Jonathan - thanks for the comments.
I would like to propose an amendment to the names, i.e. to replace
"reflectance" with "albedo".
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
and
toa_Lambertian_equivalent_albdeo_multiplied_by_cosine_solar_zenith_angle
Currently, there is no very clear definition of planetary albedo or
other related quantities in CF standard names
with which to follow. The definitions I proposed will need to be
modified to coincide with the name change
from reflectance to albedo.
Ted
Jonathan Gregory wrote:
Dear Ted
Thanks for your proposals. I have a few comments and questions.
* What is the difference between TOA reflectance and planetary albedo (the
latter is an existing standard name)?
* solar_zenith_angle is an existing std name, so that would be preferable
to sun_angle.
* "normalized" is a vague word. It means different things in various contexts.
Although I expect this is the usual technical term, for a CF standard name
it would be good to say more explicitly what it means here e.g.
multiplied_by_cosine_solar_zenith_angle
!
Cheers
Jonathan
----- Forwarded message from Ted Kennelly <[email protected]> -----
Date: Tue, 7 May 2013 13:47:33 -0400
From: Ted Kennelly <[email protected]>
User-Agent: Thunderbird 2.0.0.24 (Macintosh/20100228)
To: "[email protected]" <[email protected]>
Subject: [CF-metadata] New reflectance standard names
I would like to propose the following standard names:
?toa_lambertian_equivalent_reflectance?
?Reflectance is the ratio of the energy of the reflected to the
incident radiation. 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.?
units = 1
?toa_lambertian_equivalent_reflectance_normalized_by_sun_angle?
?Reflectance is the ratio of the energy of the reflected to the
incident radiation. 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. ?normalized_by_sun_angle? means that
the quantity has been multiplied by the cosine of the solar zenith
angle of the incident solar radiation.?
units = 1
Rationale: these are key product of the GOES-R program that are used
for downstream L2 processing and as the end-product in the imagery
product.
On GOES-R, the imagery product is referred to as the "reflectance
factor". It is equal to the "reflectence" times the cosine of the
solar zenith angle such that the signal drops off in the terminator
region as does the observed L1b radiance product. The "reflectance"
and "reflectance factor" are computed assuming diffuse reflection
off a Lambertian surface and represent the signal measured at the
top of the atmosphere. The existing CF standard name
"toa_bidirectional_reflectance" has not been adopted because it 1)
implies a directional dependence of the radiation and 2) does not
address the need for the normalized quantity. The term "lambertian
equivalent reflectance" or sometimes just "equivalent reflectance"
has been used on certain programs to specify assumptions made in the
calculation. In particular the assumption of a Lambertian surface
results in a factor of PI in the formula: rho = PI * L/ mu / E,
where L is the observed radiance, E is the solar irradiance, and mu
is the cosine of the solar zenith angle. This is
critical information for the users of the product.
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