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"D"-Value:
"D" in this case is a reference to the coefficient D found in Briegleb
et al. 1985 (1) where the relationship between albedo and solar zenith angle is defined
by a single parameter D which itself changes with scene type. There work followed on that
given in Dickinson, 1983(2). The resultant equation is:
Note this version is slightly more general than that given in Breigleb et al.(1985) as it
relates the albedo at any two solar zenith angels. They fixed their reference albedo to one
given at cos(SZA)=0.5. Using references and field experiments, we have associated a D-value
to 18 surface scene types defined by the International Geosphere, Biosphere Program. Within
each one degree grid box an IGBP type is specified then mapped to a specific d-value which
is shown on the plot to the right and supplied in the ascii text file containing the albedo
data. Hence, each clear sky surface albedo file contains the information needed to model
the given surface albedo to any desired solar zenith angle. The relationship between
parameters given in the downloadable files and the above equation are:
Since SARB processing does not use this methodology for determining ocean surface albedo
d-values for ocean are set to -999. Instead one should use the Ocean Look Up table from the
left menu bar.
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IGBP Map:
The International Geosphere Biosphere Programme (IGBP) developed a set of 17 surface
scene types and is available via the USGS on a 1km basis. The map was subsequently reduced
in spatial resolution (to 1/6 degree) and scene types (Tundra, Fresh Snow and Sea Ice) were
added to provide necessary input information for the radiation transfer being run by the
SARB group beneath the CERES footprints. (For a more detailed discussion on the developement
of the SARB version of the IGBP types for use in CERES processing see the
Surface properties description page.) For purposes here, it was again reduced in spatial
resolution to a 1 degree equal angle map where each scene type has an associated d-value.
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| References:
(1)Briegleb, B.P., P. Minnis, V. Ramanathan, and E. Harrison, 1985:Comparison of Regional Clear-Sky
Albedos Inferred from Satellite Observations and Models Comparisons, J. Clim. Appl. Meteor.,
25, 214-226
(2)Dickenson, R.E., 1983: Land surface processes and climate -- surface albedos and
energy balance. Advanc. in Geophys. 25, 305-353.
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