Size changes over the life of sea level cyclones in the NCEP reanalysis

This paper addresses the extent to which sea level pressure cyclones change size as they develop. A state-of-the-art cyclone tracking scheme has been applied to the global "reanalyses'' produced by the National Centers for En vironmental Prediction for the four-decade period 1958-97. The analysis is based on all the cyclones found in the analyses, and on those which halfway through their lifetimes are located in the 30 degrees -50 degrees and 50 d egrees -70 degrees latitude bands. Systems in both the Northern Hemisphere (NH) and Southern Hemisphere are considered, as are those in the December-F ebruary and June-August periods. The results show that the radius of surface cyclonic systems increases as t hey evolve to maturity. This finding holds for the two baroclinic domains c onsidered in both hemispheres and in both winter and summer. In the NH wint er in the 30 degrees -50 degreesN and 50 degrees -70 degreesN belts the ave rage increase in size of systems that last longer than 3 days is about 33% over 4 days. In the northern summer the rate of increase in radius is less marked, particularly in the midlatitude belt. In the Southern Hemisphere wi nter the mean rate of size increase is somewhat more modest than in the nor thern winter. The increase in size in the southern summer is greater than i n the north, particularly in the 50 degrees -70 degrees band. The small number of studies on this topic have indicated that over specific domains and limited samples the size of cyclones increase as they evolve f rom their point of first identification. The present results show that thes e increases occur in the extratropics of both hemispheres and in both winte r and summer.