L. Oreopoulos et al., Cloud three-dimensional effects evidenced in Landsat spatial power spectraand autocorrelation functions, J GEO RES-A, 105(D11), 2000, pp. 14777-14788
An analysis of nadir reflectivity spatial Fourier power spectra and autocor
relation functions for solar wavelengths and cloudy conditions is presented
. The data come from Landsat thematic mapper. (TM) observations, while Mont
e Carlo (MC) simulations are used to aid the interpretation of the observat
ions and to examine sensitivity to various factors. We show that shortwave
radiative processes produce consistent signatures in power spectra and auto
correlation functions. Power spectra take a variety of shapes not shown or
explained in previous observational studies. We demonstrate that TM spectra
call potentially be affected by radiative "roughening" at intermediate, sc
ales (similar to 1-5 km) and radiative "smoothing" at small scales (<1 km).
These processes are wavelength-dependent, with systematic differences betw
een conservative (for cloud droplets) TM band 4 (similar to 0.8 mu m) and a
bsorbing band 7 (similar to 2.2 mu m). Band 7 exhibits moro roughening and
less smoothing than band 4 and faster decrease in autocorrelation. Rougheni
ng is more prevalent at large solar zenith angles due to optical and/or geo
metrical side illumination and shadowing. MC spectra illustrate that scale
invariant optical depth fields can produce complex power spectra that take
a variety of shapes under different conditions. Radiative roughening increa
ses with decreasing single scattering albedo and increasing solar zenith an
gle (as in the observations). For low solar zenith angles, there is a clear
shift in the radiative smoothing scale to smaller values as droplet absorp
tion increases. Power spectra also show stronger decorrelations between opt
ical depth and reflectivity when cloud top variations are more pronounced.
Finally, it is shown that power spectral analysis is a useful tool for eval
uating the skill of novel optical depth retrieval techniques in removing th
ree-dimensional radiative effects. New techniques using inverse nonlocal in
dependent pixel approximation and normalized difference of nadir reflectivi
ty yield optical depth fields which better match the scale-by-scale variabi
lity of the true optical depth field.