Characteristics of the Urban Boundary Layer in Paris Under Heatwave Conditions using LES

Urban boundary layers (UBL) are characterised by larger temperatures due to higher built fractions, anthropogenic heat emissions, more heat storage and strong mixing resulting from the complex surface-atmosphere exchanges. UBL dynamics coupled with extreme weather can lead to rapid boundary layer growth, particularly during heatwaves such as during the summer of 2022 in Paris, France.

This study uses the high resolution Large-Eddy Simulation (LES) model PALM to study how the UBL and turbulent eddies develop during heatwave conditions. The domain setup is a 32km x 32km x 4km grid with 16 meter resolution, which is further refined up to 4m resolution with PALM’s self-nesting capability. The boundary conditions for wind, temperature and humidity are dynamically fed into PALM using the WRF mesoscale model output. PALM outputs are compared with LiDar and Eddy Covariance (EC) measurements conducted during the PANAME 2022 observational campaign at the SIRTA observatory and Jussieu campus. The study focuses on a 26-hour simulation period, with particular attention given to the early morning transition phase, where the UBL undergoes rapid growth, ultimately reaching a height of 2.5km during the whole simulation period.

By exploring the UBL dynamics this work aims to assess the ability of PALM to replicate the observed UBL structure. The model is able to capture the UBL’s structure and height and has good agreement with EC flux measurements taken at SIRTA. We will also make use of wavelet analysis in the finer resolution domains to study the development and distribution of the turbulent structures present in the UBL. This work highlights the importance of accurately modeling UBL dynamics for predicting urban heat risks and improving climate resilience strategies.