This research was supported in part by NSF ATM-0552215. Constructive and thoughtful comments on an earlier version of this manuscript by Kyle Swanson, Lance Bosart, David Schultz, and the three anonymous reviewers are greatly appreciated. I also thank Kate Smith for her considerable assistance with the cyclone database. I also thank the NOAA/ESRL Physical Sciences Division, Boulder, Colorado (available online at http://www.esrl.noaa.gov/psd/) for Fig. 6.
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