對流的雙重效應在台灣複雜地形下產生的夜間強降雨

Abstract

本研究旨在藉由TaiwanVVM的semi-realistic simulation結果來探究弱綜觀環境下，台灣中部以及西南部地區夜間長延時強降雨事件的發生原因及機制。 透過中央氣象局的雨量歷史資料，以及Chang et al. (2021)所篩選出的台灣夏季弱綜觀個案，長延時夜間強降雨事件大多沿著台灣中部以及西南部山腳以及河谷分布。過去研究也顯示，較早的對流會將環境調整為不同的狀態，像是加濕環境讓對流有利於持續發展；或是將對流可用位能（Convective Available Potential Energy, CAPE）消耗殆盡成對流不易發展的環境。分析結果顯示，較高比例具有長延時夜間強降雨事件的個案，其下午的熱對流持續存在之時間較長，這些持續存在的對流將濕靜能（Moist static energy, MSE）帶至二到三公里高，而建構在地面及二到三公里皆具有較高MSE的MSE雙層結構；同時在高度較高的MSE層以上建立對流不穩定以利於更晚的對流能夠發展。然而對流持續的發展有賴於其他因子。 弱綜觀的環境下，局地發展的熱對流有賴於局地環流的作用。午後熱對流透過局地環流發展成熟後產生降雨，同時伴隨下衝流，透過水滴蒸發、冰晶融化降溫，可以加強下衝流以及下衝流到地表所產生的輻散氣流或是陣風鋒面（gust front），這便是冷池（cold pool）的概念，而透過與地形、局地環流或是其他冷池的交互作用，冷池能夠激發新的對流。同時我們也發現有長延時夜間強降雨事件發生的個案中，冷池存在相當久且不斷激發新對流，也因此環境之MSE更能被調整為雙層結構。另外冷池的分布位置大多被地形鎖定在海拔五百公尺以下的台灣中部及西南部河谷中；此外，透過熱對流與地形的作用，冷池有隨時間往北邊河谷移動的現象。以上結果顯示在弱綜觀條件下，冷池能夠維持對流持續存在並且透過這些對流將環境調整為適當的熱力環境，確實能讓深夜時刻仍有對流發生。

In this study, we aim to investigate the mechanism behind a long-duration nocturnal heavy rainfall event that occurred in central and southwestern Taiwan under weak synoptic weather conditions. To accomplish this, we analyze results using semi-realistic simulations from TaiwanVVM. Based on the historical data from the Central Weather Bureau and the weak synoptic weather conditions selected by Chang et al. (2021), it has been observed that the long-duration nocturnal heavy rainfall events primarily occur along the foothills of mountains and river valleys in central and southwestern Taiwan. Previous researches have shown that preceding convection can have an impact on the environment, either impeding (through depletion of CAPE) or facilitating subsequent convection (through moistening the environment). Our study reveals that a high proportion of long-duration nocturnal heavy rainfall events are associated with a double-layer of high moist static energy (MSE) at near surface and about 2-3 kilometers above the ground brought. This double-layer of high MSE vertical structure is initiated by persistent afternoon convection. Above this high layer of high MSE is a region of convective instability (or called potential instability), which is ideal for the development of convection. Of all the semi-realistic LESs results with long-duration nocturnal heavy rainfall events, 63% of them have this double-layer of high MSE structure. However, the persistent existence of convection may be influenced by additional factors. Under weak synoptic weather conditions, local circulation plays a crucial role in the development of convection. Afternoon convection matures and starts to rain, generating downbursts and outflows or gust fronts at the near-surface layer. The downbursts cool nearby temperatures through evaporation or melting, strengthening both themselves and the outflow or gust front, leading to the formation of cold pools. The interaction between cold pools, terrain, local circulation, and other cold pools can stimulate new convection. We found that the cold pools associated with long-duration nocturnal heavy rainfall events persist for a considerable duration and trigger the formation of new convection which could create an environment with a double-layer of high MSE. Besides, these cold pools remain orographically locked in the river valleys of central and southwestern Taiwan, below an elevation of 500 meters, until late at night. Furthermore, these cold pools typically move from southern to northern river valleys through the interactions with topography and new convection. Our findings show that under weak synoptic weather conditions, cold pools play a significant role in persistently stimulating convection, thereby modifying the thermodynamic environment to favor the occurrence of long-duration nocturnal heavy rainfall events.