Abstract:

Urban areas, which are increasingly shaped by thermal instability and surface roughness, demand deeper insight into multiscale flow dynamics to mitigate heat-related risks. This presentation adopts eight cases of large-eddy simulation (LES) to systematically examine the influence of atmospheric instability on convective urban boundary layers (UBLs) interacting with idealized building roughness elements, covering thermal stratification from shear-driven (neutral) to buoyancy-driven (unstable) conditions. Spectral proper orthogonal decomposition (SPOD) is employed as a post-processing technique to analyze the dynamics within thermally unstable urban canopies. The analysis focuses on how thermal instability affects the correlation among streamwise and vertical velocity components and potential temperature, together with their contributions to coherent structures such as ejections and sweeps. The spatial characteristics of SPOD modes indicate that external, large-scale coherent structures at building roof level play key roles in triggering and transporting small-scale turbulence. By comparing forced and near-free convective conditions, changes in dominant flow scales are identified, and the collective influence of surface roughness and instability on the generation of large-scale motions (LSMs) is revealed. Furthermore, the coupling and energy exchange between small-scale turbulence within urban canopy and the overlying convective UBL are explored, highlighting critical multiscale interactions for urban heat and momentum transport.

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