Amplitude modulation of velocity fluctuations in the atmospheric flows over real urban morphology

Amplitude modulation (AM) quantifies the top-down interactions between the large-scale motions (LSMs) in the outer layer and the near ground turbulence structures. They are important to the momentum transport and pollutant dispersion in urban atmospheric surface layers (ASLs). The dataset of large-eddy simulation over a densely built region in Kowloon Peninsula, Hong Kong, therefore, is adopted to investigate the AM of small-scale eddies by LSMs in the ASL over real urban morphology. Alike its smooth-wall counterpart, the small-scale eddies are (positively) amplitude modulated by the LSMs in most regions of the roughness sublayer (RSL). However, negative AM is unexpectedly found in the RSL on the building windward side in this study, illustrating the heterogeneity of the urban surface and the flow dynamics being affected aloft. In addition, strong sweep (u ' > 0 and w ' < 0) and ejection (u ' < 0 and w ' > 0) dominate the flows, respectively, in the positive and negative AM zones. In the positive AM zones, the large-scale sweep (u(L)' > 0) leads to the surplus in the small-scale turbulence kinetic energy (TKE), while the large-scale ejection (u(L)' < 0) brings a TKE deficit to the small-scale eddies. By contrast, the large-scale sweeps result in a TKE deficit to the small-scale eddies and the large-scale ejections result in a TKE surplus in the negative AM zones. These findings could help elucidate the AM over different building designs and urban morphology in cities, promoting the momentum transport and pollutant dispersion via proper city planning.