| 摘要: | 在颱風季時，南海近岸淺水區從海表面至海底整層海水溫度通常較溫暖，由於缺乏冷水使得颱風垂直混合作用無法有效降低海溫，導致颱風登陸前強度得以維持甚至突然增強，並對沿岸地區造成嚴重災害。本研究利用WRF模式模擬2003年至2018年期間通過南海近岸淺水區之30個颱風，藉由變更淺水區海表面溫度(Sea Surface Temperature, SST)的實驗方式量化SST冷卻和淺水區對颱風登陸強度之重要性。同時也探討了SST冷卻與淺水區是否會對大氣環境產生影響，以及如何改變颱風內部結構與發展。
本文共分成兩個部分，第一部分為驗證控制組實驗所模擬的30個颱風是否合理。結果顯示颱風路徑的模擬相當接近觀測，並且透過WRF模擬的颱風能一定程度上改善再分析資料的颱風強度，因此整體而言模擬結果有掌握住颱風發展趨勢。另外也發現，模式內綜觀環境場的誤差在模擬剛開始就已存在，並非是模擬過程中突然出現的不合理現象。最後，透過計算垂直風切與颱風強度的關係並和前人研究進行比較，發現模式中強度較強的颱風有能抵抗較強風切的特徵，而前人的研究也支持相同論點，因此這間接證明了模式對於綜觀環境的模擬有一定的可靠性。
第二部分則是量化SST冷卻和淺水區對颱風登陸強度之重要性，並觀察大氣環境與颱風內部發展在這兩種作用下的變化。結果顯示，近岸SST沒有冷卻會導致颱風登陸平均強度增強6.7 ± 5.6 kt；而在淺水區重要性方面，由於淺水區缺乏冷水的混合使SST較高，導致颱風登陸時平均強度增強7.5 ± 6.3 kt。當淺水區的SST因為冷卻或限制混合作用改變時，也會影響到表面氣象場並且影響範圍集中在邊界層(~500 m)附近。將空間拉近至颱風尺度範圍能發現，颱風下方氣象場變化反映的是其結構與發展過程的改變。平均來說，相較沒有淺水區的情況，當淺水區存在時SST將上升1.87 ℃，熱通量增加137.9 W m-2，眼牆附近的垂直次環流將增強0.044 m s-1，颱風中心加熱會上升0.28 K。這些過程最終導致颱風登陸強度，如前面提到的，增強7.5 ± 6.3 kt。
;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. |
