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Abstract 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 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 θ 2 ) 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.
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Revisiting the Relation Between Momentum and Scalar Roughness Lengths of Urban SurfacesRevisiting the Relation Between the Momentum and Scalar Roughness Lengths of Urban Surfaces
Large Eddy Simulations (LES) of neutral flow over regular arrays of cuboids are conducted to explore connections between momentum (z 0m ) and scalar (z 0s ) roughness lengths in urban environments, and how they are influenced by surface geometry. As LES resolves the obstacles but not the micro‐scale boundary layers attached to them, the aforementioned roughness lengths are analyzed at two distinct spatial scales. At the micro‐scale (roughness of individual facets, e.g. roofs), it is assumed that both momentum and scalar transfer are governed by accepted arguments for smooth walls that form the basis for the LES wall model. At the macro‐scale, the roughness lengths are representative of the aggregate effects of momentum and scalar transfer over the resolved roughness elements of the whole surface, and hence they are directly computed from the LES. The results indicate that morphologically‐based parameterizations for macro‐scale z 0m are adequate overall. The relation between the momentum and scalar macro‐roughness values, as conventionally represented by log(z 0m /z 0s ) and assumed to scale with urn:x-wiley:00359009:media:qj3839:qj3839-math-0001 (where Re * is a roughness Reynolds number), is then interpreted using surface renewal theory (SRT). SRT predicts n = 1/4 when only Kolmogorov‐scale eddies dominate the scalar exchange, whereas n = 1/2 is predicted when large eddies limit the renewal dynamics. The latter is found to better capture the LES results. However, both scaling relations indicate that z 0s decreases when z 0m increases for typical urban geometries and scales. This is opposite to how their relation is usually modeled for urban canopies (i.e. z 0s /z 0m is a fixed value smaller than unity).
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- PAR ID:
- 10183078
- Date Published:
- Journal Name:
- Quarterly journal of the Royal Meteorological Society
- ISSN:
- 0035-9009
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1002/qj.3839
