2025/04/07 by Maarten van Reeuwijk, van Reeuwijk, Maarten, Jingzi Huang +1
Earth and Planetary Sciences · Environmental Science · #FOS: Physical sciences #Fluid Dynamics (physics.flu-dyn) #Meteorological Phenomena and Simulations #Tropical and Extratropical Cyclones Research #Wind and Air Flow Studies
paper · pdf · doi:10.48550/arxiv.2504.05391
openalex publication_date 2025/04/07 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
A computationally efficient multi-scale planar-averaging framework for urban areas is developed, which enables efficient computation of coarse-grained velocity and scalar fields. We apply the multi-scale framework to a large-eddy simulation of an idealised heterogeneous urban environment of 512 buildings based on a typical London height distribution. We observe that for this geometry, the characteristic urban lengthscale ℓ ≈ 50 m, which is the averaging lengthscale L at which as much variance in the mean flow is resolved as is unresolved. For L>400 m, the statistics become approximately homogeneous, suggesting that non-building-resolving numerical weather prediction (NWP) models can be applied without modification at resolutions of 400 m and above for the case under consideration. We derive the multi-scale plane- and Reynolds-averaged momentum equation and show that for neutral cases, NWP models require parameterisation of the distributed drag and the unresolved turbulence and dispersive stress. An a priori analysis reveals that the drag parameterisation from Sutzl et al. 2020, Bound-Layer Meteorol., 178:225-248 holds reasonably well for resolutions L above 200 m. Below this value, the problem becomes inhomogeneous and the parameterisation works less well. The unresolved stresses are well represented by a k-ω closure with a value of ω=0.4/s. However, an even more accurate closure can be derived from the Sutzl drag parameterisation that does not require further turbulence information.