臺灣周邊地形效應對西行颱風路徑與強度變化的影響: 海氣耦合模式之數值研究;The Impacts of Topographic Effects in the Vicinity of Taiwan on Track and Intensity Changes of Westbound Typhoons: A Numerical Study Using Atmosphere-Ocean Coupled Model

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Title:	臺灣周邊地形效應對西行颱風路徑與強度變化的影響: 海氣耦合模式之數值研究;The Impacts of Topographic Effects in the Vicinity of Taiwan on Track and Intensity Changes of Westbound Typhoons: A Numerical Study Using Atmosphere-Ocean Coupled Model
Authors:	李典宜;Li, Dian-Yi
Contributors:	大氣科學學系
Keywords:	地形效應;海氣交互作用;颱風;Topographic Effects;Air-sea Interaction;Typhoon
Date:	2018-07-20
Issue Date:	2018-08-31 11:24:47 (UTC+8)
Publisher:	國立中央大學
Abstract:	包括陸地地形與海洋在內的地形效應對熱帶氣旋的強度和路徑變化有重要的影響。本文使用Hurricane Weather Research and Forecast (HWRF) 模式對西行颱風位於海上及登陸前的路徑與強度變化展開探討。分別分析了在海洋與臺灣地形影響下的梅姬颱風（2016）路徑變化機制與海洋熱力條件對蘇迪勒颱風（2015）之強度與結構的影響。梅姬颱風（2016）向西北方移動並登陸臺灣，其路徑在登陸前向南偏折。由於颱風路徑附近相對較低的海表面溫度，相較於使用Global Forecast System (GFS) 海表面溫度資料，使用更真實的Hybrid Coordinate Ocean Model (HYCOM) 海表面溫度分析場可以改善較觀測偏北的颱風路徑模擬。相對偏暖的GFS海表面溫度則導致向北的路徑誤差以及颱風強度的高估。當大氣模式與Princeton Ocean Model 耦合，南海海表面溫度增暖導致路徑北偏，而颱風-海洋交互作用所導致的颱風附近上層海洋降溫則引起颱風路徑南偏。無論路徑偏差如何，颱風路徑在登陸前都呈現南折的現象，這主要是由中央山脈的地形效應所導致的。狹管效應導致颱風中心西側的偏北風增強，從而使路徑向南偏折。位渦趨勢診斷顯示，颱風路徑南折主要是由於登陸前指向東南方的位渦潛熱加熱趨勢。當颱風更接近臺灣時，潛熱加熱作用與氣旋式旋轉的波數一不對稱位渦垂直平流共同導致颱風路徑向南偏折。此外，中央山脈北部的波數一不對稱潛熱加熱垂直梯度呈負值，對登陸前的颱風移動有減速作用。蘇迪勒颱風（2015）穿過多個海洋中尺度渦旋，這些渦旋的存在可以影響颱風導致的海表面降溫強度與結構。由於更真實的海表面溫度與海洋渦旋等海洋條件，在耦合模式中使用HYCOM海洋分析場可以改善蘇迪勒颱風的強度模擬。颱風路徑附近更真實的海洋冷渦使模式中對颱風快速減弱的模擬得到改善。另一方面，颱風登陸前的再增強則是由於海洋暖渦的存在抑制了颱風導致的海表面降溫。當颱風穿過海洋冷渦，在海-氣耦合試驗中由於更強的颱風冷尾跡區上層海洋降溫，接近海面的大氣虛位溫量值與熱力學颱風邊界層厚度在颱風移動方向的右後象限明顯減少，而颱風流場的內流角則在該區域增大。此外，颱風移動方向右側的內流層厚度在眼牆附近有所增大，這一結構變化可能對颱風強度有積極的影響。海洋冷渦區更強的海表面降溫引起的颱風結構變化之影響予待進一步研究。 ;Topographic effects play an important role in the track and intensity changes of Tropical Cyclones. In this study, the Hurricane Weather Research and Forecast system (HWRF) was used to investigate the mechanism of the track and intensity changes over free ocean and track deflection near landfall. The influences of ocean SST and Taiwan terrain on track changes of Typhoon Megi (2016) and the influences of ocean thermal conditions on intensity and structure of Typhoon Soudelor (2015) are discussed respectively. Typhoon Megi (2016) headed northwestward toward Taiwan with southward deflection near landfall. HWRF simulations using more realistic Hybrid Coordinate Ocean Model (HYCOM) sea surface temperature (SST) analysis improve the northward-biased track compared with that using Global Forecast System (GFS) SST, due to the initial cooler SST below the storm path. The initial warmer GFS SST leads to a northward track shifting with an over-intensified typhoon. As the Princeton Ocean Model is coupled, the SST over South China Sea (SCS) becomes warmer leading to a northward track shifting compared to a southward shifting induced by the upper ocean cooling due to the typhoon-ocean interactions in the vicinity of the typhoon. Regardless of track shifting, southward deflection near landfall is mainly controlled by orographic effects of the Central Mountain Range (CMR). Cyclonic northerly is enhanced to the west of the typhoon center due to flow channeling that results in southward deflection. Diagnostics of potential vorticity (PV) tendency budget indicates that southward deflection can be explained by the southeastward tendency of latent heating effects near landfall. The combined effects of latent heating and cyclonic rotation of positive wavenumer-1 (WN-1) PV vertical advection dominate the southward deflection when the typhoon is closer to Taiwan. Furthermore, the typhoon movement near landfall is slowed down mainly due to WN-1 negative vertical differential latent heating over the northern CMR. Typhoon Soudelor (2015) passes serval sizeable preexisting ocean mesoscale eddies which can influences the intensity of typhoon-induced SST cooling. We investigate the typhoon intensity and structure when typhoon translates over different ocean thermal structures. Coupled to HYCOM analysis data can improve intensity simulation due to the more realistic ocean conditions including SST and ocean eddies. The existence of more realistic cold core eddies (CCE) below the typhoon path improves the simulation of rapid weakening of typhoon at the earlier stage. On the other hand, the re-intensification of typhoon at later times prior to landfall can be attributed to the preexisting warm core eddies which restrain the typhoon-induced upper ocean cooling. When typhoon moves over the CCE, the virtual potential temperature near the surface and the depth of thermodynamic typhoon boundary layer at the rear-right quadrant of typhoon translation is largely decreased in the coupled experiment due to the strong cold wake, while the inflow angle is enhanced at this region. Besides, the depth of inflow layer is larger near eyewall at the right-hand side of typhoon moving direction in coupled experiments which may be favorable to the intensification of typhoon. The effects of these structure changes induced by strong SST cooling over ocean eddies are worthy of further study.
Appears in Collections:	[Department of Atmospheric Sciences and Graduate Institute of Atmospheric Physics ] Department of Earth Sciences

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