This work was supported in part by the National Science Foundation Grant ATM-0645200, National Oceanic and Atmospheric Administration Grants NA05OAR431-1134, NA08OAR431-0882 (JDN and OP), and NA07OAR431-0214 (SN), and the Guggenheim Foundation (JDN). DN thanks J. L. Lin for asking what the PN06 results imply for cluster size distributions. OP would like to thank G. Pruessner, M. Gastner, and B. Oborny for helpful discussions on a related problem in ecology. We thank B. Stevens for discussions and R. Houze for comments on the manuscript.
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