Asymmetry in the intensity and frequency of extreme high temperature during daytime and nighttime in China in the summer 2022.

This study reveals the spatiotemporal evolution pattern of 2022 summertime heatwave in China, which experienced the strongest extreme heat event in east-central China since 1961, and investigates its impact on natural environment. Four extreme high temperature (EHT) indicators combining intensity/frequency and daytime/nighttime are employed to analyze the EHTs and their anomalies. Additionally, four environmental variables directly influenced by EHTs, namely soil moisture, drought index, vegetation index and fire burned area, are utilized to characterize the impact of EHTs on the environment. The results show that the average intensity of EHT in the summer of 2022 is 39.99 °C during the day and 20.54 °C at night, and the average frequency is 14.6 % during the day and 17.7 % at night. In particular, the EHT intensity is very high during the day in the Northwest China, while the EHT frequency is high during the daytime in the Qinghai-Tibet Plateau and during the nighttime in South China. The daytime EHT intensity is higher than the nighttime, but the nighttime EHT frequency is higher than the daytime. Overall, EHTs have increased in frequency and intensity, in the context of 2017-2021, with daytime and nighttime intensities increasing by 0.67 °C and 0.78 °C, and frequencies increasing by 4.5 % and 7.6 %, respectively. EHTs are increasing significantly during the day in Qinghai-Tibet Plateau and during the night in South China, in contrast, a significant portion of North and Northeast China is decreasing, especially during the day, with more than half of the region experiencing a decrease. During different months of summer, the spatial distribution of the intensity of EHT is basically invariable, with high frequency values starting in places such as northern Xinjiang and the North China Plain, then shifting to the Qinghai-Tibet Plateau and the southern coastal areas during the daytime, and also to the Northeast at night in addition to that, and finally converging in the Sichuan Basin during the daytime, and also converging to the Qinghai-Tibet Plateau during the nighttime. Notably, there is an asymmetry between daytime and nighttime EHTs, with daytime extreme heat peaks being highest in July but nighttime highest in August, and with intensity and frequency increasing more at night than during the day. Furthermore, daytime EHT intensity had the largest effect on environmental factors in general, and its interaction with nighttime EHT intensity to form an intensity-based day-night composite of EHT that has a significant effect on environmental factors. However, nighttime EHT intensity was the most influential indicator of spatial heterogeneity for the area burned by fire, and the nighttime intensity and frequency composite EHTs had the greatest effect on fire. The results of this study contribute to a deeper understanding of the evolution model of EHTs and their impacts on environmental factors in China in the summer of 2022, and provide a scientific basis and reference for future responses to EHTs.