微弱背景风在稳定分层环境下影响理想城市热岛环流的机理研究

Understanding the urban air flows in weak and calm wind conditions is crucial, as most urban heat wave and severe air pollution episodes occur when such conditions coexist with inversion, leading to formation of a heat dome or urban heat island circulation (UHIC). Such knowledge are essential for designing our built environment at the city and regional levels. At zero wind conditions, UHIC comprises a convergent inflow at the lower atmospheric level, divergent outflow at the upper, and a dome-shaped flow field resulting from entrainment and overshoot at the top. Numerous field studies worldwide have confirmed the existence of UHIC during the day and night in many cities. Most theoretical studies focus on circular or 2D heat domes. Our team has identified the effect of city shapes on the heat dome and the process of dome merging. Existing studies are limited to zero background winds. However, in the much publicized Jing-Jin-Ji haze, weak winds exist. A strong wind would destroy the dome, and breakup the inversion. However, a weak wind may only elongate the dome to become a plume or dome shadow, transport the pollutant downstream to other cities. How such a weak wind impact on the dome formation has not been studied. ..Our preliminary experimental study also identified a new shape effect for a simple square urban area: a non-uniform flow pattern with four dominant diagonal inflows at the ground level and four dominant side outflows at the upper, which has been extended to the polygon city in which the diagonal inflows are stronger at the surface level. This polygon city UHIC hypothesis is supported by the results of a 2003 field study in Beijing, a square-like city. In calm wind conditions the shape of an urban area may significantly affect the winds within a city, and thus the urban air temperature or urban haze distribution would not be uniform. This shape effect also leads to the complex merging of multiple cities, i.e. the dome cluster merging hypothesis, which is currently being studied by our team. Opportunities thus exist to optimise building density in poorly ventilated areas. As weak winds may elongate the dome, and transport it downstream, thus the interaction of multiple domes become more complex. ..Thus, we propose to study the dome formation and dome merging under weak wind conditions. We shall consider a stable stratified layer below and overlying inversion lid as commonly observed in the JJJ region haze conditions...Recognising the complexity of urban atmospheric air flows, the proposed study will first consider ideal settings and ideal city shapes, such as being round, square, triangle etc. The proposed methodologies include theoretical modelling, towed water tank modelling, large eddy simulations using a newly developed city scale method and mesoscale simulations. Our wind study will thus provide a new dataset on the impact of an urban area’s shape and morphology on city weak wind flows under ideal conditions, thereby informing urban planning at the city and regional levels, and building environment design, and paving the way for more realistic research and applications.

城市高温热浪和严峻雾霾通常发生在静风和微弱风天气中。静/弱风和逆温层的共同作用会导致城市热岛环流或者热穹窿的形成。现阶段我们已经对静风时的热岛环流机理有一定研究基础，然而，有微弱背景风时,相应的热岛环流机理尚未清楚。我们预计强风会破坏热穹窿并抑制污染环流，而微弱风则会使热穹窿扭曲变形，形成热羽流，将热和污染物传到城市下游。通过现有的研究，我们还发现对于一个多边形城市热岛环流，对角线方向的地面来流较显著，造成城市内空气温度以及雾霾的不均匀分布。城市形状的影响使得热穹窿融合机制变得更为复杂。..本课题旨在研究微弱风对热穹窿和穹窿群的影响。我们将系统研究不同形状的理想城市，并作为示例应用于具有真实形态和理想环境的方形城市。研究方法包括理论模型、改良的拖曳式水箱实验、城市尺度大涡模拟以及中尺度模拟。各类模型中均考虑大气稳定分层的天气情况。研究结果将会为将来指导城市规划和建筑环境设计提供新资料。

大多数城市高温热浪和严峻大气污染问题时常发生在静风和微弱风情况下。我们结合水箱模型，城市计算流体力学模拟和理论研究，首次研究微弱风对圆形城市热穹窿大小的影响。开展多个圆形城市产生的热穹窿流动的融合机理的研究，确定多个热穹窿可以融合的条件以及融合后的热穹窿的三维流动/温度结构。..我们发现正多边形城市，都展现出以近地面内角平分线方向入流强，远地面垂直于边方向的边出流为主的特征。当逆温层在一定厚度时，城市环流可以被解散,包括双环流，小环流翼部减缩和穿破热羽流三个过程。我们观察到两种主要的流动模式，即羽毛状和圆顶状, 确定了从羽状到该研究成果捕捉了城市尺度热羽流形态到热穹顶形态转变过程，定量确定了由热羽流到热穹顶转变的临界热源尺度。小风条件下，两个城市相互作用。两个城市的布局排列影响城市通风。当两个城市横向排列时，链流（上游城市热岛环流的出流连接到下游城市的入流）是影响两个城市间气流交换的最主要机制，可以造成污染物由上游城市向下游城市输送。上游城市热岛环流对背景风具有阻塞作用，影响下游城市的通风流量。我们研究了在2012年的一个极端城市热岛期间北京和天津上空的城市热岛环流。北京和天津的UHIC在南风作用下分别发展。由于具有多个高温地区，北京上空形成了两个热岛环流，涉及链流机制，其中近山UHIC的流出与其他UHIC沿天津方向的流入相连。在夜间，由于天津市上空的UHIC的高层流出与北京上空的UHIC的流入相关，因此出现了更强大的UHIC。由于山上的流动和下坡风造成的转子形成了北京西北部的收敛区域。特别是，我们通过用农田网格代替北京或天津市的敏感性试验，发现北京或天津市市区的存在都会改变海风环流的渗透性。城市形态对夜间UHIC的影响较大。