| dc.description.abstract | The shallow coastal regions in the South China Sea often have warmer water from the surface to the bottom during the summer season. This lack of cold water restricted the vertical mixing effect caused by typhoons, leading to the maintenance or sudden strengthening of typhoon intensity before landfall, resulting in severe disasters along coastal areas. This study useed the WRF model to simulate 30 typhoons that passed through the shallow coastal regions of the South China Sea from 2003 to 2018. By experimentally modifying the Sea Surface Temperature (SST) in the shallow water regions, the significance of SST cooling and shallow water regions on typhoon landfall intensity was quantified. The study also investigated whether these phenomena would impact the atmospheric environment and alter the internal structure and development of typhoons.
The study is divided into two parts. The first part focuses on validating the simulations of 30 typhoons using control experiments. The results demonstrate that the simulated typhoon tracks closely match observations, and the WRF model improves the issue of low-intensity typhoons commonly found in global reanalysis data. Overall, the model exhibits a certain level of accuracy in capturing typhoon development. Additionally, despite there are some errors between the simulated and observed atmospheric environments in the early stages of the simulations, these errors were already present in the early stages of the simulations and not sudden anomalies. Furthermore, by examining the relationship between vertical wind shear and typhoon intensity and comparing it with previous studies, it was found that the model exhibits characteristics of stronger typhoons being able to resist stronger wind shear, which is consistent with previous research. This indirectly validates the reliability of the model in simulating the atmospheric environment.
The second part quantifies the importance of shallow water effect and SST cooling on typhoon landfall intensity and examining the changes in the atmospheric environment and internal development under these two phenomena. The results indicate that if there is no coastal SST cooling the average typhoon landfall intensity would intense by 6.7 ± 5.6 kt. Furthermore, the presence of shallow water regions limits the mixing of cold water, resulting in higher SST and an average landfall intensity increase of 7.5 ± 6.3 kt. When SST in the shallow water region is altered due to cooling or the influence of shallow water effect, it also affects the surface meteorological fields, primarily concentrated near the planet boundary layer height (~500m). At the typhoon scale, changes in meteorological fields beneath the typhoon reflect alterations in its structure and development process. On average, the presence of shallow water effect prevents SST from decreasing by 1.87°C, increases heat flux by 137.9 W m-2, strengthens the vertical secondary circulation near the eyewall by 0.044 m s-1, and raises the heating near the typhoon center by 0.28 K. These processes ultimately lead to a typhoon landfall intensity increase of 7.5 ± 6.3 kt, as mentioned earlier. | en_US |