Large eddy simulations of Urban Turbulent Fluxes Over Enschede city, The Netherlands: Evaluation Using Eddy Covariance Tower Observations

Understanding urban energy fluxes and validating them against observations is crucial for improving urban climate models and their predictive capabilities. In this study, Large Eddy Simulations (LES) were conducted using the PALM4U model to investigate turbulent fluxes over the city of Enschede, The Netherlands, under two distinct meteorological conditions: a windy spring day (10-m wind speed: 4–8 m/s) and a calm, hot summer day (10-m wind speed: 0–3 m/s). The simulations capture the dominant turbulence mechanisms under these conditions, where mechanical shear drives turbulence on the windy day, while land surface heating drives the turbulence on the calm summer day. LES simulations were performed at a spatial resolution of 2 m on a 768 × 768 grid driven by Weather research and Forecasting (WRF) model results as the boundary conditions, covering an area of approximately 1.5 km^2. Model results were evaluated against Eddy covariance tower observations, with half-hourly averaged sensible heat flux showing a good agreement.  While average building height over Enschede city center is approximately 20 m, simulations show strong heterogeneous turbulence structures even at 60 m above ground, influenced by the neighboring built environment and local surface characteristics, affecting observational representativity. A key finding is the successful simulation of nighttime sensible heat flux, which remains a challenge for mesoscale models like WRF, even at hectometric scales. Additionally, the study assessed the impact of Enschede’s sloped terrain on  urban turbulent fluxes, highlighting terrain-induced variations in energy exchange. These results demonstrate the advantages of LES in resolving fine-scale turbulence structures and improving the representation of urban atmospheric processes. The findings emphasize the need for high-resolution modeling to enhance our understanding of urban heat flux dynamics, particularly in heterogeneous environments, and to refine model parameterizations for future urban climate applications.