Urban modification of the atmospheric boundary layer over Paris – insights from intensive ALC profiling observations along a rural-urban transect

Improving understanding of urban atmospheric boundary layer (ABL) processes and dynamics is central to improving the modelling and forecasting of weather, air quality, and thermal comfort, in particular in densely populated urban areas. The ERC (European Research Council) urbisphere-Paris measurement campaign generated extensive in-situ and remotely sensed observations of the ABL from a network of concurrently operated automated lidars and ceilometers (ALC), doppler wind lidars, radiometers, and automatic weather stations to explore surface-atmosphere feedbacks focusing on ABL dynamics upwind, over, and downwind of a metropolitan area.

Paris, one of the densest metropolitan areas in Europe, is located ~130 km from the coast in relatively flat orography, making it well-suited to study urban modifications of atmospheric boundary layer dynamics. urbisphere-Paris aims to gather consistent and coherent datasets on the magnitude and interplay of surface heat fluxes, radiation fluxes, aerosol load, and boundary layer dynamics to evaluate and improve next generation numerical weather and air quality modelling approaches.

From March 2023 to March 2024 a transect along the predominant wind direction had seven Vaisala CL61 Ceilometers operating. As the transect extended both upwind and downwind of the city centre by 60 km to include the rural periphery, modifications of the ABL aerosol profile, cloud properties, and mixed layer dynamics could be observed as air moves into, across, and beyond the metropolitan area.

The transect provides continuous profiles of attenuated backscatter and linear depolarisation ratio, with very high vertical (4.8 m up to 15.4 km) and temporal (every 5 s) resolution. From this other variables are derived, such as mixed layer height (MLH), cloud cover of boundary layer clouds (CC), and cloud base height (CBH). This contribution will present selected cases of spatiotemporal differences in ABL processes along the urban-rural continuum. Wind direction changes the upwind/ downwind effects such as the urban plume. Differences in temporal evolution of the ABL are also explored, including MLH, CC and CBH. These results will help identify both typical and unusual patterns to explore further with high resolution numerical weather prediction models.