Heat waves (HWs) and urban heat islands (UHIs) can potentially interact. The mechanisms behind their synergy are not fully disclosed. Starting from the localized UHI phenomenon, this study aims i) to reveal their associated impacts on human thermal comfort through three different definitions of HW events, based on air temperature (airT), wet-bulb globe temperature (WBGT) and human-perceived temperature (AppT) respectively, and ii) to understand the role of air moisture and wind. The analysis was conducted in four districts (NH, JD, MH and XJH) with different urban development patterns and geographic conditions, in the megacity of Shanghai with a subtropical humid climate.

Results evidenced the localized interplay between HWs and UHIs. The results indicate that less urbanized districts were generally more sensitive to the synergies. JD district recorded the highest urban heat island intensity (UHII) amplification, regardless of the specific HW definition. Notably, during AppT-HWs, the increment was observed in terms of maximum (1.3 °C), daily average (0.8 °C), diurnal (0.4 °C) and nocturnal UHII (1.0 °C). Nevertheless, localized synergies between HWs and UHIs at different stations also exhibited some commonalities. Under airT-HW, the UHII was amplified throughout the day at all stations. Under WBGT-HW, diurnal UHII (especially at 11:00–17:00 LST) was consistently amplified at all stations. Under AppT-HW conditions, the nocturnal UHII was slightly amplified at all stations. Air moisture and wind alleviated the synergistic heat exacerbation to the benefit of thermal comfort. The extent depended on geographic condition, diurnal and nocturnal scenarios, temperature type and HW/normal conditions. Stronger HW-UHI synergies indicate the necessity to develop specific urban heat emergency response plans, able to capture and intervene on the underlying mechanisms. This study paves to way to their identification.

热浪（HW）和城市热岛（UHI）可能相互作用。其协同作用背后的机制尚未完全公开。从局部UHI现象开始，本研究旨在：i）通过基于空气温度（airT），湿球温度（WBGT）和人类感知温度的三种不同的HW事件定义，揭示它们对人体热舒适性的相关影响。 （AppT），以及ii）了解空气湿度和风的作用。该分析是在亚热带湿润气候的上海特大城市的四个具有不同城市发展模式和地理条件的地区（NH，JD，MH和XJH）中进行的。

结果证明了硬件和UHI之间的局部相互作用。结果表明，城市化程度较低的地区通常对协同效应更为敏感。京东区记录了最高的城市热岛强度（UHII）放大率，无论具体的硬件定义如何。值得注意的是，在AppT-HW期间，观察到的增加幅度是最高（1.3°C），日平均（0.8°C），日间（0.4°C）和夜间UHII（1.0°C）。然而，不同站点的硬件和UHI之间的局部协同作用也表现出一些共性。在airT-HW下，UHII在所有站点全天都被放大。在WBGT-HW模式下，昼夜UHII（尤其是在LST的11：00-17：00）一直在所有站点上被放大。在AppT-HW条件下，夜间UHII在所有站均略有放大。空气湿气和风能减轻协同增热，从而带来热舒适感。范围取决于地理条件，昼夜情况，温度类型和硬件/正常条件。加强的HW-UHI协同作用表明，有必要制定特定的城市供热应急响应计划，从而能够捕获和干预潜在的机制。这项研究为他们的鉴定铺平了道路。