Structure of spatial spectra of simulated cloud scenes at infrared wavelengths

Longwave infrared imagery of cloud fields are examined in terms of their power spectral density (PSD). In order to systematically investigate the dependence of the PSD on viewing conditions, a cloud scene simulator is employed to generate images of a simulated cloud field. The cloud field is fully three dimensional and is describes by its fluctuating temperature and liquid/ice water content fields. The imaging process accurately calculates the spatially varying attenuation of blackbody emission. Several views of a single cloud field are examined to study the effect of viewing angle on the image PSD. Zenith views produce isotropic PSDs, while nearly horizontal views contain a large amount of foreshortening and a correspondingly anisotropic PSD. One possible component of the foreshortening is simply geometric and can be estimated and compared to the simulation output. We find that geometrically induced foreshortening does not describe the PSD effects observed in the simulation for the relatively thin cirrus-like cloud simulated here. Possibly this indicates that the three-dimensional cloud structure is more important in some views that in others when there are large fluctuations in the cloud optical properties. We are pursuing a more quantitative description of this behavior.