IAAFT surrogate clouds

Surrogate cloud fields share certain, typically statistical, properties with real cloud field: they are surrogates for measured clouds. Using well-known algorithms such as the Fourier method and the Bounded Cascade method the generated fields have a specified Fourier or power spectrum. This page presents a new easy iterative method that allows specifying both the power spectrum and the amplitude distribution of the parameter of interest, like liquid water content or liquid water path. Because of this, the method is ideally suited to generate cloud fields based on measured data and it is able to generate broken cloud fields. The method is based on the Iterative Amplitude Adapted Fourier Transform (IAAFT) algorithm of Schreiber and Schmitz (2000 and 1996).

On this homepage you can find.

IAAFT algorithm
A description of the main steps of the algorithm in speudo code and with a flow diagram.
Example surrogates
A range of examples of surrogate clouds.
Downscaling cloud fields
A version of the IAAFT algorithm that is able to perform downscaling of full fields.
Interpolation; local forcing
Surrogate cloud fields in which the measured values are forced upon the field. Useful for comparisons with point measurements, as this forces the clouds to be in the measured "position" on large scales.
Statistical properties
A study on the increment distributions, structure functions, annual statistics and return values of a number of different measurements. It is shown that in most of these cases surrogate time series are more accurate than the often used multifractal time series.
Validation surrogates
This page shows the results of a validation study, where we made surrogate fields from 3D LES cloud fields. In the studied cases, the radiative properties of the surrogates were not statistically significantly different.
Download
Download the algorithms and a number of surrogate clouds based amongst others on BBC cloud measurements.
Articles
Downloadable PDF articles on surrogate fields.
Surrogates intro
The main page on surrogates, linking to this page and the pages below.
Constrained surrogates
Constrained surrogates are generated with a global search algorithm. This is a more general, but slower way to generate fields.
List of generators
A list with cloud generators from literature, with links to the articles and code where available.
On cloud structure
An essay on the fractal and non-fractal nature of the structure of clouds

victor.venema@uni-bonn.de
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