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JOURNAL OF THE ATMOSPHERIC SCIENCES, 53, 2826-2837, 1996
Estimating cloud field albedo using one-dimensional series of optical depth
H. W. Barker
Abstract
This study examines the ability to estimate regional cloud albedo using 1D series of cloud optical depth tau similar
to those inferred from ground-based microwave radiometers. The investigation has two facets: use of appropriate radiative
transfer algorithms and adequate portrayal of cloud structure. Using 1024 x 1024 pixel arrays of tau inferred from 28.5-m
resolution Landsat data, regional albedos and albedos along individual scanlines are computed by a 3D Monte Carlo (MC)
photon transport algorithm. Assuming the scanlines to be proxies for 1D series of tau, a 2D MC algorithm and the
Independent Pixel Approximation (IPA) are used to compute albedos for scanlines of various lengths and resolutions.
Regarding the appropriateness of doing radiative transfer calculations on a ID series of tau, it is shown that for 1D
series of tau containing 1024 pixels (similar to 30 km), lack of information about cloud structure adjacent to the series
yields root-mean-square errors for 2D MC albedos of about 2% for a stratocumulus and 20% for two cumulus cloud fields.
For the cumulus cases there is a marked tendency to over (under) estimate albedos for relatively bright (dark) scanlines.
For the same series, the IPA performs very well relative to the 2D MC for stratocumulus conditions. For broken cumulus clouds,
however, notable biases between the 2D MC and IPA results stem mostly from the neglect of cloud sides by the IPA. In all cases,
random errors are small.
The IPA is then used to investigate the accuracy of estimating regional cloud albedo with 1D datasets containing various amounts
of information. It is demonstrated that for series with less than 100 pixels (less than or similar to 3 km) at full resolution,
the probability of attaining good estimates of regional cloud albedo is very low regardless of cloud type. Ideally, series
with more than 1024 pixels should be used. Regarding sensitivity to data resolution, estimated regional albedos are almost
resolution independent for pixel sizes up to about 2 kin (similar to 70 pixels). At coarser resolutions, loss of cloud
structure information important for radiative transfer is great and the quality of regional albedo estimates degraded,
particularly for oblique sun.
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