Simulation Of Storm Surge Water Level Based On Data Assimilation Method

Storm surge is a huge disaster phenomenon that originates from strong atmospheric disturbances.The combination of storm surges and astronomical surges can lead to rapid sea level rise and cause disasters.Its impact and the adoption of early warning and preventive measures should be of great concern.Wind stress is a key factor in the formation and development of storm surge,and the Wind Drag Coefficient（C_d）has a significant impact on its nature.Therefore,it is important to understand the wind drag coefficient to study the storm surge model in depth.When numerical simulations of storm surges are performed,two methods are used to deal with the wind drag coefficient:one is to take a constant value and assume constant surface roughness;the other is to calculate the wind drag coefficient based on different forms of empirical formulas,and several different forms of wind drag coefficient expressions have been given by researchers.The wind stress drag coefficient in the storm surge model is taken as a constant or calculated according to the empirical formula is relatively easy to implement,but this approach may lead to less accurate simulation results,so the numerical simulation is not the best method.In this paper,based on the two-dimensional numerical storm surge model and the accompanying assimilation model,the wind stress drag coefficient in the form of spatial distribution is inverse performed using the data assimilation method,and the storm surge level of Typhoon 7303 is simulated.The main research content and results contain two parts.Firstly,the effects of different forms of wind stress drag coefficients on storm surge levels are investigated.Based on the two-dimensional numerical storm surge model and the accompanying assimilation model,the wind stress drag coefficients in the form of spatially distributed wind stress drag coefficients are inverse performed by using the data assimilation method.Then,the effects of the spatially distributed form of wind stress drag coefficient and three empirical formulations of wind stress drag coefficient on the storm surge levels during the occurrence of Typhoon 7303 in the Bohai,Yellow,and East China Seas were investigated.The results show that the storm surge levels calculated using the spatially distributed form of wind stress drag coefficients are closer to the observed levels than those calculated by the empirical formulas of wind stress drag coefficients,which indicates that the data assimilation method can effectively improve the simulation accuracy of the storm surge levels.Further,the influence of astronomical tides on storm surge levels during data assimilation is discussed.The results show that the storm surge level calculated based on the data assimilation method is the closest to the observed level when the split tide is considered.Therefore,the influence of astronomical tides should be considered in the simulation of storm surge.Next,the effects of wind stress drag coefficients based on constant and linear form inversions on storm surge levels are investigated.Based on the concomitant assimilation model,the effects of wind stress drag coefficients in constant form and wind stress drag coefficients linearly related to the wind speed on the storm surge levels during Typhoon7303 are investigated,and the effects of the initial values of,in the linear empirical formulation of wind stress drag coefficients on the storm surge simulation results are also examined.The results show that the effects of different parameters in the linear form of wind stress drag coefficients with and as initial values do not differ significantly on the simulation results.The simulated water levels obtained by applying Wu’s linear empirical formula wind stress drag coefficient in the data assimilation method are closer to the actual observed water levels than using the constant wind stress drag coefficient0.0026,indicating that the inversion capability of the linear empirical formula wind stress drag coefficient helps to improve the accuracy of the storm surge simulation.The study also investigates the influence of four components of the astronomical tide（M₂,S₂,K₁,₁O）on the storm surge level during the data assimilation based on the linear empirical formula wind stress drag coefficient.The experimental results show that the estimated storm tide level by the data assimilation method is the closest to the observed level when the astronomical tide component is considered.Therefore,when using the data assimilation method to invert the storm surge,a constant or linear form of the wind stress drag coefficient needs to be considered in conjunction with the astronomical tide factor.