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REMOTE SENSING OF ENVIRONMENT 54, 113-120, 1995
A spectral-analysis of albedo and bidirectional reflectances for inhomogeneous clouds
H. W. Barker
Abstract
Power spectra of optical depth tau for horizontally inhomogeneous clouds were compared to power spectra of corresponding
near zenith-bidirectional reflectances and albedos. Although spectra for bidirectional reflectances resembled closely those
of tau, albedo spectra were steeper. These bidirectional reflectances are similar to what nadir-viewing satellites would observe,
whereas the albedos are approximately those that would be measured by aircraft-mounted pyranometers skimming cloudtops.
Thus, tau inferred from zenith radiances should resemble true tau better than those inferred from albedos.
Power spectra were also computed for albedos that would be measured at various heights above cloudtop. Spectra for fluxes at
100 m bore a strong resemblence to those measured at cloudtop: steeper than the corresponding tau spectrum. At 3000 m above
cloudtop, however, series of albedo can be very complex and so too can their spectra. At barge spatial scales albedo spectra
resembled that for tau, but at scales smaller than about 10 times the horizontal resolution of both tau and the albedo data,
albedo spectra signified white noise. Thus, it is pointless to infer detailed cloud structure from albedo observations made
high above cloudtops.
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