以系統追蹤分析探討雲凝結核對於臺灣複雜地形夏季日夜降雨之影響

Abstract

由於與日俱增的人為活動，氣膠作為雲凝結核對於雲及降水的影響成為近年來受重視的研究議題。前人研究指出，氣膠、雲、降水交互作用的結果取決於環境型態。在本研究中，我們將環境型態聚焦於綜觀天氣影響較弱時、臺灣複雜地形之上的日夜降水系統。為了解析細微尺度的大氣物理過程，我們利用具有高解析度臺灣地形的渦度向量方程雲解析模式(TaiwanVVM)進行半真實大渦流模擬。我們從弱西南風或弱綜觀的天氣型態當中選取13個個案，並以其簡化之觀測探空資料作為模擬的初始條件。在控制組(乾淨環境)中，氣膠數量混合比為每公斤3×108；而在實驗組(一般環境)中，氣膠數量混合比增加至每公斤3×1010。13個個案的合成降水模擬結果顯示，阿里山山脈區域是臺灣島上最顯著的降水熱點，此現象與前人的觀測分析相符。我們以阿里山山脈的區域平均降水率時序變化，分辨出兩種不同的降水型態：強降水型及弱降水型；並利用「雲物件連結」及「降水系統追蹤」方法，從對流發展生命期的觀點，檢驗雲、雨特性。對於強降水型而言，雲凝結核濃度上升的影響更為顯著：有能力製造強降水的系統出現頻率提高，且其對總降雨量的貢獻增加；降水系統開始與結束的時間延後；降水系統成熟期的降雨率、降雨面積、雲厚、雲體積皆增加，並伴隨更集中且更強的雲內上升區。因此，雲凝結核濃度增加會讓複雜地形之上的夏季日夜降水系統出現「強者愈強」的反應。進一步討論兩種降水型態的差異與局部環流的關係，發現強降水型在降雨發生前有較弱的近岸底層風場。本研究呈現，半真實大渦流模擬及系統追蹤分析，對於瞭解雲凝結核濃度如何影響複雜地形之上的日夜降水，足以提供實用而嶄新的分析觀點。

The influence of aerosols, serving as cloud condensation nuclei (CCN), on clouds and precipitation becomes a highlighted research topic in recent years due to increasing human activities. Previous studies have suggested that the results of aerosol-cloud-precipitation interaction are regime-dependent. In this study, the regime of interest is focused on the diurnal precipitation over complex topography in Taiwan with weak synoptic-scale weather forcing. Semi-realistic large-eddy simulations (LESs) were carried out using the vector vorticity equation model with high-resolution Taiwan topography (TaiwanVVM) to resolve fine-scale atmospheric processes. The simulations of 13 cases were driven by the simplified observational soundings, selected under weak southwesterly flow or weak synoptic weather events. In the control groups (clean scenarios), the aerosol concentration was fixed at 3×108 kg-1 in the entire domain, while in the experimental groups (normal scenarios), the value was 100 times higher. The composite of the simulated results reveals a precipitation hotspot around Alishan Mountain Range (AMR), which is consistent with the observed climatology. Two different types of precipitation patterns by the AMR regional-averaged rain rate evolution are identified: the STRONG type and the WEAK type. By performing cloud object connecting and rain cell tracking analyses, the properties of cloud and precipitation are examined from the perspective of the life cycle of convection. Several responses due to increasing CCN are highlighted especially for the STRONG type. First, the diurnal precipitating systems with a greater ability to produce heavy rain rates occur more frequently and contribute more to the total precipitation. Also, the initiation and the ending time of the diurnal precipitating systems are delayed. Moreover, the maximum rain rate, rain area, cloud depth, and cloud size become stronger with a more concentrated and vigorous updraft in the clouds during the mature stage of the diurnal precipitating systems. An overall “strong get stronger” response to the diurnal precipitating systems over complex topography is identified with increasing CCN. The relationship between the intensity of local circulation and the precipitation patterns in the AMR region is discussed, with the STRONG type having weaker near-coast low-level southwesterly before the initiation of precipitation. This research shows that semi-realistic LES and tracking of precipitating systems provide novel and useful insights to the understanding of the responses to diurnal precipitation resulting from increasing CCN under relatively weak synoptic weather regime over complex topography.