Assessing the synergies between heat waves and urban heat islands of different local climate zones in Guangzhou, China

Urban heat island (UHI) effects and heat waves (HWs) increase, posing a threat to human health and urban ecological systems. However, the synergistic mechanisms of UHI and HWs are yet to be fully elucidated. This study aims to assess the characteristics of UHI under HWs in different Local Climate Zones (LCZs) using tem-perature observations from Guangzhou, China. Fixed-point measurements were employed to simultaneously collect air temperature from nine LCZs in 2021. The results showed a localised interplay between HWs and UHI. During HWs, the UHI effect was significantly amplified in built-type LCZ, where the UHI intensity (UHII) was amplified by 0.1-0.5 degrees C throughout the day, 0.4-1 degrees C at night, but slightly reduced by 0-0.4 degrees C during daytime. The UHII was significantly reduced in land-cover type LCZ, and the urban cold effect was amplified during HWs. Furthermore, with strong solar radiation and low wind speed in HWs, the daily maximum UHII of LCZs increased 0.4-1.1 degrees C. Finally, correlations between UHII and morphological characteristics of LCZs are examined in HWs and non-heat waves (NHWs). During HWs, the increase in impervious surface fraction (ISF) and building surface fraction (BSF) and the decrease in sky view factor (SVF) exacerbate UHII, particularly at night. The augment in surface vegetation fraction (Fveg), does not significantly contribute to reducing the UHI and is less effective than NHWs at night. Therefore, different measures should be taken in different LCZs to mitigate UHII amplification during HWs. This study can serve as a reference for preventing responses to climate extremes.