Large-Scale Characteristics and Probability of Rapidly Intensifying Tropical Cyclones in the Western North Pacific Basin

The China Meteorological Administration (CMA) and the National Centers for Environmental Prediction (NCEP) reanalysis datasets are employed to examine the large-scale characteristics of rapidly intensifying western North Pacific tropical cyclones (TCs). The results show that of all 27 581 samples for the period 1970-2007, 85%, 65%, and 47% of all tropical depressions (TDs), tropical storms (TSs), and typhoons (TYs), respectively, intensify. Of the 1214 TCs, 18%, 70%, 30%, and 10% of all tropical cyclones, supertyphoons, severe typhoons, and typhoons, respectively, underwent rapid intensification (RI) at least once during their lifetime. Three kinds of cases-RI, slow change in intensity (SC), and rapid decay (RD)-during the period 1982-2007 are used to analyze the large-scale conditions associated with them. The comparison shows that the RI cases tend to occur farther south and east than the non-RI cases. In addition, the RI cases have a more westerly component of motion and intensify more during the preceding 12 h than do the non-RI cases. For the non-RI cases, the SC cases tend to have a lower initial intensity and a lower speed of motion than do the RD cases. Also, the RI cases are farther from their maximum potential intensity and develop in warmer water, lower vertical shear, and more easterly upper-tropospheric flow than do the non-RI cases. The probability of RI for TCs is estimated by using the rapid intensify index (RII) developed in this study for the western North Pacific basin. The verification based upon the cross validation shows that the RII is skillful relative to climatology.