水文氣候條件對熱浪–都市熱島協同作用之調控

Abstract

熱浪為氣溫異常偏高的極端事件，可能對自然環境與人類社會造成多方面的負面影響。在熱浪發生期間，由於都市熱島效應的作用，都市地區的氣溫通常會比周圍的鄉村地區更高，也因此更容易遭受到熱浪事件的衝擊。除此之外，過去研究指出，熱浪與都市熱島之間可能存在協同作用（synergy），當兩個現象同時發生時，會產生比兩者單獨作用的線性相加還要更強的高溫異常。隨著全球都市人口持續增加，理解熱浪–都市熱島協同作用的形成機制，對於提升都市居民的健康及制定有效的調適策略而言相當重要。然而，熱浪–都市熱島協同作用是否存在因城市而異，且目前仍缺乏一套完整的物理機制，能同時解釋此協同作用在部分城市出現而在其他城市缺失的原因，因此有待進一步研究。 本研究利用臺灣歷史氣候重建資料（TReAD）探討臺北、臺中與高雄三個主要城市中熱浪–都市熱島協同作用的形成機制。利用TReAD提供的多項氣候變數，我們建立了降水量、土壤濕度，以及潛熱與可感熱通量分配之間的物理連結，以解釋都市與鄉村地區在熱浪期間的增溫幅度差異。在臺北，降水的季節分佈相對均勻，有助於維持較高的土壤濕度，使其鄉村地區在熱浪期間能夠增加潛熱通量，而都市地區的不透水地表限制了潛熱通量，使可感熱通量增加較多，都市和鄉村地區的通量差異進而導致熱浪–都市熱島協同作用。反觀臺中與高雄，這兩個城市的降水較集中於夏季，全年多數時間的土壤濕度也因此偏低。當熱浪發生時，水分不足限制了鄉村地區潛熱通量的增加，使該地區的通量特徵與城市地區類似、以可感熱通量為主。這使得都市和鄉村地區之間的差異較小，因此削弱甚至抑制了熱浪–都市熱島協同作用的形成。 本研究進一步利用英國哈德里中心整合地面觀測資料庫（HadISD），分析全球118對都市與鄉村測站，發現降水集中於夏季的城市同樣較不易出現熱浪–都市熱島協同作用，驗證了臺灣城市案例中所揭示之降水季節分佈的重要性。最後，透過分析複合乾旱熱浪事件（compound drought–heatwave events），並引入能量限制（energy-limited）與水分限制（water-limited）的概念架構，我們指出熱浪–都市熱島協同效應主要由都市與鄉村在潛熱通量調節機制上的差異所驅動。整體而言，本研究提出一套能同時解釋熱浪–都市熱島協同作用發生與否的物理機制，強調降水季節分佈與地表能量通量分配所扮演的關鍵角色。

Heatwaves (HWs) are events of abnormally high air temperature that may cause adverse impacts. During HWs, urban regions are more vulnerable due to the urban heat island (UHI) effect, which generally puts cities under larger heat stress. HWs are also shown to have synergistic interactions with UHIs, amplifying the added heat stress in urban regions compared with rural regions. Given the large and expanding urban population, understanding HW–UHI synergy is essential for the well-being of city residents and the development of mitigation strategies. However, whether HW–UHI synergy exists is shown to be dependent on cities, and further research is necessary due to the absence of a comprehensive mechanism for this phenomenon. Using the Taiwan Re-Analysis Downscaling dataset (TReAD) over Taiwan, we investigated the mechanisms behind HW–UHI synergy in three major cities: Taipei, Taichung, and Kaohsiung. Taking advantage of the TReAD’s broad set of variables, we established a physical mechanism linking precipitation, soil moisture, and the partitioning of latent and sensible heat fluxes, which affects how urban and rural regions respond to HWs. In Taipei, evenly distributed precipitation maintains soil moisture, allowing rural latent heat flux to increase during HWs, while impervious urban surfaces restrict additional latent heat flux. This contrast enhances urban sensible heat flux, producing daytime HW–UHI synergy. In Taichung and Kaohsiung, precipitation is more concentrated in the summer, and soil moisture remains low during much of the year. When HWs occur, this insufficient soil moisture limits rural latent heat flux, thereby suppressing HW–UHI synergy. Extending this analysis globally using in-situ observations from the Hadley Centre Integrated Surface Database (HadISD), we examined 118 urban–rural pairs and found that summer-concentrated precipitation similarly inhibits HW–UHI synergy, confirming the critical role of precipitation timing and distribution identified in Taiwan. Finally, by analyzing compound drought–heatwave events with an energy-limited and water-limited framework, we show that HW–UHI synergy is driven by urban–rural contrasts in latent heat flux regimes. Overall, our results provide a unified physical mechanism explaining both the presence and absence of HW–UHI synergy, highlighting the critical roles of precipitation seasonality and surface flux partitioning.