Venema, V., Meyer, Steffen, Gracia, S.G., Kniffka, A., Simmer, C., Crewell, S., Loehnert, U., Trautmann, T. and Macke, Andreas (2006) Surrogate cloud fields generated with the Iterative Amplitude Adapted Fourier Transform algorithm. Tellus A: Dynamic meteorology and oceanography, 58 (1). pp. 104-120. DOI 10.1111/j.1600-0870.2006.00160.x.

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Abstract

A new method of generating two-dimensional and three-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◦. The cumulus surrogate clouds ended up to be identical to the large
eddy simulation (LES) clouds on which they are based, except for translations and reflections. The root mean square
differences of the stratocumulus transmittance and reflectance fields are less than 0.03% of the radiative budget. The
radiances and mean actinic fluxes fit better than 2%. 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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