Department of Meteorology, University of Reading, Reading, RG6 6ET, UK.
Faraday Discuss. 2021 Mar 1;226:149-172. doi: 10.1039/d0fd00098a. Epub 2020 Dec 21.
Air quality in cities is influenced not only by emissions and chemical transformations but also by the physical state of the atmosphere which varies both temporally and spatially. Increasingly, tall buildings (TB) are common features of the urban landscape, yet their impact on urban air flow and dispersion is not well understood, and their effects are not appropriately captured in parameterisation schemes. Here, hardware models of areas within two global mega-cities (London and Beijing) are used to analyse the impact of TB on flow and transport in isolated and cluster settings. Results show that TB generate strong updrafts and downdrafts that affect street-level flow fields. Velocity differences do not decay monotonically with distance from the TB, especially in the near-wake region where the flow is characterised by recirculating winds and jets. Lateral distance from an isolated TB centreline is crucial, and flow is still strongly impacted at longitudinal distances of several TB heights. Evaluation of a wake-flow scheme (ADMS-Build) in the isolated TB case indicates important characteristics are not captured. There is better agreement for a slender, shorter TB than a taller non-cuboidal TB. Better prediction of flow occurs horizontally further away and vertically further from the surface. TB clusters modify the shape of pollutant plumes. Strong updrafts generated by the overlapping wakes of TB clusters lift pollutants out of the canopy, causing a much deeper tracer plume in the lee of the cluster, and an elevated plume centreline with maximum concentrations around the TB mean height. Enhanced vertical spread of the pollutants in the near-wake of the cluster results in overall lower maximum concentrations, but higher concentrations above the mean TB height. These results have important implications for interpreting observations in areas with TB. Using real world ceilometer observations in two mega-cities (Beijing and Paris), we assess the diurnal seasonal variability of the urban boundary layer and evaluate a mixed layer height (MLH) empirical model with parameters derived from a third mega-city (London). The MLH model works well in central Beijing but less well in suburban Paris. The variability of the physical meteorology across different vertical scales discussed in this paper provides additional context for interpreting air quality observations.
城市空气质量不仅受到排放物和化学转化的影响，还受到大气物理状态的影响，这种状态在时间和空间上都有所变化。越来越多的高楼大厦（TB）成为城市景观的常见特征，但它们对城市气流和扩散的影响尚未得到很好的理解，并且在参数化方案中也没有适当考虑到它们的影响。在这里，使用两个全球特大城市（伦敦和北京）内区域的硬件模型来分析 TB 对孤立和集群环境中流动和传输的影响。结果表明，TB 会产生强烈的上升和下降气流，从而影响街道层面的流场。速度差异不会随距离 TB 的距离单调衰减，尤其是在近尾迹区域，那里的流动特征是回旋风和射流。从孤立 TB 中心线的横向距离至关重要，即使在几个 TB 高度的纵向距离处，流仍然受到强烈影响。在孤立 TB 情况下评估尾流流模型（ADMS-Build）表明，重要特征并未被捕捉到。对于较细长、较短的 TB，预测结果要好于较高的非立方体 TB。在远离表面的水平和垂直方向上，预测的流动情况会更好。TB 集群会改变污染物羽流的形状。TB 集群重叠尾流产生的强烈上升气流将污染物抬出冠层，导致在集群背风侧形成更深的示踪剂羽流，以及在 TB 平均高度周围的高架羽流中心线，最大浓度出现在此处。在集群的近尾迹中，污染物的垂直扩散增强，导致整体最大浓度降低，但在 TB 平均高度以上的浓度升高。这些结果对解释 TB 区域的观测结果具有重要意义。我们使用两个特大城市（北京和巴黎）的真实测云仪观测结果，评估了城市边界层的日季节变化，并使用第三个特大城市（伦敦）的参数评估了混合层高度（MLH）经验模型。该 MLH 模型在北京市中心运行良好，但在巴黎郊区效果较差。本文讨论的不同垂直尺度上的物理气象变化为解释空气质量观测结果提供了额外的背景。
