Effect of nocturnal urban boundary layer stability and mixing on temperature contrasts between built-up environments and urban parks

Abstract

City centres are almost always warmer than their rural surroundings. During the day, the near-surface air temperature contrasts between built-up areas and rural settings are moderate. At night however, under calm and cloud-free skies, the air in the urban canopy layer can be several degrees warmer than in rural areas. This is known as the urban heat island (UHI), a widely studied urban effect. Excess heat in cities has impacts on human comfort, productivity, and mortality. However, UHI is a highly variable phenomenon that depends both on internal characteristics of the city and external weather conditions. We study the UHI and its variability in the Paris region at multiple spatial scales, including intra-urban contrasts between built-up areas and urban parks, and at regional scales compared to rural locations. During a one-week, cloud-free heat wave event in summer 2022, we find that intra-urban and regional UHI in the Paris region vary widely from 1 to 5°C and 1 to 8°C, respectively. Using turbulent vertical mixing conditions derived from observations of a wind profiling Doppler Lidar above the city and thermodynamic profiles derived from Windsonds launched at several locations, we relate the intensity of both intra-urban and regional UHI to atmospheric stability of the nocturnal urban boundary layer. We find three different stability and mixing regimes that explain the variability in UHI. We also identify the conditions that lead to the strongest evening cooling in both rural settings and urban parks, and the weakest cooling in the built-up environment, hence associated with the strongest intra-urban contrasts. Finally, we show that the cooling potential of urban parks after sunset depends on their characteristics, such as their size, but also on atmospheric stability regimes in the nocturnal urban boundary layer that drive the relative importance of radiative and turbulent cooling processes in the urban canopy layer.