Characteristics of Size Change of Tropical Cyclones Traversing the Philippines

Abstract

AbstractThis study investigates the size changes of tropical cyclones (TCs) traversing the Philippines based on a 37-yr statistical analysis. TC size is defined by the radius of 30-kt (≈15.4 m s?1) wind speed (R30) from the best track data of the Japan Meteorological Agency. A total of 71 TCs passed the Philippines during 1979?2015. The numbers of size increase (SI; 36) and size decrease (SD; 34) cases are very similar; however, the last 15 years have seen more SI cases (17) than SD cases (11). SI and SD cases mostly occur along northerly and southerly paths, respectively, after TCs pass the Philippines. Before landfall, SI cases have small initial sizes and weak intensities, but SD cases have larger initial sizes and stronger intensities. After landfall, most SI cases are intensifying storms, and most SD cases are nonintensifying storms. Composite analyses of vertical wind shear, absolute angular momentum flux, relative humidity, and sea surface temperature between SI and SD cases are compared. All of these values are larger in SI cases than in SD cases. Furthermore, the interdecadal difference in the ratio of the numbers of SI to SD cases reveals an unusually high number of SI cases during 2001?15. The synoptic patterns between 1979?2000 and 2001?15 are analyzed. The high SI ratio in the latter period is related to strong southwesterly wind in the south of the South China Sea that raised relative humidity, warmed the sea surface, and increased import of angular momentum flux.