An improved azimuth-dependent Holland model for typhoons along the Zhejiang coast prior to landfall based on WRF–ARW simulations

The Holland model is a common and efficient parametric model for constructing the typhoon wind field and involves two critical parameters: the radius of maximum wind (RMW) and the Holland B parameter, which are sensitive to the azimuth of a typhoon prior to landfall. However, their azimuth dependencies cannot be well addressed in the original mathematical expression. This study developed a framework for improving the Holland model by considering the azimuth-dependent RMW and Holland B parameter for typhoons along the Zhejiang coast prior to landfall. The Weather Research and Forecasting (WRF)–Advanced Research WRF (ARW) model was applied to construct the precise database of wind fields for six historical typhoons that passed through the Zhejiang coast. The azimuth-dependent functions were proposed to describe the asymmetry of the RMW and Holland B parameter, of which the coefficients were obtained by regression analyses of the precise database of historical typhoons. The improved azimuth-dependent model was then illustrated to assess the coastal hazards along Zhejiang Coast, including wind speeds with 50- and 100-year return periods and storm surges. The asymmetries of the RMW and Holland B parameter were expressed as functions of the heading direction and latitude of the typhoon center, respectively. The improved model showed good agreement with the observations during Typhoon Winnie (9711), Prapiroon (0012), and Khanun (0515). Including azimuth dependency in typhoon wind fields can provide more specific and reliable information in estimating coastal hazards under typhoons.