposed standard
names. Note CF does not seem to have a definition for albedo though
it is used
in a handful of standard_names.
-ted
toa_lambertian_equivalent_albedo
Albedo is the ratio of the outgoing to the incoming power per unit
area (irradiance). A coordinate variable can be used to specify the
radiation wavelength or frequency. toa means top of atmosphere.
lambertian_equivalent means the quantity is evaluated for a
diffusely reflecting surface.
canonical units = 1
toa_lambertian_equivalent_albedo_multiplied_by_cosine_solar_zenith_angle
Albedo is the ratio of the outgoing to the incoming power per unit
area (irradiance). A coordinate variable can be used to specify the
radiation wavelength or frequency. toa means top of atmosphere.
lambertian_equivalent means the quantity is evaluated for a
diffusely reflecting surface.
multiplied_by_cosine_solar_zenith_angle means that the quantity
has normalized to remove the angular dependence of the incoming
shortwave irradiance
canonical units = 1
Jonathan Gregory wrote:
Dear Ted
Thanks for this explanation:
Reflectance is the ratio of outgoing radiance to incoming irradiance
giving it units of per steradians.
Albedo is the ratio of outgoing irradiance to incoming irradiance
and is unitless.
Because the proposed quantity is unitless, albedo is the better term.
However, albedo is the integral over all angles of the bidirectional
reflectance. In general, true
planetary albedo would account for this angular dependence. To make clear the
assumption regarding the Lambertian surface I still propose using the term
Lambertian equivalent in the name. Thus:
toa_lambertian_equivalent_albedo
toa_lambertian_equivalent_albdeo_multiplied_by_cosine_solar_zenith_angle
OK, with small L in lambertian as you had before (because stdnames are all
in lower case). Thanks!
Cheers
Jonathan
--
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