This study characterizes the variation of the urban heat island (UHI) within the planetary boundary layer (PBL) and investigates the horizontal advection effects on the UHI variation throughout a heat wave (HW) event over Shanghai municipality in August 2019 based on numerical model simulations. It is found that the UHI intensifies under HW conditions with the UHI intensity gradually weakening from the surface upwards. The daytime UHIs during the HW are 3.68 K (29.27 %), 1.41 K (33.52 %) and 1.04 K (36.97 %) higher than those during the pre-HW at the surface, near-surface and in the PBL, respectively, while the nighttime UHIs have no significant response to the HW at least for this case. The mean UHIs during the HW period at the surface, near-surface (2 m) and in the PBL are 6.83 K (2.68 K), 2.45 K (1.34 K), and 1.73 K (0.04 K) in the daytime (nighttime), respectively. The PBL UHI generally exists only in the daytime potentially caused by the thermal convective diffusion. The near-surface and PBL UHIs are significantly affected by the horizontal advection, resulting in different UHI intensities and variations against rural regions in different orientations. Cold advection from the Yangtze River (the ocean) in the riverside (coastal) rural region corresponds to the great UHI intensification, while the strong cold advection in the urban region well explains the fast UHI weakening. This study highlights that, besides local thermal factors, synoptic circulation also play an important role in the interaction between the UHI effect and HW events.

中文翻译：

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行星边界层内城市热岛效应对上海上空热浪和水平平流影响的响应


本研究表征了行星边界层 （PBL） 内城市热岛 （UHI） 的变化，并基于数值模型模拟研究了 2019 年 8 月上海市热浪 （HW） 事件中水平平流对 UHI 变化的影响。研究发现，UHI 在 HW 条件下增强，UHI 强度从表面向上逐渐减弱。HW 期间的白天 UHI 分别比 HW 期间的表面、近表面和 PBL 中的 UHI 高 3.68 K （29.27 %）、1.41 K （33.52 %） 和 1.04 K （36.97 %），而夜间 UHI 对 HW 没有显着响应，至少在这种情况下。在 HW 期间，地表、近地表 （2 m） 和 PBL 中的平均 UHI 分别为 6.83 K （2.68 K）、2.45 K （1.34 K） 和 1.73 K （0.04 K） 白天（夜间）。PBL UHI 通常只存在于白天，可能是由热对流扩散引起的。近地表和 PBL UHI 受水平平流的显着影响，导致不同方向的农村地区具有不同的 UHI 强度和变化。来自江边（沿海）农村地区长江（海洋）的冷平流对应于 UHI 的大增强，而城市地区的强冷平流很好地解释了 UHI 的快速减弱。本研究强调，除了局部热因素外，天气环流在 UHI 效应和 HW 事件之间的相互作用中也起着重要作用。