Multi‐scale flow analysis for scale‐aware urban canopy models

Abstract As numerical weather prediction (NWP) models approach hectometric resolution, they operate increasingly in a regime where urban heterogeneity, specifically the morphological heterogeneity and resulting flow heterogeneity, is only partially resolved and the assumptions underlying conventional urban canopy models (UCMs) become questionable. To address this scale gap, we apply the multi‐scale coarse‐graining framework to building‐resolving large‐eddy simulations (LES) of the University of Bristol campus, a realistic heterogeneous urban environment. Two related morphologies are considered: an original layout containing large open‐space contrasts and a modified configuration with these regions infilled. By filtering the LES fields systematically, we quantify how flow heterogeneity is partitioned between resolved and unresolved contributions as the averaging length changes and identify a characteristic urban length‐scale of flow heterogeneity at which resolved and unresolved variability are comparable. This scale is strongly morphology‐dependent, with m for the original layout and m for the modified case, demonstrating that neighbourhood‐scale organisation can remain dynamically important at resolutions relevant to next‐generation NWP. Using this framework, we perform an a priori assessment of distributed drag and turbulent‐stress parameterisations. The results show that parameterisations derived from idealised geometries perform reasonably well only at sufficiently large averaging length‐scales (, corresponding to coarse resolutions), where horizontal transport is negligible and the flow appears approximately homogeneous. At smaller averaging lengths, their fidelity degrades rapidly due to increasing heterogeneity and filter‐to‐filter variability in morphology. These limitations are more pronounced in realistic layouts than in idealised cuboid arrays. Overall, the results highlight that the applicability of urban parameterisations depends critically on the relationship between model resolution and the characteristic urban length‐scale of morphology‐dependent flow heterogeneity. The framework provides a systematic route to diagnose this scale and to guide the development of scale‐aware urban canopy models for high‐resolution NWP.