低頻渦度對熱帶氣旋形成之研究

Abstract

為了解特定環境因素影響下、熱帶氣旋形成之物理決定程度，本研究利用WRF數值模式3.2版，在相同模式設定下、模擬2008-2009年於西北太平洋形成之52個熱帶氣旋。分析時以不同時間尺度(低頻和高頻)之渦度值定量區分熱帶氣旋個案，將所有模擬個案區分為伴隨較強和較弱低頻渦度兩類，以探討數值模式對此兩類熱帶氣旋形成之模擬能力與其中之原因；研究中同時分析不同數值模式中對此兩類熱帶氣旋形成之預報程度。此外在此兩類個案中挑選14個極端個案，進行不同積雲參數化之敏感度實驗，以驗證整體模式掌握度與物理決定程度之關係；並針對2008年之Nuri颱風及2009年之Dujaun颱風，分別進行解析度及濾除高頻環境之敏感度實驗，以凸顯不同環境下熱帶氣旋形成之重要物理過程。 結果顯示，伴隨較弱低頻渦度之熱帶氣旋個案(如2008之Nuri)，模擬結果對選用之積雲參數化法及模式解析度相當敏感，即採用之積雲參數化及模式解析度顯著影響此類熱帶氣旋形成過程中、模擬之對流系統配置與強度，進而影響熱帶氣旋形成與否。反之，針對於低頻渦度較強環境下形成之熱帶氣旋(如2009年Dujuan)，模擬結果對選用之積雲參數化法及初始資料中之高頻環境場並不敏感，實驗中都可模擬出熱帶氣旋之形成，但伴隨顯著路徑誤差；而在此有利之低頻環境下，高頻系統似僅影響初期系統中心位置，並影響渦度增強過程，進而影響後續系統之渦度值。整體而言，在較強低頻渦度環境下形成之熱帶氣旋個案，模擬過程中之對流系統分布，顯著受大尺度環境影響，並將渦度集中至擾動系統，模式較易掌握熱帶氣旋之形成過程；反之，在較弱低頻渦度環境下形成之熱帶氣旋個案，近中心之對流過程是氣旋系統主要之渦度來源，因此對流過程對此類形成過程中較為重要。

The WRF Model is used to simulate 52 tropical cyclones (TCs) formed in the western North Pacific during 2008-2009 to study the influence of the specific environmental features on the degree of deterministic nature of TC formation (Vmax ~ 25kts). All simulations, using the same model setting, are repeated at four distinct initial times and with two different initial datasets. These TCs are classified into two groups based on the environmental 850-hPa low-frequency vorticity (using a 10-day low-pass filter) during the period 24-48 hours prior to TC formation, that to discuss the overall simulation results in these two groups of TC. In order to discuss the relationship between the capability of models and the and deterministic nature of TC formation and to emphasize the physical process of TC formation. The forecast results also analyzed for 52 TCs in three models, and fourteen selected TCs are further simulated to examine the sensitivity of previous results to different cumulus parameterization schemes in WRF model. Moreover, two extremely TCs (Nuri and Dujuan) are simulated to examine the sensitivity of TC formation to model resolution and high-frequency environments, respectively. Results show that the numerical models are less capable of simulating (or forecasting) the TC formation process for TCs formed in an environment with lower low-frequency vorticity (LTCs), but with smaller track errors. The simulated convection pattern and strength are very sensitive to the model resolution and the cumulus scheme used; therefore, model simulation capability for LTC depends on the cumulus scheme used. In contrast, the models are more capable of simulating (or forecasting) the TC formation process, but with larger track errors for TCs formed in an environment with higher low-frequency vorticity (HTCs). And the capability of the WRF model to simulate HTC formation is not sensitive to the choice of cumulus scheme, and also not sensitive to the high-frequency environment in initial conditions. The high-frequency systems could determine the position of an initial vortex, which decide the value of vorticity tendency and affect the strength of Dujuan in simulations. Results of this study reveal that the convection process is not a dominant factor in HTC formation, which highly controlled by low-frequency environments, but is very important for LTC formation as the major source of vorticity.