Identifying the rapid intensification of tropical cyclones using the Himawari-8 satellite and their impacts in the Philippines

Tropical cyclones (TCs) that undergo rapid intensification (RI), defined as the upper 95th percentile increase of TC maximum winds in a 24-hr period, are likely to pose a bigger threat to affected countries in the western North Pacific (WNP). In the Philippines, of the 522 TCs that made landfall from 1951 to 2020, 146 TCs (28%) underwent RI. The majority (82%) of these RI TCs made landfall with at least typhoon intensity, in contrast, only 12% of landfalling non-RI TCs were typhoons. As the region in the WNP basin where most TC RI occurs (bounded by 123 degrees-140 degrees E and 10 degrees-20 degrees N) is located just to the east of the Philippines, TC RI is typically followed by landfall in the Philippines, many of which do so at their peak intensity. Consequently, results suggest that TCs undergoing RI have a larger impact on the country. Comparison of socioeconomic data between landfalling RI and non-RI TCs showed that the former tend to have larger impacts in terms of population affected and number of deaths. Hence, determining TC RI at its onset can aid in the early warning of impending TC intensification. In this study, we explored the use of infrared brightness temperature (Bt) satellite data in the identification of TC RI onset. The 10.4 mu m window band of the Himawari-8 satellite was used to measure the amount of active convection that typically occurs before or during the onset of TC RI. RI onset is determined if 100% of pixels within a 50 km radius from the TC centre have Bt <= 225 K. Assessment of satellite Bt data that fit this set of criteria followed by 25 and 35 kt RI in the next 24 hr have positive Pierce skill scores of 0.47 and 0.48, respectively, false alarm ratio of 0.13-0.20, and probability of false detection of 0.02-0.03.