Objective:
To examine the distribution of tropical cyclone (TC) intensity changes for all named North Atlantic TCs that formed from 1989-1997 and to investigate whether there were common climatological and synoptic characteristics that were associated with the TCs that intensified most rapidly.
Rationale:
Tropical cyclone (TC) intensity forecasting has proven to be considerably less skillful than has TC track prediction (DeMaria and Kaplan 1999). In particular, the forecasting of rapid intensification (RI) has been especially difficult as was underscored by the unexpected RI of 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.
Method:
The National Hurricane Center (NHC) best track data were employed to identify the maximum 24 h pressure fall (Pfall24) that occurred during the lifetime of each named TC that formed within the North Atlantic basin from 1989-1997. These data were then stratified by the maximum intensity that each TC attained during its lifetime. A stratification comprised of all named TCs (ATLNST) and another that consisted solely of TCs that attained hurricane intensity (ATLHUR) were obtained and compared with a stratification consisting of all Northwest Pacific typhoons (PACTYP) determined previously by Holliday and Thompson (1979). In addition, the magnitude of a variety of climatological and synoptic characteristics (Table 1) were obtained for all North Atlantic TCs by employing the SHIPS data base constructed by DeMaria and Kaplan (1999). These data were subsequently used to determine if there were specific climatological and synoptic characteristics that were commonly observed in rapidly intensifying TCs.
Accomplishment:
Figure 1 shows the cumulative frequency distribution of Pfall24 for each of the above stratifications. The figure indicates that the PACTYP stratification exhibits higher rates of Pfall24 than either the ATLNST or ATLHUR stratification. Holliday and Thompson (1979) defined RI for the Northwest Pacific basin as being any typhoon whose Pfall24 was within the upper 25 % of the total sample. Thus, any typhoon with a Pfall24 of 42 mb/day was said to have undergone RI in their study. However, applying their definition to the present study yielded RI thresholds of ~28 mb/day and ~23 mb/day for the ATLHUR and ATLNST stratifications, respectively. It is worth noting that the finding of higher rates of intensification for the Pacific basin is consistent with previous research which has shown that the Northwest Pacific basin is generally a more favorable basin for TC formation than the Atlantic.
To ascertain whether there were common characteristics that distinguished TCs that experienced RI from those that intensified at much slower rates, the mean climatological and synoptic characteristics of TC that underwent RI and those that experienced slow intensification (SI) were computed. The RI (SI) TC samples were comprised of TCs which had values of Pfall24 that were within the upper (lower) 25% of the ATLNST sample. Table 2 shows that there were statistically significant differences observed between several of the climatological and synoptic characteristics computed for the mean RI and SI samples. Specifically, the analysis showed that the RI TCs were stronger and commenced intensification earlier in the day than the SI TCs. Moreover, the RI TCs were associated with lower than average values of 850-500 mb and 850-200 mb vertical shear as well as more easterly flow at 200 mb than the SI TCs. Finally, the RI TCs tended to have lower 200 mb fluxes of relative eddy angular momentum.
Key reference:
DeMaria, M., and J. Kaplan , 1999: An Updated Statistical Hurricane Intensity Prediction Scheme (SHIPS) for the Atlantic and Eastern North Pacific Basins. Wea. Forecasting., in press.
Holliday,C.R., and A.H. Thompson, 1979: Climatological characteristics of rapidly intensifying typhoons. Mon. Wea. Rev., 107, 1022-1034.
Kaplan, J., and M. DeMaria, 1999: Climatological and Synoptic Characteristics of Rapidly Intensifying Tropical Cyclones in the North Atlantic basin. Preprints, 23rd Conf. on Hurricanes and Tropical Meteorology, Miami, FL, Amer. Meteor. Soc. 592-595.
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