The flow in the atmospheric boundary layer (ABL) over cities is crucial for the urban climate as it controls the exchange of heat, moisture and pollutants within the ABL and with the surroundings. In particular for cities in mountainous terrain, the mesoscale flow shows a high spatial and temporal variability, which poses great challenges to the means of observation. We used the dual-Doppler lidar scan strategy of virtual towers (VT), to measure profiles of the horizontal wind in the ABL over the city of Stuttgart in southwestern Germany. To study the mesoscale variability of the horizontal wind, we placed six VTs in the topographically structured investigation area, which is characterised by a basin-shaped valley opening into a major valley. Comparisons with radiosonde data show that reliable wind information can be retrieved from the VT measurements after careful processing. A statistical analysis reveals a strong dependence of the flow in the valleys below ridge height on the bulk Richardson number (BRN). A critical BRN of 1.25 is identified for the area, below which the flow below ridge height is dynamically unstable and coupled to the ambient flow above. For BRNs greater than 1.25, the flow is dynamically stable and the flow below ridge height is dominated by thermally driven down-valley winds, which are decoupled from the ambient wind. This study shows that the VT technique is applicable in highly complex terrain and a promising tool for the investigation of the flow in areas which are difficult to access by traditional in situ or single lidar measurements, like complex urban terrain.

中文翻译：

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用双多普勒激光雷达廓线研究斯图加特周围复杂城市地形的中尺度风型

城市上空的大气边界层（ABL）中的流动对于城市气候至关重要，因为它控制着ABL内以及与周围环境之间的热量，水分和污染物的交换。特别是对于山区的城市，中尺度流量表现出很高的时空变化性，这给观测手段提出了极大的挑战。我们使用虚拟塔（VT）的双多普勒激光雷达扫描策略来测量德国西南部斯图加特市ABL上的水平风廓线。为了研究水平风的中尺度变化，我们在地形结构化的调查区域中放置了六个垂直VT，其特征是向主要山谷开放的盆状山谷。与无线电探空仪数据的比较表明，经过仔细处理后，可以从VT测量中获取可靠的风信息。统计分析表明，山脊高度以下的山谷中的水流对总体理查森数（BRN）的依赖性很大。该区域的BRN临界值为1.25，低于该高度时，低于脊高的流量是动态不稳定的，并耦合到上方的环境流量。对于大于1.25的BRN，流量是动态稳定的，并且低于山脊高度的流量由热驱动的向下谷风控制，该降谷风与环境风分离。这项研究表明，VT技术适用于高度复杂的地形，是一种有前途的工具，可用于研究传统的原位或单激光雷达测量难以进入的区域中的流量，