The Influence of the Tropical Cyclone Outer-Core Wind Strength for Landfalling Storm in Taiwan

In 2015 and 2016, there were four typhoons with different intensities landed in Taiwan, including severe typhoon Dujuan (2015) and Nepartak (2016) and moderate typhoon Soudelor (2015) and Megi (2016). Although the first two are stronger typhoons, the associated wind and rainfall impacts on the island are much less than the latter two storms. This situation doesn’t in line with the traditional impression that the stronger landfalling typhoon which could led to more damages over the island. This study is aimed to access the relationship between the tropical cyclone (TC) structure and associated wind and rainfall distribution during landfall for above mentioned four TCs based on observational and numerical simulation framework. For the observation part, the TC size and outer-core wind strength (OCS) is determined by several available data (e.g, global analyses, ASCAT scatterometer wind, and dropsonde data), and three rainfall and three wind indices are determined for representing the general impact of the landfalling TC. For the examination by the numerical model simulation, the WRF 3.3-km high resolution simulation which initialized from the NCEP FNL analysis for these TCs are conducted.

Based on the observational analysis for these four TCs, results show that the OCS of both Soudelor and Megi are much stronger than that of Dujuan and Nepartak, although their intensity is much weaker. It is found that the strong-OCS TC could induce stronger wind and rainfall during the passage of the storm, the correlations of the TC OCS and several wind and rainfall indices are significantly correlated (r>0.97, p<0.05). Furthermore, for the numerical model examinations, the results also clearly demonstrate that strong-OCS TC could lead to stronger wind and heavy rainfall over the island, the relationships between TC OCS and associated wind and rainfall indices are highly correlated (r>0.94¸ p<0.10). Detailed analyses show that for a stronger OCS storm implies larger potential vorticity that it exhibited which could further resist the friction effect from the topography, thus maintains storm’s vorticity and increases the devastation time of strong wind and heavy rainfall over the island. Based on the results from both the observation analysis and numerical simulations, it is proposed that TC OCS could play a dominated factor for determining the overall wind and rainfall impact of the landfalling storm in Taiwan.