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| Titel |
Bounded cascade clouds: albedo and effective thickness |
| VerfasserIn |
R. F. Cahalan |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 1, no. 2/3 ; Nr. 1, no. 2/3, S.156-167 |
| Datensatznummer |
250000008
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| Publikation (Nr.) |
 copernicus.org/npg-1-156-1994.pdf |
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| Zusammenfassung |
| If climate models produced clouds having liquid water amounts
close to those observed, they would compute a mean albedo that is often much too large,
due to the treatment of clouds as plane-parallel. An approximate lower-bound for this
"plane-parallel albedo bias" may be obtained from a fractal model having a range
of optical thicknesses similar to those observed in marine stratocumulus, since they are
more nearly plane-parallel than most other cloud types. We review and extend results from
a model which produces a distribution of liquid water path having a lognormal-like
probability density and a power-law wavenumber spectrum, with parameters determined by
stratocumulus observations. As the spectral exponent approaches -1, the simulated cloud
approaches a well-known multifractal, referred to as the "singular model", but
when the exponent is -5/3, similar to what is observed, the cloud exhibits qualitatively
different scaling properties, the socalled "bounded model". The mean albedo for
bounded cascade clouds is a function of a fractal parameter, 0 << 1, as well as the usual
plane-parallel parameters such as single scattering albedo,
asymmetry, solar zenith angle, and mean vertical optical thickness. A simple expression is derived to
determine from the variance of the logarithm
of the vertically-integrated liquid water. The albedo is shown to be approximated well by the plane-parallel albedo
of a cloud having an "effective" vertical optical thickness,
smaller than the mean thickness by a factor χ(f), which is given as an analytic function of f.
California stratocumulus have
a mean fractal parameter (f) ≈ 0.5, relative albedo bias of 15%, and an effective thickness 30% smaller than the
mean thickness (χ ≈ 0.7). For typical observed values of mean liquid water and (f), the effective thickness approximation
gives a plane-parallel albedo within 3% of the mean albedo. |
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