The research is supported by NOAA Grant NA77WA0571 and NSF Grant ATM-9812729. The authors would like to thank Drs. E. Kalnay and J. Sela for their persistent support and encouragement, and Dr. Olivier Talagrand and Dr. Qin Xu for their suggestions that were useful for improving the original manuscript. Thanks go to two anonymous reviewers for thorough and helpful comments and suggestions.
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