The authors thank one of the anonymous reviewers for insightful comments. D. K. was supported by a Rising Star Fellowship from the National Institute for Aerospace, Hampton, Virginia. This work was also supported by a National Oceanic and Atmospheric Administration THORPEX Grant, the Army Research Office, a James S. McDonnell 21st Century Research Award, the NPOESS Integrated Program Office (IPO), the Office of Naval Research (Physics), and the National Science Foundation (Grants 0104087 and PHYS 0098632). E. J. K. gratefully acknowledges support from the National Science Foundation (Grant DMS-0408102).
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