Changes in the Characteristics of Tropical Cyclone Size Traversing the Philippines

The development of tropical cyclone (TC) size is affected by several complex factors but have provided a sufficient understanding of climatological characteristic and environmental conditions that affect it. Synoptic flow patterns exert more effects on TC size than the development of inner structures does. On the aforementioned basis, the present study attempts to investigate the features of change in TC size before and after crossing the Philippines based on the 37-year statistical analysis.

The TC size is defined by radius of 30-kt wind speed (R30) from JMA best track data. For the characteristic change of TC size, it is found that there are 71 TCs which passed the Philippines during 1979-2015. The numbers of size increasing (SI) and size decreasing (SD) cases are very close (36:34), and the proportion of SI case tends to increase in recent period 2001-2015 (17:11). Most of the SI cases occur in more northerly path and SD cases in further southerly path when regarding to TC landfall location. Further, before landfall, there are smaller initial size and weaker intensity in SI cases, while larger initial size and more intense intensity in SD cases. After landfall, more intensifying storms are found in SI cases, and non-intensifying ones are shown in SD cases.

Detailed examination of environmental forcing are conducted to explain the change of TC size. The composite analyses of vertical wind shear, angular momentum flux, relative humidity, and sea surface temperature between SI and SD cases are compared. Results show that the values of vertical wind shear, relative humidity, sea surface temperature, and import of angular momentum flux in SI cases are larger than those in SD cases. Furthermore, the decadal difference of proportion of SI cases is also revealed, there are more SI cases found in recent period. The synoptic patterns between 1979-2000 and 2001-2015 are compared, results show that more SI cases are related to higher relative humidity, warmer sea surface temperature, stronger monsoon trough, and weaker subtropical high in recent period. Most of the above analyzed characteristics regarding to change in TC size are statistically significant at a 95% confidence level.