Urban Boundary-Layer Dynamics During Extreme Heat in Paris

Urban boundary-layer (UBL) processes are important to understand the complex interactions between the surface and atmosphere. Increased surface rough-ness and heat storage are typically attributed to the larger fraction of builtsurfaces in urban areas. Heatwaves lead to stronger heat storage and to extremeconvective UBLs, which is one of many applications where large-eddy simu-lations (LES) offe an accurate modelling methodology of complex turbulentcharacteristics in urban flw studies. This study investigates the structure, tur-bulence 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 the PANAME 2022 campaign,including eddy-covariance fluxe (EC), Doppler wind lidar (DWL) profilesand near-surface measurements from the M´et´eo-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 fluxe at the SIRTA observatory with an 67.9 W m−2 and a bias of 34.5 W m−2, while momentum-related quantities show larger deviations, particularly during weaknighttime stratification Near-surface temperatures are well captured across the M´et´eo-France network, with an RMSE of 1.18 ◦C and mean bias of -0.63 ◦C. DWL comparisons show that PALM captures the growth and decay of the UBL at urban and suburban sites, with maximum UBL heights reach-ing 2.4 km in the city centre, approximately 10% higher than in more ruralconditions. Neighbourhood-scale turbulent motions (O(100 m)) dominate ver-tical transport during daytime convective conditions, with contributions fromlarger scales depending on measurement height and location. These resultsdemonstrate the capability of high-resolution LES to represent convective UBL dynamics during extreme heat and to assess the representativeness of urbanatmospheric measurements. Overall, this study provides one of the firs city toneighbourhood-scale LES analyses of an extreme convective UBL supportedby multiple observations. By linking resolved turbulent scales to vertical cou-pling and measurement height, the results infer when and where urban ECobservations remain representative under strongly unstable conditions. Thefinding offe practical guidance for the interpretation and design of urbanmeasurements and highlight the importance of resolving neighbourhood-scaleturbulence in urban flw modelling applications.