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Downscaling with surrogate fields

The flow chart of the downscaling algorithm. The result of the operations is illustrated by 2D horizontal cross sections at the height of the maximum LWC, just below the top of this stratocumulus cloud.

The IAAFT algorithm to generate surrogate clouds can be extended to perform adownscaling. Two changes are needed. First of all the power spectrum of the coarse field needs to be extended to smaller scales. This extrapolation can be performed by assuming a power law (fractal) power spectrum. Any other extrapolation function, e.g. based on small scale measurements (in part of the field), can also be utilized. Secondly, the the adjustment of the distribution, has to be replaced with an adjustment of the surrogate field to match the coarse field.

The basic idea of the algorithm is illustrated below with a flow chart. For applications to cloud fields, it can be useful to include information on the sub-scale cloud fraction (e.g. from satelite retrievals) as well. For more information on the algorithm, please see Venema et. al. 2010.

Examples of a cumulus (left column) and a stratocumulus (right columns) LES cloud and their corresponding coarse-grained and surrogate fields. The first row depicts the original 3D LES clouds; the largest square subplot is the average LWC seen from the top, the bottom subplot is a front view and the right subplot a side view. The second row represents the coarse-grained cloud fields. The third row shows surrogates with an extrapolated spectrum.

Example cloud fields

The figure to the left shows two example cloud fields, one cumulus and one stratocumulus field (If you click on the figure, you will see a larger figure with two more example fields). In the article, radiative transfer computations are performed on the original LES clouds and the downscaled surrogate fields to study the suitability of these clouds for 3D radiative transfer studies.

References

The downscaling article: Venema, V.K.C., S. Gimeno García, and C. Simmer. A new algorithm for the downscaling of cloud fields. Quart. J. Royal. Meteorol. Soc., 136, no. 646, pp. 91-106, doi: 10.1002/qj.535, 2010.

Background information on surrogate clouds: Surrogate cloud fields generated with the Iterative Amplitude Adapted Fourier Transform algorithm (color | black/white)
Tellus 58A, no 1, pp. 104-120, 2006a
Victor Venema, Steffen Meyer, Sebastián Gimeno García, Anke Kniffka, Clemens Simmer, Susanne Crewell, Ulrich Löhnert, Thomas Trautmann, and Andreas Macke