利用氣候變遷故事鏈探討臺灣北部冬季極端降水

Abstract

在北台灣，冬季東北季風與地形的交互作用之下，有出現致災性的極端降水事件之可能性。然而，氣候模式面臨解析局地氣候變遷的困難性，尤其是在複雜的地形之下，局地環流的特徵及降水的機制在氣候模式之下不容易掌握，也導致了在過去區域氣候研究上的不確定性增加。在這篇研究當中，我們使用了氣候變遷故事鍊，透過分析現今氣候資訊及觀測資料中，控制北台灣冬季降水型態及強度的重要因子，並理解在氣候變遷之下，這些重要因子如何透過物理機制被影響，以理解未來的北台灣極端降水可能的變化。 為了區分北台灣地區的降水型態，我們使用階層式聚類分析，針對25年間秋季及冬季的網格化觀測降水資料進行分類。聚類分析結果顯示出四種不同的降水型態，並且在其中兩個型態當中具有較高頻率的極端降水事件出現，分別為宜蘭 (YL) 及宜蘭 – 北海岸 (YL-NC) 兩個類型。四個聚類分別顯示了不同的綜觀天氣條件及東北季風上游垂直結構中的熱力學及動力學特徵。研究結果顯示，邊界層中的垂直積分水汽輸送（IVT）和估計逆溫強度（EIS）是影響臺灣北部地區降水型態差異的關鍵因素，通過不同的流場型態和不同的層積雲活躍程度，能夠對區域的環流造成影響，進而控制臺灣北部的降水模式。在研究中使用臺灣地球系統模式 (TaiESM1) 中共享社會經濟途徑 (SSP5-8.5) 的情境進行對未來北台灣極端降水控制因子的評估。結果顯示，在本世紀初至世紀末，邊界層中的垂直積分水汽輸送有隨全球暖化而出現較高頻率的高水氣傳送事件，且傳送方向偏向北風的頻率具有增加的情形。同時，估計逆溫強度的增加也指示了大氣穩定度的提升，並改變層積雲的活躍程度。在未來，北台灣地區冬季受到環境特徵改變的影響，增加極端降水的發生頻率，並且在北海岸地區可能出現較顯著的提升。 為了釐清北臺灣地區宜蘭及北海岸地區的降水機制，我們進行理想化 TaiwanVVM 的初步模擬。分析結果顯示，北海岸地區的山地地形在穩定的逆溫層環境之下引發亂流，形成活躍的層積雲並造成降水；宜蘭地區則由於東北季風無法通過中央山脈及雪山山脈，在宜蘭平原地區形成阻擋流，在和環境的東北季風輻合後形成對流，造成宜蘭地區的降水。

In northern Taiwan, extreme winter precipitation caused by the interaction between the northeast winter monsoon and topography can lead to disasters. However, the climate model has difficulties to resolve the complex circulation and the precipitation process on the local scale, especially under complex topography. In this study, we employ the storyline approach to analyze the factors that control rain patterns and precipitation intensity in northern Taiwan during winter and assess how climate change may alter these factors. To classify the rain patterns in northern Taiwan, we apply hierarchical agglomerative clustering to gridded daily precipitation data observed during the fall and winter seasons in 25 years period. The clustering results reveal four distinct rain patterns, with a higher occurrence of extreme events in Yilan (YL) and both Yilan and the north coast (YL-NC). These four clusters offer insights into the varying synoptic weather regimes and the dynamic and thermodynamic characteristics of the vertical structure upstream. The results showed the integrated vapor transport in the boundary layer (IVT) and estimated inversion strength (EIS) controlled the rain patterns in northern Taiwan by the different flow regimes and stratocumulus activity. The extreme precipitation in the future can be evaluated with the change of controlling factors in the climate model. For Taiwan Earth System Model (TaiESM; Lee et al., 2020) Version 1 in the Shared Socioeconomic Pathways (SSP5-8.5) scenario, the IVT showed an increased frequency of northerly and high transport events from the early century to the late century. Also, the higher EIS indicated increasing stability in the late century. The changes in factors related to dynamics and thermodynamics suggest an increased frequency of extreme precipitation for the northern coast in the future. From the preliminary results of the idealized TaiwanVVM simulation, the mechanism of precipitation in Yilan and the north coast can be clarified. The analysis showed that the precipitation on the north coast was caused by the terrain-induced turbulence below the inversion layer, which is different from the convergence of prevailing wind and the block flow in Yilan.