Variations in urban boundary layer height at regional and European scales from lidar network observations

The dynamics of the Atmospheric Boundary Layer (ABL) play a crucial role for the transport of heat, moisture, and pollutants within urban environments. In this study, we use long-term observations from ground-based remote sensing to examine indicators of ABL dynamics for multiple cities in Europe. Two independent methods are employed to monitor the Mixed Layer Height (MLH): (1) aerosol-based layer detection using observations from automatic lidars and ceilometers (ALC) and (2) turbulence-based layer detection that exploits profiles of the vertical velocity variance obtained from Doppler wind lidar (DWL) measurements. We compare aerosol-derived layer heights at the European scale and layer heights from both methods at a suburban and an urban site in the Paris region.

While the general agreement of the layer results obtained with these two approaches gives  confidence to the automatic MLH detection, a detailed comparison provides valuable insights into the relationship between instantaneous mixing and aerosol vertical transport. We present an extensive analysis of MLH variability across diurnal to seasonal timescales as well as spatial variations at European and regional scales. Particular attention is given to the morning evolution of the MLH, examining its dependence on nocturnal atmospheric stratification and the presence and characteristics of the Low-Level Jet (LLJ). By integrating high-resolution lidar observations with advanced analysis of mixing processes, this work contributes to a better understanding of urban climate dynamics and provides a framework for studying the interactions between the city and boundary layer processes. 

The study is linked to the RI-URBANS project and the European Research Infrastructure ACTRIS, the ABL testbed project implemented at AERIS with support of the European COST Action PROBE, and the PANAME initiative in the Paris Region (France).