Intensification and rapid intensification of North Atlantic tropical cyclones: geography, time of year, age since genesis, and storm characteristics



Tropical cyclone best track data in the Atlantic from 1950 to 2009 were analysed to determine whether tendency to rapidly intensify varied among geographical sub-regions, 15-d intervals within the season, or when the storm displayed different characteristics of age, strength, or time of day. Wind speed increases of 7.7 m s−1 (15 kt)/24 h and 15.4 m s−1 (30 kt)/24 h were used to define intensification events; these ended up showing similar patterns. Cyclones were most likely to begin intensifying in the Gulf of Mexico and least likely in the northeast Atlantic. Likelihood did not peak during the early September peak of cyclone occurrence; cyclones were most likely to begin intensification on their initial observations, and at that time were most likely to do so if their wind speed was 18.0 m s−1 (35 kt). Older cyclones showed peaks of intensification onset at 15.4 m s−1 (30 kt) and 36.0 m s−1 (70 kt). Intensification rarely began immediately after wind speed had declined over the previous 6 h; when winds had been unchanged, intensification most often occurred when central pressure had dropped by 1 or 2 mb. Intensification onset was more likely when winds were slower than average relative to the storm's central pressure. In three time zones, onset of intensification (7.7 m s−1 [15 kt]/24 h) appeared to be most likely shortly after midnight local time, and least likely shortly before midnight. Cyclogenesis was least frequent around local midnight, and most likely in the diurnal morning hours.