Hi Nan and Maarten,
Maarten is correct that my definition was considering only reflection from a
plane surface (a better term than my ("reference surface"). My definition was
from the perspective of satellite-based remote sensing of a plane target,
usually horizontal, such as surface, cloud-top or top of atmosphere. I agree
with Maarten that it would be better to allow more general geometries.
I think the following two issues need to be considered, although I'm not firmly
committed to a resolution for either:
1) Should the definition of albedo be restricted to shortwave? The
upwelling longwave at the surface includes a component that is atmospheric
emission that began as downwelling and was reflected by the surface. However, I
have most often seen the equations describing the longwave fluxes at the
Earth's surface refer to the fraction of flux reflected as (1 - emissivity)
rather than albedo. Because this reflected longwave exists, I think that if we
agree to restrict the definition of albedo to shortwave then "shortwave" should
be explicitly included in the definition. I would be comfortable restricting
the definition of albedo to shortwave.
2) Do we need to distinguish reflection from transmission? If we do, then
it is probably necessary to include "reflected" in the definition as Nan
suggests. In the restricted case of reflection from a surface, the distinction
can be made, as I did, by stating that the outgoing radiation is towards the
hemisphere from which the radiation was incident. My experience is largely with
plane surfaces. But for a sphere or Maarten's aerosol particle both incident
and reflected radiation can be from / to the full 4 pi of directions. Are there
cases like that in which it is not clear or not sensible to distinguish
reflection from transmission? Perhaps it makes sense to distinguish reflection
from transmission for a plane (such as a leaf or an optical filter) but not a
small particle (such as when defining the aerosol single scattering albedo).
I'd welcome Maarten's or anyone else's thoughts.
Another option that cuts across both these issues would be to include
"reflected" but omit "shortwave" to accommodate reflected thermal emission.
I suggest we are converging to:
"Albedo is the ratio of (exiting|outgoing|reflected) [shortwave] radiant flux
to incident [shortwave] radiant flux"
where we must choose one of the words in the parentheses () and choose whether
to include "shortwave" in [].
My inclination is towards:
"Albedo is the ratio of reflected shortwave radiant flux to incident shortwave
radiant flux"
to exclude thermal radiation and exclude transmission in those cases where
transmission makes sense.
Regards,
Ian
Ian Grant | Satellite Specialist
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Observations & Infrastructure Division | Science & Engineering Section
Bureau of Meteorology
GPO Box 1289 Melbourne VIC 3001
Level 5, 700 Collins Street, Docklands VIC 3008
Tel: +61 3 9669 4080 | [email protected]
www.bom.gov.au
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