The authors would like to thank Jan Barkmeijer, Ron Errico, Ron Gelaro, Tony Hollingsworth, Ernst Klinker, and Florence Rabier for helpful discussions. The three-level quasigeostrophic model was developed by Franco Molteni. Jan Barkmeijer provided much of the software. Support of the sponsor, Office of Naval Research, and the program manager, Naval Research Laboratory, Project Element 0601153N, is gratefully acknowledged. Much of this work was done while the first author was visiting the European Centre for Medium-Range Weather Forecasts.
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