The Modulation of Synoptic Weather Patterns and Human Activities on the Diurnal Cycle of Summertime Canopy Urban Heat Island in Yangtze River Delta Urban Agglomeration, China

Abstract. The pronounced excess urban warming phenomenon during summer in the Yangtze River Delta Urban Agglomeration (YRDUA) has emerged as a significant challenge to the health and economy of urban residents amidst the accelerating urbanization process. Despite its undeniable importance, few studies have investigated the diurnal variability patterns of the canopy urban heat island (CUHI) in this region from the perspective of synoptic weather patterns (SWP) and human activities. This study integrated multiple source datasets, including meteorological station observations, high-resolution satellite imagery, and reanalysis data, to systematically analyze the diurnal patterns of the CUHII in YRDUA. Using objective classification and a machine learning model, we discovered notable diurnal patterns of CUHI intensity (CUHII), particularly higher levels of CUHII at night compared to day. Further analysis revealed that among six SWPs, type 2, dominated by subtropical high pressure, generated the strongest CUHII, while type 4, influenced by the combined effects of southwestern moisture transport and southward cold air incursions, resulted in the lowest CUHII. Additionally, the study found that key indicators such as landscape percentage (PLAND), largest patch index (LPI) and anthropogenic heat flux (AHF) exhibited an increasing trend over recent years, with higher values in the east and lower in the west, aligning well with the spatio-temporal patterns of the CUHII. These findings collectively confirmed the central roles of SWPs and human activities as the main drivers of CUHI phenomena. Simulations using the RF model further indicated the diurnal asymmetry in the modulation of the CUHI by SWPs and human activities: SWPs exerted a more pronounced influence on day CUHII, while human activities dominated night CUHII. Furthermore, the study delved into the impact mechanisms of heatwave (HW) events on the diurnal cycle of the CUHII. During HW periods, the amplification effect of day CUHII was stronger than night CUHII, and the presence of HW events significantly reduced the diurnal amplitude of the CUHII. In conclusion, this study not only provided scientific insight into the complex driving mechanisms of the CUHI diurnal cycle in YRDUA, but also offered a theoretical foundation for evaluating urban overheating issues and developing effective mitigation strategies.