Air-sea Momentum Exchange Law Under Extreme Wind Conditions And Its Effects On The Simulations Of Tropical Cyclone,Storm Wave And Surge

Typhoon(or hurricane)is one of the most frequent,deadliest and costliest natural disasters.Once it makes landfall in coastal areas,the damaging wind,and its induced storm surge and wave will threaten life and property securities.As a significant impact factor during typhoon process,the air-sea momentum exchange(also named as wind stress)generates storm surge and wind wave,and it also impedes typhoon from continuously strengthening.Therefore,the simulation accuracy of disaster forecast largely depends on the reasonability and validity of wind stress evaluation methods.The thesis is aimed to investigate the air-sea momentum exchange law under extreme wind conditions,and to systematically analyze its effects on the numerical results of typhoon intensity,storm surge and wave height.Based on the momentum and energy conservation equations inside the atmospheric wave boundary layer,a wind stress evaluation model(called WBLM)for typhoon conditions is established.The model takes into account the physical details of the wind-wave interaction process as well as the energy dissipation due to the stratification effect of suspended sea spray.The computed wind stress coefficientC_d increases with the wind speed at a moderate range and peaks when the wind speed reaches 40 m/s.After that,C_d gradually decreases.This result shows fairly good consistency with the recent field observations.The atmosphere-ocean coupled model WRF-ROMS is employed to simulate the hurricane processes of Katrina(2005)and Matthew(2016).When compared to the uncoupled model,the coupled runs yield more reasonable results on the evolution of the sea surface temperature and hurricane intensity.Three momentum flux schemes are examined in the simulations.It is found that atmospheric near-surface wind speed is highly sensitive to the parameterization ofC_d,while relatively weaker sensitivity is found for the hurricane track.The scheme with the lowestC_d at high wind speeds produces the most intense hurricane,particularly for the maximum 10-m wind speed.Among the coupled runs in both hurricane cases,the WBLM-based flux option yields the most consistent intensity with the observations,which indicates that it has certain applicablity for typhoon forecast.The surface wave model SWAVE is employed to simulate the storm waves under the forcing of Katrina.When the wind stress evaluation model WBLM is embedded into SWAVE,the simulated significant wave height agrees well with the observed data,which also shows its applicability for wave modeling.Then five wind stress schemes are investigated.The results in deep water areas display large anomalies of wave height around the hurricane core among different schemes.However,the waves inside the shallow water regions are insensitive to C_d.They are mainly controlled by the depth-induced wave breaking.The wave model SWAVE and the wind stress evaluation model WBLM are both coupled to the ocean model FVCOM to simulate the storm surge under the forcing of Katrina.The surge evolutions are in good consistency with the observations measured by tide gauges.When the effect of the wave radiation stress on the current is considered in the model,the surge is larger around the regions with the rapidly changing bathymetry.Further investigation shows that different values of C_d at extreme wind speeds cause significantly different surge at the areas where the surge is originally large,while make little impact on the areas where the wind forcing is relatively weak.