Uncertainty of extreme storm surge estimation by high wind sea surface drag coefficient and future typhoon change

Japan has been constructing long coastal defense since the storm surge disaster with a loss of 5,000 lives by Typhoon Vera in 1959. The defense is designed for the storm water level including the storm surge of the standard typhoon based on Typhoon Vera. Stochastic typhoon model, simulating various typhoon track and intensity with Monte Carlo method, is one of useful tools to estimate the return period. According to recent research output the return period of the storm surge of the standard typhoon is near 100 years or more at three major bays in Japan. But there is uncertainty by some of parameters and models in the stochastic simulation. Sea surface drag coefficient under high wind speed and future change in typhoon intensity and track are critical to extreme values of the storm surges.