A spectral analysis of albedo and bidirectional reflectances for inhomogeneous clouds

Power spectra of optical depth τ 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 τ, 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, τ inferred from zenith radiances should resemble true τ 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 τ spectrum. At 3000 m above cloudtop, however, series of albedo can be very complex and so too can their spectra. At large spatial scales albedo spectra resembled that for τ, but at scales smaller than about 10 times the horizontal resolution of both τ 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.