Urban Boundary Layer Growth and Turbulence in Paris Using Large-Eddy Simulations

Abstract Urban boundary layer (UBL) processes are important to understand the complex interactions between the surface and atmosphere. Increased surface roughness and heat storage are typically attributed to the larger fraction of built surfaces in urban areas. Heatwaves lead to stronger heat storage and to extreme convective UBLs, which is one of many applications where large-eddy simulations (LES) offer an accurate modelling methodology of complex turbulent characteristics in urban flow studies. This study investigates the structure, turbulence characteristics and measurement implications of UBL in Paris, France, during a heatwave in July 2022. High-resolution LES simulations (4–16 m) produced with PALM model system are evaluated against observations from an obervation campaign, including eddy-covariance fluxes (EC), Doppler wind lidar (DWL) profiles, and near-surface measurements from the Météo-France observation network. In addition, wavelet analysis is used to examine resolved turbulent scales and vertical coupling at typical urban measurement heights. PALM reproduces the temporal variability of sensible heat fluxes at the SIRTA observatory with a root mean squared error (RMSE) of $$73.4~\mathrm {W~m^{-2}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>73.4</mml:mn> <mml:mspace/> <mml:mi>W</mml:mi> <mml:mspace/> <mml:msup> <mml:mi>m</mml:mi> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> and a bias of $$45.0~\mathrm {W~m^{-2}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>45.0</mml:mn> <mml:mspace/> <mml:mi>W</mml:mi> <mml:mspace/> <mml:msup> <mml:mi>m</mml:mi> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> , while momentum-related quantities show larger deviations, particularly during weak nighttime stratification. Near-surface temperatures are well captured across the Météo-France network, with an RMSE of $$1.18 ^\circ {\textrm{C}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>.</mml:mo> <mml:msup> <mml:mn>18</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> <mml:mtext>C</mml:mtext> </mml:mrow> </mml:math> and mean bias of - $$0.63 ^\circ {\textrm{C}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>0</mml:mn> <mml:mo>.</mml:mo> <mml:msup> <mml:mn>63</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> <mml:mtext>C</mml:mtext> </mml:mrow> </mml:math> . DWL comparisons show that PALM captures the growth and decay of the UBL at urban and suburban sites, with maximum UBL heights reaching 2.4 km in the city centre, approximately 10% higher than in more rural conditions. Neighbourhood-scale turbulent motions dominate vertical transport during daytime convective conditions, with contributions from larger scales depending on measurement height and location. These results demonstrate the capability of high-resolution LES to represent convective UBL dynamics under strong surface heating and to assess the representativeness of urban atmospheric measurements. Overall, this study provides one of the first city to neighbourhood-scale LES analyses of a convective UBL supported by multiple observations. By linking resolved turbulent scales to vertical coupling and measurement height, the results infer when and where urban EC observations remain representative under strongly unstable conditions. The findings offer practical guidance for the interpretation and design of urban measurements and highlight the importance of resolving neighbourhood-scale turbulence in urban flow modelling applications.