Study On The Simulation Of Super Typhoons' Boundary Layer Based On The Nudging Data Assimilation Method

Typhoon is a natural disaster with high frequency and serious damage.Located in the west bank of the Pacific Ocean,China is one of the countries affected by typhoon seriously.The landfalls of super typhoon can cause significant economic loss and bring hazard to people's production and life.In the field of civil engineering,in order to analyze the disaster-causing mechanism of the structure under the action of typhoon,as well as to enhance the wind resistant capability of buildings,the restoration of typhoon wind field in the atmospheric boundary layer becomes necessary.However,the commonly used Computational Fluid Dynamics method in wind engineering can only solve micro-scale wind,in which the boundary conditions are hard to determined.On the other hand,the observation of typhoon data is difficult to obtain.On this background,reanalysis data based on a variety of observations and the commonly used mesoscale model for regional downscaling provide the possibility for this restoration.In order to determine the main factors affecting results from mesoscale model,the WRF(Weather Forecast and Research)Model is used for simulation.First,the comparison tests of the moving nest and the basic nest are set up to check their applicabilities,as well as the influence of the resolution on simulated wind field.With respect to the physical processes,we mainly focus on the heat exchange and the boundary layer process.Results show that coupling the OML(Ocean Mixed Layer)model can effectively improve typhoon intensity,and physical schemes can also impact the results but it's far from deterministic.Moreover,results also show that the tracks and intensities are highly correlated.As the errors of track are large,the errors of intensity will also increase.In order to restrict the model's error,the nudging method is adopted for the simulation of typhoon Rammasun and Mujigae.Based on the characteristic of nudging method,a conception of ‘Optimal Nudging Strategy' is proposed.First,we optimize the size of horizontal domain,and then change the nudging strength and nudged variables for different cases.Moreover,both grid and spectral nudging are used for all the cases.By turning on the nudging,results show that track errors will decrease as the domain size become s smaller.By increasing nudging strength,track errors will also decrease.For the choice of variables,nudging both velocity and temperature is better than nudging velocity alone.Finally,spectral nudging performs better for the improvement of typhoon track,which indicates its good applicability in tropical cyclone downscaling.The end of this thesis focus on the wind engineering application.Typhoon Hato is simulated using the foregoing method.In this section,the asymmetric structure for both dynamic field and thermodynamic field of typhoon is confirmed.To investigate how the model can restore the wind field at various height,the field data from the 350-meter-high meteorological tower in Shenzhen is used,which include wind speed,wind direction,and wind profile index ?.We find the errors of wind speed and wind direction grow larger for the near-ground region,and ? also tends larger as distance from typhoon center becomes bigger.Finally,greater horizontal wind shear seems to appear around the typhoon center and steep topography area,which needs to be taken care of.