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TELLUS A, 58, 104-120, 2006.
Surrogate cloud fields generated with the Iterative
Amplitude Adapted Fourier Transform algorithm.
V. Venema, S. Meyer, S. Gimeno Garcia, A. Kniffka,
C. Simmer, S. Crewell, U. Lohnert, and T. Trautmann
Abstract
A new method of generating 2-dimensional and 3-dimensional cloud fields is presented,
which share several important statistical properties with real measured cloud fields. Well-
known algorithms such as the Fourier method and the Bounded Cascade method generate
fields with a specified Fourier spectrum. The new iterative method allows for the specification
of both the power spectrum and the amplitude distribution of the parameter of interest, e.g. the
liquid water content or liquid water path. As such, the method is well suited to generate cloud
fields based on measured data, and it is able to generate broken cloud fields. Important
applications of such cloud fields are e.g. closure studies. The algorithm can be supplied with
additional spatial constraints which can reduce the number of measured cases needed for such
studies. In this study the suitability of the algorithm for radiative questions is evaluated by
comparing the radiative properties of cloud fields from cloud resolving models of cumulus
and stratocumulus with their surrogate fields at nadir, and for a solar zenith angle of 0 and 60
degrees. The cumulus surrogate clouds ended up to be identical to the LES clouds on which
they are based, except for translations and reflections. The root mean square differences of the
transmittance and reflectance fields are less than 0.03 % of the radiative budget. The
radiances and mean actinic fluxes fit better than 2 percent. These results demonstrate that
these LES clouds are well-described from a radiative point of view, using only a power
spectrum together with an amplitude distribution.
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