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JOURNAL OF CLIMATE, 8, 2620-2630, 1995
Inferring optical depth of broken clouds from Landsat data
H. W. Barker, and D. M. Liu
Abstract
Optical depths tau(pp) for broken, shallow clouds over ocean were inferred from Landsat cloud reflectances R(cld) (0.83 mu m)
with horizontal resolution of 28.5 m. The values tau(pp) were obtained by applying an inverse, homogeneous, plane-parallel
radiance model to each pixel value of R(cld). The primary objective of this study was to estimate optical depth errors
incurred by the homogeneous, plane-parallel, independent pixel paradigm. This was achieved by computing reflectances R(mc)
with a 3D Monte Carlo photon transport algorithm that employed tau(pp) and cloud geometric thicknesses h > 0.
A single cloud was isolated for study in which the solar zenith angle was 30 degrees and average tau(pp) was 5.8. This cloud
measured about 1.2 km in diameter but h had to be estimated. In the Monte Carlo simulations, h was set to be uniform for the
entire cloud. For h between 150 and 300 m, cloud-average reflectance R(mc) was about 15% less than R(cld). It was found that
use of tau(pp)(1/delta(h)) in the Monte Carlo algorithm yielded R(mc) approximate to R(cld). For h = 225 m, 1/delta (h = 225)
approximate to 1.11, and this increased average tau(pp) to similar to 8.0, which was a 35% increase. At the pixel level,
however, random errors associated with fields of R(mc) - R(cld) were reduced only slightly when tau(pp)(1/delta(h)) was
used rather than tau(pp). Finally, tau(pp)(1/delta(h)) applied to numerous neighboring clouds. When the aspect (height to width)
ratio A of neighboring clouds was assumed to be constant, tau(pp) for each cloud received a unique scaling, and this yielded
Landsat mean reflectances to within 4% for A < 0.3. This suggested that grid-averaged tau(pp) was likely about 4 rather than 3,
as was the plane-parallel, independent pixel estimate.
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