Interactions between the large-scale atmospheric circulation and the urban boundary layer during current and future heatwaves

In recent years, Europe has been exposed to recurrent heatwaves with significant social, economic and environmental impacts. These impacts have been more severe in cities, where the majority of the population lives. Heat risks in cities are augmented by the urban heat island effect associated with the high density of buildings, the lack of vegetated surfaces and human activities. The urban heat island intensity is especially high during nights with weak vertical mixing and low wind speeds (Céspedes et al. 2024). It is therefore necessary to implement measures, such as greening streets or parks, to mitigate health risks during future heat waves, which are expected to become more frequent, intense and prolonged with global warming. However, also the cooling potential of urban greening depends on the stratification of the atmosphere. During stagnant conditions, the cooling effect of parks remains very local due to reduced ventilation and does not allow the surrounding streets to be refreshed (Haeffelin et al., 2025).

It is therefore important to understand the combined impact of urban buoyancy and synoptic-scale stratification on the dynamics in the Urban Boundary Layer (UBL) and further to assess how these processes will evolve in the future. To this end, we select a number of heat waves that hit Paris (France) between 2015-2023 and study the link between synoptic conditions and atmospheric stratification in the urban boundary layer, thanks to a combination of observations and reanalysis (ERA5), in order to assess this link to future climate simulations. We particularly take advantage of numerous observations that are available at the SIRTA observatory, southwest of Paris, at the QUALAIR-SU platform in the city center, in addition to the Météo-France surface station network and measurements conducted in the context of the PANAME measurement campaign during summers 2022 and 2023.