Urban Heat Mitigation: The Influence of Turbulent Mixing in the Nocturnal Boundary Layer on Vegetation Cooling Effects

Excess heat in cities, also known as the urban heat island (UHI) effect, has impacts on human comfort, productivity, and mortality. To mitigate this urban heat risk, one of the most prominent measures is increasing the vegetation fraction in urban environments. However, cooling effect intensity of urban vegetation compared to surrounding built-up environments, is highly variable as it depends on internal characteristics of the city, on the design of urban greening and on large-scale weather conditions.

We quantify the nocturnal cooling effect intensity of urban parks in the Paris region and its day-to-day variability (1 to 5°C cooling) based on in-situ temperature measurements during three consecutive summer periods (2022-2024). Using a wind profiling Doppler Lidar to measure turbulent vertical mixing conditions above the Paris city centre, we relate the park cooling intensity to the turbulent mixing conditions in the nocturnal urban boundary layer. We identify three mixing regimes that explain the day-to-day variability in park cooling intensity for several green spaces across the Paris Region, as the relative importance of radiative and turbulent cooling processes in the urban canopy layer differ between regimes. This regime analysis allows us to characterise the park cooling intensity of green spaces of various sizes and configurations (about 1 to 20 ha) and we further assess how this cooling benefits the surrounding built-up areas. Finally, we show that the mixing regimes are valuable when linking the air temperature UHI with spatial contrasts in satellite-derived surface temperatures.

Our results highlight that the stability regimes provide a powerful framework for the assessment of nocturnal urban heat risk and the quantification of urban vegetation cooling intensity as they enable more meaningful comparisons between different locations, conditions, and periods of investigation.