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REMOTE SENSING OF ENVIRONMENT, 42, 51-64, 1992
Cumulus cloud radiative properties and the characteristics of satellite radiance wave-number spectra
H. W. Barker, J. A. Davies
Abstract
Visible and infrared wavenumber spectra were examined for two AVHRR images (256 x 256 pixels; 1.1 km resolution) of
single layer, shallow-cumuliform clouds from off the east coast of North America. For each image, both spectra are
almost identical in form for scales between 2 and 256 km. For scales greater than approximately 5 km, spectral slopes
can vary widely between cloud types, but for smaller scales they follow approximately k-3 in constancy with previous results.
It is argued that abrupt scaling changes are to be expected at about typical cloud cell size, thus implying that satellite
radiance variability is scale dependent. Furthermore, it is argued that application of existing generalized scale invariance
techniques to satellite imagery should avoid crossing over the scale associated with typical cloud cell size. Monte Carlo
methods of photon transport were used to demonstrate the similarities between spectral slopes for a cloud's vertical integral
of optical depth tau(upsilon) and its corresponding reflected and emitted radiation fields. This suggests that the k-3 radiance
spectra for shallow cumuliform clouds derives from cloud optical depths varying as approximately k-3 also. This, in turn,
implies that if in shallow cumulus liquid water content (LWC) variability is isotropic, spectra of LWC transects should be
close to k-2, as is often the case. Despite the spectral similarities, it is shown that the many-to-many relation between
reflected radiance and tau(upsilon) can be quite pronounced, thus making inference of tau(upsilon) difficult.
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