We are grateful to Fabio D'Andrea for providing a code of the QG3 model and information on its performance. Andrew W. Robertson and Padhraic J. Smyth provided the codes for the k-means method and the Gaussian mixture model, respectively; it is a pleasure to thank them for the codes and their advice on using these. We also wish to thank Sergey Kravtsov and Ricardo Todling for helpful discussions, and Daan Crommelin for sharing preprints of his recent work. Gilbert Brunet and an anonymous reviewer provided constructive comments on a previous version of the paper. Our study was supported by NASA Grant NAG5- 9294 and NSF Grant ATM00-8213. This is publication 5825 of UCLA's Institute of Geophysics and Planetary Physics.
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