受地形影響颱風的風雨關聯診斷和追蹤－模擬分析研究

Wind-Rain Relation and Trajectory Analyses of Typhoon under Mountain Range Influence, a Simulation Study

鄭曉陽(Hsiao-Yang Cheng)

指導教授 ： 李天浩

國立臺灣大學/工學院/土木工程學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.10358

摘要

本研究以垂直速度場為指標，探討垂直速度和降雨場的相互關聯，以及兩者受地形效應的影響，供雷達外延預報參考和應用。充分了解指標與強降雨的關係，亦可能對於選擇ETQPF系集成員和決定權重方式有助益，並可作為極端降雨區域雨量估計的輔助資訊。
本研究使用平滑的三維高斯地形代替台灣真實地形，聖帕颱風（SEPAT）的初始場與環境場，以Weather Research and Forecasting（WRF）模式模擬，利用模式輸出的氣象資訊，以垂直速度和強降雨為指標，探討強降雨生成的機制；其次，使用TREC（Tracking Radar Echo by Correlation）法做為追蹤手段，追蹤強降雨系統受地形影響的時空變化。
由風雨指標分析可知，在高層較大的垂直速度與低層較大的雷達回波資訊，兩者線性相關程度不高；但是高層的垂直速度越大，則低層發生強回波、強降雨的機率越高。由垂直速度與雷達回波在空間中的關聯，可以發現地形對風雨關係影響顯著，地形上方降水機制較海面複雜，使得高層的垂直速度與與低層的雷達回波相關性較差。在地形上區域的比較，迎風面的風雨相關性極佳，且迎風面為十分有利於強降雨發生的條件，在高層垂直速度相若時，迎風面在低層發生強降雨的機率越高；背風面難以產生較大的垂直速度或是強降雨，風雨關聯極差。比較山區與平地的風雨行為可知，地形上方低海拔地區風雨關聯較佳，而高海拔的降雨機制受地行效應影響較大，風雨關聯不明顯。
以高層大垂直速度區塊型態作分類，可分為點狀、帶狀、塊狀大垂直速度區塊；點狀垂直速度區塊主要出現於海上以及迎風面，伴隨著對流雨胞而產生，移動方式近於周圍水平風場；此外，點狀垂直速度區塊風雨關聯極佳，海上生成的部份，有較充足的時間發展其對流結構，故在地形上幾乎都會產生強降雨；塊狀與帶狀大垂直速度區塊主要因地形對颱風環流的影響而產生氣流輻合，產生大垂直速度而形成，其輻合角度決定塊狀與帶狀的型態；塊狀以及帶狀垂直速度移動速度低，且維持時間可能較長或是不斷生成，伴隨著低層大範圍強雷達回波區域，易造成特定地區長延時強降雨，且造成TREC在對雨胞移動場分析上的困難。在大垂直速度與強雷達回波出現的地區，可以發現強降雨行為與對流結構的發展密切相關，兩者相輔相成，互相為增強對方之條件。地形影響水平流場，除了沿山流與繞山流以外，符合特定條件的過山流會以垂直山脊的方向過山；另外，水平氣流幅合提供發生大範圍大垂直速度的條件，因此相似流型、路徑特徵的颱風，在大垂直速度出現的位置可能有相似的表現。
本研究指出，以往台灣易發生極端降雨的地區，常被認為是地形效應所影響，但是這些地區通常也是水平風場的輻合區域，故極端強降雨應為地形與風場輻合共同作用的結果。

關鍵字

地形 ； 垂直速度 ； 風場 ； 颱風 ； 降雨

並列摘要

In this study, generation mechanism of torrential rain during typhoon is studied numerically by means of vertical wind and heavy rainfall. The Weather Research and Forecasting model is applied, the smooth three-dimensional Gaussian terrain is adopted instead of complex terrain of Taiwan, and meteorology information of typhoon Sepat is regarded as initial field. Tracking Radar Echo by Correlation (TREC) method is chosen for tracking and studying spatial and temporal evolution of typhoon rainfall influenced by terrain.
According to the index of wind-rain, we found that strong vertical wind at higher level has less liner correlation with high radar echo at lower level, but stronger vertical wind has a higher possibility to occur accompanied with a higher echo. According to the connection between vertical wind and radar echo, the effect induced by terrain on the relation between wind and rain is very conspicuous, strong vertical wind might not coincide with torrential rain on the land, and others factors that could induce or reduce precipitation on the terrain; therefore, the wind-rain correlation on the sea is better than on the land. On the terrain, the relation between wind and rain on mountain range is less correlated than on plains range because precipitation mechanics are interfered by terrain efficacy more. We found that very high wind-rain correlation occurs on the windward side, as windward side is a beneficial fuel to cause strong precipitation because the possibility to cause heavy rain with the vertical wind of same strength is higher.
Strong vertical wind region at higher level can be classified into three types: spot, belt, or lump. Spots are generated by convection systems mostly and usually discovered with a convective rain cell on windward side and sea. Its movement is similar to ambient horizontal wind field. Spot vertical wind regions are always accompanied with heavy rainfall but the area of the torrential rain would be small. Spots formed on the sea are given enough time to develop the convection structure, thus likely to brought heavy rainfall when it move on the land. Belts and lumps are formed by convergence of the horizontal wind field induced by the effect of terrain, and the angle of convergence of horizontal wind would determine which type the vertical wind region is. Belt and lump regions are likely to cause hazardous rainfall on some specific place easily because they were held longer duration or generated rapidly with slower movement, and usually accompanied with strong and wide-spread precipitation on the same location. Furthermore, the occurrence of Belts and globs near or on the land can cause difficulties in TREC tracking because of special radar echo pattern and rainband movement resulted from those types. There is a very strong linkage between the behavior of heavy rainfall and the development of convection structure, and the two components can be reinforced by each other on the location with strong vertical wind and radar echo. Horizontal wind field of typhoon can be altered by the terrain effect, and some wind flow cross the mountain ridge with a specific direction perpendicular to the ridge possibly. In addition, the convergence of horizontal wind is an important condition for wide-spread strong vertical wind region to occur, and similar location and timing with the existence of strong vertical wind can be performed if the condition of path and flow style is similar in some specific typhoon cases.
Some particular places in Taiwan often sustains extreme heavy rainfall and the phenomenon was sometimes regarded as the interference by terrain and landform features like valleys or fan-shaped pockets in the past; however, those places are usually also the locations with convergence of horizontal wind field, so extremely hazardous rain is generated by the interactive combination of both terrain features and wind flow convergence effects.

並列關鍵字

terrain ； vertical wind ； wind field ； typhoon ； precipitation

參考文獻

11. 林佑蓉，2009：颱風風雨型態分析辨識與分類氣候法定量推估降雨之研究，台大土木工程研究所論文。

2. 于宜強、鄭兆尊、林李耀，2011：颱風定量降雨技術發展與改進，國家災害防救科技中心。建國百年天氣分析預報與地震測報研討會論文彙編，中央氣象局，167-172。

12. 林忠義，2005：利用雷達回波分析降雨系統三維結構之研究，台大土木工程研究所論文。

23. 鐘德霖、李天浩，2005：利用雷達回波資料與TREC方法估計降雨雨胞移動場，中國土木水利工程學刊。

24. Manfred H. Bauer, Georg J. Mayr, Ignaz Vergeiner, and Helmut Pichler，2000：Strongly Nonlinear Flow over and around a Three-Dimensional Mountain as a Function of the Horizontal Aspect Ratio，Journal of the Atmospheric Sciences, Volume 57, Issue 24。