A SPECTRAL-ANALYSIS OF ALBEDO AND BIDIRECTIONAL REFLECTANCES FOR INHOMOGENEOUS CLOUDS

Power spectra of optical depth tau for horizontally inhomogeneous clou ds were compared to power spectra of corresponding near zenith-bidirec tional reflectances and albedos. Although spectra for bidirectional re flectances resembled closely those of tau, albedo spectra were steeper . These bidirectional reflectances are similar to what nadir-viewing s atellites would observe, whereas the albedos are approximately those t hat would be measured by aircraft-mounted pyranometers skimming cloudt ops. Thus, tau inferred from zenith radiances should resemble true tau better than those inferred from albedos. Power spectra were also comp uted for albedos that would be measured at various heights above cloud top. Spectra for fluxes at 100 m bore a strong resemblence to those me asured at cloudtop: steeper than the corresponding tau spectrum. At 30 00 m above cloudtop, however, series of albedo can be very complex and so too can their spectra. At barge spatial scales albedo spectra rese mbled that for tau, but at scales smaller than about 10 times the hori zontal resolution of both tau and the albedo data, albedo spectra sign ified white noise. Thus, it is pointless to infer detailed cloud struc ture from albedo observations made high above cloudtops.