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JOURNAL OF GEOPHYSICAL RESEARCH, 111, D17201, doi:10.1029/2005JD006668, 2006.
Effect of satellite-derived cloud property errors
in computing domain-averaged irradiances with a 1D radiative transfer model.
S. Kato, A. Cheng, and L. M. Hinkelman
Abstract
The process of retrieving cloud optical thickness and effective radius from radiances mea-
sured by satellite instruments is simulated to determine the error in both the retrieved
properties and in the irradiances computed with them. The radiances at 0.6 µm and
3.7 µm are computed for three cloud fields – stratus, stratocumulus, and cumulus– gener-
ated by large eddy simulation models. When the horizontal flux is neglected in the retrieval
process and overcast pixels are assumed, the error in the retrieved domain averaged optical
thickness from nadir is 1% to -32% (1% to -27%) for the solar zenith angle of 30° (50°)
and the error in the effective radius is 0% to 67% (0% to 63%) for solar zenith angles
of 30° (50°). Using the radiance averaged over the 1 km size pixel also introduces error
in the optical thickness because of the non-linear relation between the reflected radiance
and optical thickness. Both optical thickness and effective radius errors increase with in-
creasing horizontal inhomogeneity, which is expressed by the shape parameter of a gamma
distribution fit to the optical thicknesses over the domain. The retrieved shape parameter
varies from 0.7 to 62 for three cloud scenes used in this study. When the 0.6 µm albedo
is computed with the independent column approximation using retrieved properties from
nadir (oblique) view for a solar zenith angle of 50°, the error is -0.3% to 14% (-5% to -30%)
compared to the albedo from 3D radiative transfer computations. The albedo error occurs
even though the radiance at one angle is forced to agree because retrieved cloud proper-
ties have errors and a plane parallel cloud cannot consistently match the radiance angular
distribution with a single value of optical thickness and effective radius. When albedos
computed with cloud properties derived from nadir and oblique views are averaged, the
albedo error can partially cancel. For two cloud cases of which retrieved shape parameter
of a gamma distribution is not significantly less than 10, the error in the narrowband 0.64
µm (3.7 µm) albedo averaged over a domain is less than -5% (-4%) when albedos computed
with cloud properties derived from viewing zenith angles between 0 and 60 are averaged
and when the solar zenith angle is between 10° and 50°. When the solar zenith angle is
70°, the error increases to up to +24% (+37%) for all three cases.
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