The Sixth Report of the IPCC states that global warming is expected to result in more loss and damage (Miao et al., 2024), increasing the probability of extreme climate events (Yang et al., 2011; Xu et al., 2024; Mariusz et al., 2025). Heatwaves, one of the most destructive extreme climate events, have worsened in recent decades (Stott et al., 2004; Alexander et al., 2006). There has been a significant increase in the number of heatwaves on a global scale (Perkins and Alexander, 2012), which are increasingly showing continuity, longer-travel, and slower-movement (Luo et al., 2024a), posing greater threats to human societies, ecosystems, and human health, among other concerns (Rohini et al., 2016; Perkins-Kirkpatrick et al., 2020; Wang et al., 2020; Bian et al., 2022; Cheng et al., 2023). High-temperature heatwave events, as reported by Yin et al. (2022), have caused unprecedented harm globally, attracting widespread attention from the public and scientists around the world (Karl and Knight, 1997; Robine et al., 2008; Wu et al., 2023a; Luo et al., 2024b).

Global warming has resulted in increasing diurnal warming, with daily temperatures (daytime and nighttime) becoming more extreme (Donat and Alexander, 2012). Moreover, NHWs are increasing more than DHWs (Thomas et al., 2020; An and Zuo, 2021). Cooling measures are often taken in response to daytime heat. However, nighttime heat is also extremely dangerous. NHWs severely disrupt the traditional diurnal pattern of heat and cold, making it difficult for people to recover from the cooler nighttime hours (García-Herrera et al., 2010), resulting in a failure of thermoregulation, which can have detrimental effects (Gosling et al., 2009; An and Zuo, 2021). When extreme daytime high temperatures are combined with nighttime high temperatures (i.e., CHWs), more dangerous situations occur (Wang et al., 2020). Therefore, the identification of DHWs, NHWs, and CHWs, as well as understanding their evolution, is important in reducing the adverse effects of heatwaves.

Furthermore, urbanization has significantly altered the spatial and temporal variability of extreme weather (Li et al., 2020; Wang et al., 2022; Wei et al., 2024). The process of urbanization leads to an increase in impervious areas, and a rise in surface temperatures, resulting in the creation of urban heat islands (Giorgio et al., 2017; Yuan et al., 2023). Anthropogenic greenhouse gas-driven heat risks are sweeping globally (Zhang et al., 2023). The concentration of human activities in cities has created a variety of environmental challenges (Hou et al., 2023). The majority of heatwave-related deaths, as well as disease risks and other related factors, are found in urban areas (Kovats and Kristie, 2006; Huang et al., 2010; Wouters et al., 2017; Hamdi et al., 2020). However, the inequality in heatwave risk between the Global North and the South has raised concerns (Zhang et al., 2023). Urbanization exacerbates the occurrence of HWs (Sun et al., 2019; Wang et al., 2020, 2023). The monsoon climate in eastern China is significant (Ge et al., 2016). After 1990, China’s urbanization entered a phase of rapid development (Luo and Lau, 2021), leading to a significant increase in impermeable areas and urban populations. The Yangtze River Delta urban agglomeration (YRDUG) is not only one of the top three high-temperature heatwave-prone regions in the world (Sun et al., 2022), but also one of the fastest urbanizing regions in China in the past decades (Yang et al., 2017). HWs frequently occur in YRDUG, yet there remains a lack of clarity regarding the dynamics and distinctions among various types of heat waves in both urban and rural areas undergoing urbanization (Mishra et al., 2015; Luo and Lau, 2018).

Studies have discussed the formation mechanisms of extreme high-temperature heat waves (Hong et al., 2018; Mukherjee and Mishra, 2018; Thomas et al., 2020; Wu et al., 2023b). However, the kinetic and thermodynamic processes underlying their formation mechanisms remain poorly understood (Li et al., 2020). The possible causes of DHWs and NHWs need further investigation. Considering the rapid pace of urbanization, there is a need for research focusing on the evolution of daytime and nighttime heatwaves. This motivated using three types of HWs to investigate the impact of increased urbanization on temperature distributions and heatwaves, as well as analyzing the total cloud cover, specific humidity, precipitation, soil moisture, and other mechanisms of impact.

Many studies have investigated into the formation mechanisms of HWs (Ding et al., 2010; Freychet et al., 2017; Luo and Lau, 2017; Luo and Lau, 2019; Sun and Gao, 2022), most of them have focused on the study of a single type of HW, and there have limited studies on different types of HWs, resulting in a relative scarcity of assessments and systematic evaluations. The occurrence mechanism of DHWs is typically associated with atmospheric conditions (Lau and Nath, 2012), while NHWs are usually linked to a humid atmosphere, precipitation, and increased cloud cover (Gershunov et al., 2009; Bumbaco et al., 2013). Therefore, this study revealed the impact mechanism of diurnal heatwaves in the Yangtze River Delta region.

A high-pressure anticyclone emerged in the YRDUG (Figure 7c). This abnormal anticyclone triggered adiabatic warming. The enhanced downdraft also led to a reduction in the overall cloud cover in the region. During the daytime, there was a relatively low cloud coverage (Figure 7b), and the increased shortwave solar radiation (Figure 8a) warmed the air near the surface, creating a favorable environment for the development and persistence of heatwaves during the daytime. China’s coastal areas were influenced by subtropical high-pressure systems when heatwaves occurred from May to September. Meanwhile, the southeast monsoon brought ample water vapor from the ocean, leading to an extremely humid atmosphere with exceptionally high specific humidity (Figures 7a and 7d). Both the nighttime specific humidity (Figure 7d) and long-wave radiation (Figure 8e) in the Yangtze River Delta region were elevated, and the NHWs were primarily attributed to long-wave radiation. Since the heat accumulated from the sun’s shortwave radiation during the day was challenging to dissipate, the air near the surface warmed at night, leading to unusually high humidity in the atmosphere (Figure 7d). This, in turn, exacerbated the greenhouse effect and the downward longwave radiation at night, trapping more longwave radiation (Figure 8e) and re-emitting it to the surface during the night. Consequently, this elevated the air temperature near the surface and heightened the likelihood of nighttime heatwaves, further intensifying the occurrence of nocturnal heatwaves.

The CHWs were influenced by shortwave (Figure 8c) and longwave radiation (Figure 8f). The characteristics of the CHWs surpassed those of DHWs and NHWs. Rainfall anomalies during the three HWs were predominantly positive (Figures 8g-8i). During DHWs, intense solar shortwave radiation led to smaller rainfall anomalies compared to DHWs and NHWs. The relative humidity was low, leading to soil moisture anomalies. The reduced soil moisture resulted in a decrease in surface latent heat flux, (Figure 8j), leading to significantly negative flux anomalies (Figure 8m) (i.e., upward heat transfer).

In addition, the weather or climatic conditions of HWs may be favorable for the occurrence of heavy precipitation in the afternoon. High surface temperatures and sensible heat fluxes associated with HWs can enhance convective effective potential energy, providing favorable conditions for the development and maintenance of heavy precipitation (Hao, 2022). Higher soil moisture, accompanied by nighttime rainfall (Figure 8k), and the surface sensible heat flux in the YRDUG generally correlated with soil moisture and specific humidity (Figure 8). An increase in sensible heat flux favored elevating surface temperatures, which were mostly negative during DHWs and positive during the NHWs (Figures 8p and 8q). This indicates that NHWs characteristic was more pronounced than DHW characteristics in the YRDUG (Figure 12).

To further analyze the impact of impervious surfaces and urban green areas on HWs, we analyzed trend changes in impervious surfaces and green spaces within the buffer zones of meteorological stations (Figure 9). The impervious area was negatively correlated with temperature and positively correlated with the green land (Figure 9b). The impervious area (green land) of the meteorological stations buffer zone increased (decrease) by 65.5% (33.2%) between 1990 and 2019 (Figure 9a). For every 1% increase (decrease) in the impervious area (green land), the intensity of DHWs, NHWs and CHWs increased (decrease) by 0.02°C (0.05°C), 0.04°C (0.08°C) and 0.19°C (0.04°C), respectively. This indicated that the intensity of three HWs had been increasing in the post-1990 urbanization.

After 1990, the DHWs and CHWs in urban areas were greater than those in rural areas. While the NHWs was increasing, the NHWs in the rural areas around the urban areas was higher than that in the urban areas, and the eastern part of the study area was most typical. This is contrary to the trend of DHWs and CHWs. This paper discusses this phenomenon to a certain extent for the following reasons.

First, the wind speed anomalies during NHWs events in the YRDUG consistently exceeded those during DHWs and CHWs. Specifically, nighttime wind speeds in urban areas along the east coast of the YRDUG were higher than those in the surrounding areas (Figure 10). High wind speeds in urban areas transport hot air from urban areas to neighboring regions through urban-rural circulation, acting as a heat diffuser (Figure 11) (Kurppa et al., 2018), resulting in higher nighttime air temperatures in the rural areas around the urban areas of the YRDUG compared to their urban counterparts.

Additionally, the urban areas in the eastern part of the YRDUG were susceptible to the influence of the southeasterly monsoon during hot summer days (Figure 7). This was due to the interaction of sea and land winds, which brought cooling to the urban areas and transfered heat from urban areas to the surrounding countryside. Moreover, urban areas in the eastern part of the YRDUG had achieved a higher level of economic development and prioritized urban greening. Greenery in the urban environment of the eastern YRDUG had been expanding since 1990 (Gao et al., 2019), and vegetation played a crucial role in absorbing and locally dissipating atmospheric heat by providing shade. At the same time, ambient heat was converted into latent heat, a process that reduced the temperature in the surrounding environment (Tyrväinen et al., 2005; Kong et al., 2014; Alan and Evgeni, 2019).

Finally, in the economically developed urban areas of the eastern YRDUG region, there was a trend of relocating relevant enterprises and industries to the surrounding rural areas and rapidly developing urban zones. This relocation was driven by factors, such as land scarcity, energy efficiency, emissions reduction, and high operating costs (Zhang and Wang, 2010). The reduced presence of secondary industries in the eastern urban areas led to decreased energy consumption and heat emissions, resulting in lower temperatures in these urban areas and higher temperatures in the surrounding rural regions (Jia et al., 2017; Huang et al., 2021). Therefore, the rate of NHWs increased in the eastern urban areas, but the increase was not as pronounced as in the neighboring rural areas and the rapidly expanding urban zones.

Based on meteorological data from 159 stations in the YRDUA, ERA5 reanalysis data and so on, this study analyzed the spatial and temporal evolution of different HWs types in the region. The aim was to reveal distinctions between urban and rural areas, and to investigate the primary influencing mechanisms. The main conclusions are as follows: Due to the influence of increased solar shortwave radiation and relatively low cloud cover, the duration, frequency, and intensity of DHWs in the YRDUG mainly exhibited a decreasing trend, with the most significant decrease was observed in the southwestern part of the region due to its terrain. Conversely, the NHWs were intensified by the increase in long-wave radiation and the sensible heat flux of the atmosphere. This situation hampered the dissipation of heat from the ground, particularly due to the region’s high humidity, which exacerbated the greenhouse effect resulting from the elevated water vapor carried by the southeast monsoon. Influenced by wind direction, precipitation and humidity, the HWs in the YRDUG appeared more humid and hotter. The duration, frequency, and intensity of NHWs increased by 25.8%, 19.5%, and 37.1% respectively, particularly in the eastern and southern parts of the region. The proportion of HWs was gradually shifting from daytime-dominated to nighttime- dominated. Under the urbanization process, the disparity between urban and rural DHWs was diminishing, the NHWs in urban and rural areas continued to rise and the characteristics of NHWs in urban areas were lower than those in rural areas. Under the combined effect of DHWs and NHWs, CHWs showed a significant increasing trend, surpassing both DHWs and NHWs, while the characteristics of CHWs in urban areas were higher than those in rural areas. Under the rapid process of urbanization, the duration, frequency, and intensity of CHWs increased by 47.8%, 81.7% and 73.6%, respectively.