Publication

Heat waves and urban heat islands (UHIs) may interact together, but the dependence of their interaction on background climate is unclear. Hourly meteorological observations in June to August from 2013 to 2015 collected in the megacities of Beijing (temperate semihumid monsoon climate), Shanghai (subtropical humid monsoon climate), and Guangzhou (marine subtropical monsoon climate) in China were used to study the interaction. At each megacity, eight rural stations and eight urban stations, respectively, were selected to study the UHI. Although under different background climates, UHIs in Beijing and Guangzhou shared a similar diurnal variability, that is, higher in the nighttime. However, the diurnal cycle is opposite for Shanghai if rural coastal stations were selected as rural reference stations. During heat wave periods, daytime (10:00-16:00) UHIs were intensified by 0.9 +/- 0.13 (mean +/- 1 standard deviation) degrees C in Shanghai, nighttime (22:00-4:00) UHIs were intensified by 0.9 +/- 0.36 and 0.8 +/- 0.20 degrees C in Beijing and Guangzhou, respectively. The surface solar radiation during the heat wave period was approximately 1.5 times to that under normal conditions in each city. The enhanced solar radiation during heat waves, which was absorbed by the urban canopy in the daytime and released at night, was closely related to nighttime UHIs in Beijing and Guangzhou and daytime UHIs in Shanghai. Additionally, changes in wind direction were observed in Shanghai under heat waves, that is, with more than 63% (wind direction) of the wind originating from neighboring hot cities in the southwest instead of the cool sea breeze from the southeast, which led to a significant increase in daytime UHIs during heat wave periods. Plain Language Summary Heat waves are one of the most damaging natural disasters in the world. Generally, urban areas tend to experience more severe heat stress under heat waves due to the urban heat island (UHI) effect (i.e., urban areas being warmer than rural areas). The impacts of heat waves on UHIs under different local background climates remain unclear, especially for the diurnal cycles of UHIs. In this study, hourly UHIs were analyzed under heat waves in three megacities: Beijing, Shanghai, and Guangzhou. The results show that nighttime UHIs were obviously intensified in Beijing and Guangzhou under heat waves, whereas in Shanghai, the intensified UHIs were much stronger during daytime than during nighttime. Increased solar radiation under heat waves was an important factor for the amplified UHIs. In addition, changes in wind direction also played an essential role in the intensified daytime UHIs in Shanghai.