Diurnal variation of amplified canopy urban heat island in Beijing megacity during heat wave periods: Roles of mountain-valley circulation and urban morphology

Abstract. In the context of global warming and rapid urbanization, heat waves (HW) are becoming more frequent, which is amplifying canopy urban heat island (CUHI) via various driving mechanisms. While the roles of local circulation and urban morphology remain unclear in the synergistic interaction between HW and CUHI. By utilizing the data from high-density automatic weather stations in the Beijing megacity, this article explored spatiotemporal patterns of the interactions between HW and CUHI. The average daily CUHII during HW periods exhibited a significant increase of 59.33 % compared to the non-heat wave (NHW) periods. Mountain-valley breeze significantly modulated the spatiotemporal patterns of CUHI intensity (CUHII). In particular, on an urban scale, the turning mountain-valley breeze caused horizontal transport of heat inner-city, resulting in the north-south asymmetric pattern of urban excess warming during HW periods. On a street scale, the amplified CHUII was closely associated with urban morphology in the inner city, especially for the vertical characteristics of buildings. During the mountain breeze phase, the amplification of CUHII in the high-rise street zone was significantly stronger than that in the low-rise street zone. During the valley breeze phase, the amplification of CUHII in high-rise street zones exhibits weaker effects in the afternoon compared to the low-rise street areas, while demonstrating stronger amplification during the nighttime. Our findings provide scientific insight to understand the driving mechanisms of urban excess warming and mitigating the escalating risks associated with extreme high-temperature events over megacities in the transitional zone of mountains and plains.