上海城市热岛与热浪协同作用及其影响因子

在全球变暖的背景下,热浪变得更加频繁。城市地区由于城市热岛效应在热浪事件中承受更大的风险。然而城市热岛与热浪之间的相互作用还少有研究。利用2016-2017年两个夏季（6-8月）上海城、郊气象站逐时气温、风速、相对湿度资料、城区徐家汇和郊区奉贤涡动相关通量观测资料对比分析了热浪和非热浪期间城市热岛强度的差异,并利用一个平流扩散解析模型揭示了城市热岛与热浪相互作用的物理机制。结果表明,热浪期间的城市热岛强度相比非热浪期间明显增强,且白天增强大于夜间,显示出城市热岛与热浪之间的协同效应。热浪期间城、郊相对湿度比值相对非热浪期间明显减小,表明热浪期间城区地表相对郊区变得更干,从而抑制了蒸发作用,加剧了城市热岛强度;平均风速也明显减小,平流降温效应减弱,对城市热岛强度起正反馈效应。热浪期间城区净辐射通量的增加大于郊区,使城区获得更大地表辐射输入;由于城区更少的植被覆盖和更多的不透水下垫面,热浪期间城区潜热通量略有降低,而郊区明显增加;城区感热通量增幅则大于郊区,这种感热和潜热通量分配的改变也加剧了城市热岛强度。本研究对城市如何应对加剧的热风险具有重要的科学指示意义。

Synergistic interaction between urban heat island and heat waves and its impact factors in Shanghai

Under the background of global warming, heat waves are expected to be more frequent and long lasting. Cities endure greater risk under heat wave events because of the pre-existing urban heat island. However, research on the interaction between heat wave and urban heat island is still lacking. Based on hourly data of air temperature, wind speed, relatively humidity, and eddy covariance energy flux data from Shanghai urban (XJH) and rural site (FX) during two summers (June-August) in 2016-2017, the difference of urban heat island (UHI) between heat wave (HW) and non-heat wave (NHW) conditions is analysed. In addition, an advection-diffusion analytical model has been used to unravel the mechanism of the interaction between UHI and HW. Results show that the UHI intensity is obviously enhanced during HWs, and the enhancement is stronger during daytime than that of nighttime, which indicates the synergistic effect between UHI and HW. The relative humidity ratio of urban and suburban areas during HWs significantly decreases compared with NHW conditions, indicating the urban surface becomes even drier than suburban areas during HWs that suppresses evaporation and intensifies UHI intensity. The mean wind speed also has an obvious decrease, leading to weaker advection cooling effect, which has a positive effect on UHI intensity. The increase of net radiation at the urban site is larger than that of the suburban site during HWs so that urban area receives more radiation input. Due to low vegetation cover and more impervious surfaces, the latent heat flux at the urban site has a slight decrease while it has an obvious increase at the suburban site. The increase of the urban sensible heat flux is larger than that of the suburban site. This change of the partition between sensible and latent heat flux also exacerbates the UHI intensity. This study has important implications for cities to cope with intensified thermal risks.