颱風利奇馬 Lekima(2019)通過臺灣的數值研 究:地形對不同颱風路徑的影響

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題名:	颱風利奇馬 Lekima(2019)通過臺灣的數值研究:地形對不同颱風路徑的影響
作者:	沙聖浩;Sha, Sheng-Hao
貢獻者:	大氣科學學系
關鍵詞:	FV3;颱風利奇馬(2019);FV3;Typhoon Lekima(2019)
日期:	2021-01-25
上傳時間:	2021-03-18 16:40:48 (UTC+8)
出版者:	國立中央大學
摘要:	本研究利用 TFV3 全球模式模擬颱風利奇馬 Lekima (2019)，於 2019 年 8 月 8 日至 9 日在接近台灣北部海面出現的路徑偏折現象。於多組不同物理參數化實驗，模式皆能掌握西北向颱風靠近臺灣地形時路徑出現向北後、再向西的偏折現象。當颱風出現向北偏折時，未移除臺灣地形的實驗顯示颱風環流經過臺灣地形南側繞流後，於颱風中心南側輻合並延伸至颱風內核區域，導致颱風徑向入流明顯增強，使角動收收支中對稱水平平流及科氏力力矩於颱風底層之貢獻增強，但又受到臺灣地形背風側風速較弱，於半徑 2.5 度外產生不對稱之負切向動量平流，進而導致颱風內核切向風速明顯增強，在半徑 2.5 度外增強不明顯。在動能收支中亦受到底層徑向入流增強，對稱平均徑向水平平流及平均壓力梯度力於底層的正趨勢貢獻更明顯，亦有利於颱風強度維持。渦度收支不對稱量分析指出颱風移動主要由水平平流項主導，當颱風接近地形時，有、無地形時的波數一駛流風場出現繞行渦旋中心的一對迴流(gyre)，使水平平流產生的渦旋移動方向產生氣旋式旋轉，使颱風路徑出現向北、後再向西偏折。利用理想化 WRF 進行路徑偏折機制的敏感度實驗探討，顯示出在較弱的西向駛流風下，當橢圓型山脈長軸向西旋轉 50 度時(似西北向侵臺颱風，情境一)，颱風初始縱軸位置愈靠近地形中心時，路徑於北偏折程度愈大，當颱風初始縱軸位置較地形中心以南，路徑將出現明顯北偏，過地形後又出現明顯南偏，若距離 450 公里以上，偏折現象則不明顯。當駛流強度增至兩倍時，路徑偏折明顯減弱。當橢圓形山脈不旋轉時(似西行侵臺颱風，情境二)，若初始位置於地形中以北時，颱風於地形上游處略微北偏，過地形中心後出現向南偏折，其中登陸前夕颱風中心位置近靠地形中心時，會受渠道效應影響出現明顯南偏，若颱風初始縱軸位置位於地形中心以南，於地形上游則會傾向於南偏，過地形後再出現北偏。當橢圓地形長軸旋轉 90 度時(似北向侵臺颱風，情境三)，若颱風初始位置於地形中心以南，於地形上游時出現輕微向南偏折，於更靠近地形時各組實驗均出現北偏，其中初始位置較靠近地形中心之颱風會被地形吸引出現南偏。 I 進一步使用位渦分析並指出，於情境一下，當颱風中心位於地形以南，颱風靠近地形時，水平平流項增強會出現延遲且非絕熱加熱項與其作用反向，導致路徑偏折較晚出現，當颱風中心於地形以北，水平平流項增強時間將提早並造成颱風移動先北偏、後南偏，非絕熱加熱項會指向遠離地形方向，減緩北偏作用。於情境二下，颱風在靠近地形時，水平平流項亦出現先減弱、後增強，且非絕熱加熱作用抵銷其作用，導致偏折不明顯。在登陸前夕，若颱風位置靠近於地形中心，會出現渠道效應，且非絕熱加熱項與垂直平流項明顯增強，導致颱風大幅向南偏折，過地形後，若颱風初始位置於地形以南，水平平流項會導致颱風向北移動，反之，初始位置於地形中心以北時，則會向南移動。在情境三下，當颱風接近於地形時，水平平流項出現向北分量明顯增強並主導颱風向北移動，其中當颱風初始位置於地形中心以南時，非絕熱加熱項會指向北，當颱風初始位置於地形中心以北時，非絕熱加熱項會指向南，並與水平平流作用反向減緩颱風移動。;The global model FV3GFS is used to simulate Typhoon Lekima (2019) with track deflection when approaching near Taiwan from the southeast during 8th to 9th August 2019. FV3GFS successfully captures the observed northward track deflection of the northwestward Lekima in sensitivity experiments with different physical parameterization schemes among which the old simplified Arakawa-Schubert cumulus scheme performs best. When the typhoon begins to deflect northward, its intensity is enhanced with split flow around the southern end of the topography to converge with the southern flank of the typhoon vortex, thus producing intense southerly radial inflow into the inner vortex at low levels and causing the vortex to move northward, which enhanced typhoon intensity of inner core caused by the radial horizontal advection of mean AM, mean Coriolis force term in angular momentum budget and mean radial advection of MTKE (RMTKE), work rate by mean radial pressure gradient force (WMRP) in kinetic energy budget at low levels is strengthen, but the radial horizontal advection of eddy AM and eddy radial advection of tangential momentum of TKE getting negative made typhoon intensity of outer core enhanced slowly. From the diagnostics of vorticity budget, the typhoon movement was dominated by horizontal advection that induces a pair of gyres around the vortex center in the asymmetric wavenumber-one flow difference between the simulated flow with and without Taiwan terrain, which tends to rotate the vortex motion vector cyclonically with first northward movement and followed westward movement. Sensitivity experiments using idealized WRF were used to explore how the track of an approaching typhoon will be changed in response to the topographic effects for faster and slower steering flows over a long mountain range at different orientations from the north. For the mountain at an orientation of 50o (mimicking northwestward typhoons), a northward III deflection will be induced and becomes more significant and occurs later as the initial meridional departure position of the typhoon is closer to the center of the mountain. There will be no apparent track deflection if the meridional departure exceeds 450 km.The northward track deflection is also greatly reduced as the steering flow speed is doubled to 8 m s-1. When the mountain orientation is 90o (mimicking northward typhoons), the track deflection becomes more apparent, but without a clearer time delay for larger meridional departure distances. When the mountain orientation is 0o (mimicking westward typhoons), the typhoon will first deflect northward as closing to the mountain and then southward near landfall ahead the mountain, and finally rebound back (northward) after passing over the mountain. In wavenumber-one potential vorticity budget analysis, typhoon movement was dominated by horizontal advection, but diabatic heating was also stronger and contribution of typhoon movement caused by diabatic heating was opposite to horizontal advection, which made idealized typhoon move slower.
顯示於類別:	[大氣物理研究所 ] 博碩士論文

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