2009年8月8日莫拉克颱風影響台灣期間，台灣海峽出現不斷新生或近乎滯留的東西向雨帶，此雨帶進入台灣陸地後與地形產生交互作用。同時，在中央山脈南部持續出現南北走向的雨帶。這些雨帶為台灣西南部沿海地區及山區皆帶來了豪大雨以及重大災害。本研究利用七股雷達與馬公雷達以及地面雨量站觀測資料探討這些雨帶與地形之間的關係，並且利用新的多都卜勒雷達合成方法了解分析範圍內的三維風場結構。從雷達觀測資料可將台灣南部山區的降雨情形分為兩種：第一種為分析範圍北部區域，該區域的山區為較和緩的地形，而東西向雨帶在外海輻合後，強風帶將雨帶內的對流胞向東輸送至陸地後，對流胞在緩坡上受地形抬升發生增強或維持其強度；另外一種為分析範圍南部山區，地形較為陡峭，入流受地形強迫抬升後產生上升運動，且在大氣不穩定的狀態下，有助對流發展。但兩種情況下，最強回波皆發生於山頂之前，在背風坡時強度皆迅速減弱。利用地面雨量站觀測資料發現在中央山脈迎風坡(西側)的累積降雨量比背風坡(東側)高出許多，顯現中央山脈在此個案中對降雨的空間分布具有相當的關鍵性。在迎風面地區，當東西向雨帶內對流胞移入時降雨強度迅速增加，而在海拔越高的山區因有強上升運動有助對流維持或發展，平均降雨強度大於平地、沿海地區。最後，利用偏極化雷達參數定量估計降雨量，發現以馬公雷達KDP(比差異相位差)參數估計降雨強度結果最好，但在六小時降雨量估計方面，大雨會發生低估情形。On August 8 2009, a few east-west oriented rain bands of typhoon Morakot continuously re-formed or were nearly stationary in the Taiwan Strait. These rain bands interacted with the terrain when reaching Taiwan. In the same period, due to the terrain effects a north-south oriented band persistently exhibited near Central mountain ridge. These rain bands brought heavy rainfall and significant damage in the mountain area. The present study collects data from RCCG S band Doppler radar, RCMK C band polarimetric/Doppler radar and surface rain gauge observations. An advanced multiple-Doppler radar synthesis method is applied to determine the structure of the rain band in the analysis domain and to study the orographic effect on precipitation.From the results of wind field synthesis, the east-west oriented rain band was induced by convergence between the north-west flow of the typhoon’s circulation and the south-west flow from the South China Sea. A belt of strong wind associated with the rain band was strengthened by the embedded convective cells. Then this strong wind belt extended to the mountains. The radar reflectivity observations in the mountain area can be described in two conditions: (i) in the northern part of the domain where the mountains has relatively gentle slope, precipitating systems moving eastward toward land maintained their strength over the slopes; (ii) in the southern part of the domain where has steep terrain, deep convection was developed by the uplifting of the cross-barrier flow near the mountains. The strongest reflectivity happened on the windward slope of the mountain in both conditions, and weakened in the leeside. From rain gauge observations, more precipitation distributed in windward side, it reveals that Central Mountain Ridge imposes a profound influence on the spatial distribution of the rainfall. In the windward side, the rainrate increases rapidly when the cells are transported into the areas. The average rainrate is more when the height of the area is higher due to the uplifting by the terrain. Finally, we estimate rainfall quantitatively by using many dual-polarimetric radar parameters, and the KDP(specific differential phase shift ) has the better results.
