Impact-Based Ensemble Flood Forecasting in Japan: Effects of Typhoon Intensity Modification on Flood Damage Mitigation during Typhoon Hagibis (2019)

Typhoon Hagibis (2019), one of the most powerful storms to strike Japan in recent years, caused widespread flooding and severe damage. Impact-based forecasting play a critical role in planning effective mitigation measures and enhancing disaster preparedness and responses. In this study, we employ the Integrated Land Simulator (ILS) coupled with the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) to evaluate the effects of typhoon intensity modification on flood damage mitigation associated with Typhoon Hagibis.

To systematically assess uncertainties in typhoon forecasts, we conducted ensemble simulations consisting of a control run and ten ensemble members. The results show that the spatial distribution of heavy rainfall and flooding is closely linked to the typhoon track. When the typhoon track shifted westward, heavy rainfall and flooding expanded over southwestern Japan. In contrast, eastward shifts in the typhoon track led to increased heavy rainfall and flooding in central Japan, with particularly strong impacts over the densely populated Kanto region.

To further investigate the effects of typhoon intensity modification on flood damage mitigation, the central pressure of the typhoon was artificially increased by 1 to 15 hPa at 1-hPa intervals on 10 and 11 October.  These intensity modification experiments demonstrate that human intervention generally led to reductions in heavy rainfall and flood damage across Japan. Moreover, modifications applied on October 10 resulted in greater reductions in both heavy rainfall and flood damage than those applied on October 11.

These findings highlight the critical importance of both the intensity and timing of human intervention in influencing flood risk. By simulating different modification intensities and timings and explicitly evaluating the role of weather modification, this study advances our understanding of flood hazards and provides valuable insights for improving disaster preparedness and flood mitigation strategies.