In tropical cyclone (TC) risk assessment, many of previous studies have attempted to construct the statistical relationship of TC damage and three risk elements–exposure, vulnerability, and hazard. For hazard parameters, central minimum pressure, maximum wind speed, and size have been widely utilized. This study shows that, in a decision tree analysis for TCs that made landfall in South Korea during 1979–2014, impact angle and extratropical transition (ET) information are the most important factors to decide whether the TC risk to be activated as a catastrophe or to stay in the potential status.
We first divided the landfall TCs in to to groups: weak TC (WTC, defined as maximum wind speed <17 m s–1 including potential TC and post TC) and strong TC (STC, >17 m s–1). WTC, which is even ignored in the TC warning system of Korean Meteorological Agency, appeared to be more damaging than STC for western provinces. WTCs have significantly different tracks and landfall locations compared to STCs, so that western provinces of South Korea suffers more from WTCs than from STCs. Second, we conducted decision tree analysis for STCs and WTCs separately. Decision tree analysis revealed that the most distinctive differences that divide damaged and non-damaged TCs were 1) for STCs, if the STC is west-approaching or east-approaching to South Korea, and 2) for WTCs, whether the WTC undergoes ET around the Korean Peninsula or not. The importance of impact angle can be explained by different physical geography experience, duration of influence, and relative position of dangerous semicircle side of the TC, according to different impact angle. For ET, it is hypothesized to be related to re-intensification of the system during ET.
Our findings reaffirm that risk communication should focus more on the local hazards that residents in different areas actually experience (e.g. rainfall) rather than one representative intensify (e.g. central wind intensity) that is concentrated on the inner-core of a TC. The physical mechanism of ET around the East Asia and Western North Pacific is not fully understood, and a better forecasting of ET is urged as it is one of the most critical factor for damage in the region. Also, the sensitivity of TC damage depending on its impact angle indicates that large uncertainty in future track projection may seriously mislead future TC risk modeling because trivial track projection error alone can produce severe error in damage projection even when TC frequency and intensity value is precise and accurate.