Maximum wind radius estimated by the 50 kt radius: improvement of storm surge forecasting over the western North Pacific

Abstract. Even though the maximum wind radius (R_max) is an important parameter in determining the intensity and size of tropical cyclones, it has been overlooked in previous storm surge studies. This study reviews the existing estimation methods for R_max based on central pressure or maximum wind speed. These over- or underestimate R_max because of substantial variations in the data, although an average radius can be estimated with moderate accuracy. As an alternative, we propose an R_max estimation method based on the radius of the 50 kt wind (R₅₀). Data obtained by a meteorological station network in the Japanese archipelago during the passage of strong typhoons, together with the JMA typhoon best track data for 1990–2013, enabled us to derive the following simple equation, R_max  =  0.23 R₅₀. Application to a recent strong typhoon, the 2015 Typhoon Goni, confirms that the equation provides a good estimation of R_max, particularly when the central pressure became considerably low. Although this new method substantially improves the estimation of R_max compared to the existing models, estimation errors are unavoidable because of fundamental uncertainties regarding the typhoon's structure or insufficient number of available typhoon data. In fact, a numerical simulation for the 2013 Typhoon Haiyan as well as 2015 Typhoon Goni demonstrates a substantial difference in the storm surge height for different R_max. Therefore, the variability of R_max should be taken into account in storm surge simulations (e.g., R_max  =  0.15 R₅₀–0.35 R₅₀), independently of the model used, to minimize the risk of over- or underestimating storm surges. The proposed method is expected to increase the predictability of major storm surges and to contribute to disaster risk management, particularly in the western North Pacific, including countries such as Japan, China, Taiwan, the Philippines, and Vietnam.