Numerical Modelling Of Typhoon Waves In The South China Sea And Waves In The Pacific Ocean

Surface waves have a direct impact on oceanic and atmospheric boundary layer structures,and the variations in the surface waves are important in oceanic and atmospheric dynamics.At present,wave model has developed to the third generation.Among them,WAVEWATCH III offers a high degree of precision and is very stable,and has become widely used in practical marine forecasts.At first,we use WAVEWATCH III to study the physical mechanism of typhoon waves in South China Sea.Buoy-based observations of surface waves during three typhoons in the South China Sea were used to obtain the wave characteristics.With the local wind speeds kept below 35 m/s,the surface waves over an area with a radius five times that of the area in which the maximum sustained wind was found were mainly dominated by wind-wave components,and the wave-energy distribution was consistent with fetch-limited waves.Swells dominated the surface waves at the front of and outside the central typhoon region.Next,the dynamics of the typhoon waves were studied numerically using a state-of-the-art third-generation wave model.Wind forcing errors made a negligible contribution to the surface wave results obtained using hindcasting.Nearrealistic wind fields were constructed by correcting the idealized wind vortex using in situ observational data.If the different sets of source terms were further considered for the forcing stage of the typhoon,which was defined as half inertial period before and after the typhoon arrival time,the best model performance had mean relative biases and root-mean-square-errors of-0.7% and 0.76 m,respectively,for the significant wave height,and-3.4% and 1.115 s,respectively,for the peak wave period.Different sets of source terms for wind inputs and white-capping breaking dissipation were also used and the results compared.Finally,twin numerical experiments were performed to investigate the importance of nonlinear wave–wave interactions on the spectrum formed.There was evidence that nonlinear wave–wave interactions efficiently transfer wave energy from high frequencies to low frequencies and prevent doublepeak structures occurring in the frequency-based spectrum.Secondly,we use WAVEWATCH III to study the temporal and spatial variations in the wave fields in the Pacific Ocean.In previous studies in which Pacific Ocean wave simulations have been established,rather more attention has been paid towave variations in small parts of the Pacific Ocean than to the ocean as a whole.In this dissertation,the temporal and spatial variations in the wind and wave fields in the Pacific Ocean between 2002 and 2011 are analyzed using WAVEWATCH III.The model performance for a significant wave height is validated using in situ buoy data.The results show that the wave model effectively hindcasts the significant wave height in the Pacific Ocean,but the errors are relatively large in the mid-and low-latitude regions.The spatial distributions and temporal variations in a wind speed and the significant wave height in the Pacific Ocean are then considered after dividing the Pacific Ocean into five regions,which show meridional differences and seasonal cycles.Regional mean values are used to give yearly average time series for each separate zone.The high latitude region in the Southern Hemisphere had a stronger significant wave height trend in the model results than regions at other latitudes.The sources and sinks of wave energy are then investigated.Their regional mean values are used to quantify variations in surface waves.Finally,the spectral analyses of the daily mean wind speeds and the significant wave heights are obtained.The significant wave height and the wind speed spectra are found to be connected in some ways but also show certain differences,and variation trends are not the same different regions.Previous researches showed that,the parameterization scheme taken account of waves can improve the existing wave model to some extent.Some climate studies using ocean-atmosphere coupled model have shown that,the climate wave simulation can get a certain degree of improvement if the effects of waves are considered.The results of this dissertation not only assess the existing wave model hindcast skills,but also reveal the temporal and spatial variations in regional wave fields,providing direct reference of the synoptic-scale wave modelling and the study of distribution characteristics of regional wave fields.Primary discussion for the influence of waves on the air-sea momentum flux is progressed based on the previous parameterization schemes considered the two-dimensional wave spectrum.We calculate and discuss the influence of waves on the air-sea momentum flux by ideal cases and observed data.The results indicate that parameterization schemes considered the two-dimensional frequency-direction wave spectrum can reduce the air-sea momentum flux exchanges.