Predictability of passive scalar dispersion in atmospheric surface layers with urban-like roughness : A large-eddy simulations study

The predictability of passive scalar dispersion is of both theoretical interest and practical importance, for example for high-resolution numerical weather prediction and air quality modeling. However, the implications for the numerical modeling of urban areas remain relatively unexplored. Using obstacle-resolving large-eddy simulations (LES), we conducted twin experiments, with and without a velocity perturbation, to investigate how the presence of urban roughness affects error growth in streamwise velocity (u) and passive scalar (θ) fields, as well as the differences between error evolutions in u and θ fields. The predictability limit is characterized using the signal-to-noise ratio (SNR) as a continuous metric to indicate when error reaches saturation. The presence of urban roughness decreases Τ_p of the passive scalar by around 20% compared to cases without them. The error statistics of θ indicate that urban roughness-induced flow structures and different scalar source locations affect the scalar dispersion and relative fluctuations, which subsequently dictate the evolution of the SNR. Analysis of the passive scalar error energy (ϵ_θ²) budget indicates that the contributions from advective transport by the velocity and velocity error dominate. The error energy spectra of both u and θ exhibit a −5/3 slope in flat-wall cases, but not in the presence of urban roughness, thereby highlighting the deviation from the assumption of locally isotropic turbulence. This study reveals that urban roughness can decrease the predictability of the passive scalar and destroy the similarity between the error statistics of the velocity and the passive scalar. © 2023 Royal Meteorological Society.