Abstract
Tropical cyclone (TC) intensity forecasting has proven to be considerably less skillful than has TC track prediction. In particular, the forecasting of rapid intensification (RI) has been especially difficult as was underscored by the unexpected RI of Hurricane Opal (1995) which strengthened from a minimal category 2 to a strong category 4 hurricane on the Saffir/Simpson scale in just 18 hours. Fortunately, Opal weakened dramatically after this period of RI and made landfall as only a minimal category 3 hurricane. Nevertheless, the unexpected RI of Opal so close to the United States illustrates the need for improving our understanding of the rapid intensification process. In the only previous comprehensive study of RI, the climatological characteristics of rapidly intensifying TCs in the Northwest Pacific basin were examined for the period 1956-1976 by Holliday and Thompson (1979). Some important findings from their study were that RI was only observed when a sufficiently deep layer of warm water was present and that RI was more prevalent during the nighttime than than during the daytime. More recently, an analysis of the climatological and synoptic characteristics of rapidly intensifying TCs from 1989-1992 was performed during the development of a statistical hurricane intensity prediction scheme (SHIPS) for the North Atlantic basin (DeMaria and Kaplan 1994). This analysis indicated that RI occurred in TCs that were over warmer than average sea-surface temperatures (SSTs) and that RI typically occurred in regions where the 850-200 mb vertical shear of the horizontal wind (SHR) was lower than the sample mean.
In this study, the climatological characteristics of TCs that underwent RI in the North Atlantic basin have been investigated for the period 1975-1996. This time interval was chosen for consistency since the Dvorak satellite classification scheme was used for the entire time period. Additionally, the synoptic characteristics associated with rapidly intensifying TCs have been examined for a subset of that time period (1989-1996) in an attempt to identify the large-scale physical mechanisms favorable for RI. The time interval 1989-1996 was chosen since the magnitude of a variety of synoptic characteristics were available from the SHIPS data base for all named storms that formed during this period. A comparison of the intensification rates determined in this study with those obtained previously for the Northwest Pacific basin showed that lower rates of intensification existed for the Atlantic basin. Moreover, a statistical analysis of the SHIPS and best track data indicated that tropical cyclones that underwent RI tended to have common climatological and synoptic characteristics that were significantly different from those of the non-rapidly intensifying tropical cyclones. Specifically, tropical cyclones that experienced RI tended to be stronger and began intensifying earlier in the day than non-rapidly intensifying TCs. In addition, TCs that underwent RI were typically located over higher than average SSTs and were also embedded in an environment of lower than average SHR, more easterly 200 mb wind flow, and warmer than average 200 mb temperatures. Finally, RI TCs typically occurred in regions of lower than average 200 mb fluxes of relative eddy angular momentum.