On 25/08/15 19:48, Nan Galbraith wrote:
Thanks, Maarten. Do you want to propose a definition for Albedo from the
TROPOMI Algorithm Team document?
Right now I'm swamped with deadlines, so I'll try.
What I find there is:
Albedo: The ratio: outgoing irradiance / incoming irradiance. This is in
contrast
to the reflectance which is the ratio: outgoing radiance / incoming irradiance.
That doesn't stand alone, in my opinion, and would need to include the TROPOMI
definitions for the supporting terms (or at least for irradiance):
Radiance: radiative energy density in a particular direction.
Irradiance: the integral of the radiance weighted with m over 2p steradians.
There are 2 possible problems with this that I can see. First, Karl said:
the proposed definition of albedo doesn't mention that the photons included in
the
calculation are emitted by the sun. Radiation originating at the surface or in
the
atmosphere ("longwave"), is not included in the fluxes considered.
That is not surprising, given the origin of this document with its
GOME/Sciamachy/OMI/GOME-2/TROPOMI heritage. Our spectral coverage basically starts at
~260 nm and continues to 800/~2300/500/800/2400 nm (with gaps for Sciamachy and
TROPOMI). I don't mind limiting the definition to shortwave radiation.
This doesn't seem to be addressed by the TROPOMI definition. I hate to start
mucking around with it, but could the term 'outgoing irradiance' be replaced
by 'reflected irradiance' to answer this point?
I think limiting the definition to shortwave carries fewer risks of non-intentional
alteration the meaning of the definition. The evil is in the details.
Second CF contains this definition for irradiance, inside the solar_irradiance
definition:
"Irradiance" means the power per unit area (called radiative flux in other
standard
names), the area being normal to the direction of flow of the radiant energy.
Ian Grant suggested this:
Albedo is the ratio of the radiant flux exiting a reference surface into the
whole
hemisphere to the radiant flux incident on the same surface from the
hemisphere. The albedo,
also called the bihemispherical reflectance, is often the ratio of upwelling
flux to
downwelling flux at a horizontal surface, such as the reflected and incident
> radiation at an interface.
This is more the definition of a plane albedo (although "reference surface" should be
replaced by "plane surface" in the above definition). The most generic definition of
albedo is "the ratio of exiting radiant flux to incident radiant flux".
The object of which you want to define the albedo can be a plane surface, a sphere or
even a small irregular particle (i.e. aerosol). The incoming radiation can come from
all (4 pi) directions.
Any specific suggestion on a definition based on the document (or on anything
else)?
I hope my comments above are useful, I'll let others assemble a full description for
the standard_name list.
Best,
Maarten
On 8/20/15 1:22 PM, Maarten Sneep wrote:
On 10/08/15 19:37, Maarten Sneep wrote:
On 07/08/15 21:15, Nan Galbraith wrote:
Hi All, and happy Friday.
It's been almost a month since Maarten Sneep posted 'A few definitions seem off'
and there have been just a couple of replies.
Can we please add definitions for albedo and air_pressure to the standard names
table? Do people think we don't need these defined - is that the problem?
I have no particular preference about the content of the definitions, here's
what's
been proposed.
We have a document with definitions we use for our remote sensing work. This
document
is currently in the signatures loop, but I hope to share it with you in the
coming
week. That gives at least our definitions, from a perspective of a group that is
strictly looking in the short-wave spectrum. Note that the document uses maths
to
define the quantities, which is a bit awkward for the textual definitions used
in CF.
But at least it can provide a basis, for quite some quantities. Also note that
there
are quantities defined in the document that would be nice in CF from our
perspective,
but I don't feel like 'defending' them here (a slant column, anyone?).
Here is the list of terms & definitions we use for satellite remote sensing
development at KNMI. I hope you find this useful.
Best,
Maarten Sneep
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