On the relation of the number of extratropical cyclones to their sizes

Abstract

Extratropical cyclones were identified on the basis of sea level pressure NCEP/NCAR reanalysis data for the Northern Hemisphere from 1948 to 2004. Cyclone positions were determined with a time interval of 6 h. Cyclone sizes were obtained with the use of a numerical scheme based on a rotation of the spherical coordinate system such that the pole of the new coordinate system coincided with the cyclone center. Cyclone sizes were determined at each step of the trajectory. The last closed isobar was assumed to be the outer boundary of the cyclone. The pressure deficit in the cyclone center was regarded as a characteristic of the intensity of a synoptic formation. The interrelation between the number of cyclones and their sizes was estimated for all extratropical cyclones of the Northern Hemisphere regardless of the stage of their development. The number of cases being analyzed is 1.5 × 10⁶. Cyclone areas vary from 0.13 × 10⁶ to 6.4 × 10⁶ km², and 80% of extratropical cyclones have an intensity of 1–15 hPa. The distribution of the number of cyclones depending on their intensities is shown to be of an exponential character. The distributions of the number of cyclones were approximated with a very high accuracy, so that the regularities obtained are very stable during the past several decades.