Renato Vitolo, Willis Research Fellow, University of Exeter

David Stephenson, WRN Senior Academic, University of Exeter

Ian Cook, Willis Re

Kirsten Mitchell-Wallace, Willis Re

This paper investigates how clustering of wintertime extra-tropical cyclones depends on the vorticity intensity of the windstorms, and the sampling time period over which storm transits are counted.

This study has investigated how clustering of wintertime extra-tropical cyclones depends on the vorticity intensity of the windstorms, and the sampling time period over which storm transits are counted. Clustering is characterised by the dispersion (ratio of the variance and the mean) of the counts of eastward transits of storm tracks obtained by objective tracking of 850hPa vorticity features in NCEP-NCAR reanalyses. The counts are aggregated over non-overlapping time periods lasting from 4 days up to whole 6 month long October-March winters over the period 1950-2003.