The Urban Wind Island (UWI), a small but persistent positive mean boundary-layer wind anomaly over the city as a whole, has previously been revealed using a simplified conceptual model of the convective atmospheric boundary layer. This study extends the UWI research into less idealised cases by using the three-dimensional WRF mesoscale model for Amsterdam (The Netherlands) and its surroundings, at 500 m grid spacing. Two summers of forecast results for in total 173 days are used to identify whether the UWI persists in a three-dimensional modelling environment, and which conditions are optimal for its formation and persistence. In order to focus only on wind modified by surface processes, large-scale influences which modify wind speed, such as frontal passages, are identified and eliminated from the dataset. We then find that a positive UWI is present roughly half the time, with an order of magnitude that is similar to the previous work (∼ 0.2–0.5 ms−1). In addition we find an evening UWI that is caused by the delayed onset of the transition from an unstable to a stable or a neutral boundary layer in the urban area, while the rural area is already stable and calm.

中文翻译：

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三维视角下的都市风岛


城市风岛 （UWI） 是整个城市上空的一个小而持续的正平均边界层风异常，之前已使用对流大气边界层的简化概念模型进行了揭示。本研究通过使用阿姆斯特丹（荷兰）及其周边地区的三维 WRF 中尺度模型，以 500 m 的网格间距将 UWI 研究扩展到不太理想化的案例中。使用总共 173 天的两个夏季预报结果来确定 UWI 是否在三维建模环境中持续存在，以及哪些条件最适合其形成和持续。为了只关注由表面过程改变的风，确定了改变风速的大尺度影响因素，例如锋面通道，并从数据集中消除了。然后我们发现，正 UWI 大约有一半的时间存在，其数量级与之前的工作相似 （∼ 0.2–0.5 ms−1）。此外，我们发现夜间 UWI 是由于城市地区从不稳定边界层过渡到稳定或中性边界层的延迟开始引起的，而农村地区已经稳定和平静。