Atmospheric Flows and Pollutant Transport over Real Urban Morphology: Empirical Mode Decomposition

Momentum transport and pollutant dispersion in the atmospheric surface layer (ASL) are governed by broad-spectrum turbulence, which, however, have not been explicitly investigated in the context of real urban morphology. The building-resolved large-eddy simulation (LES) of winds and pollutants over the Kowloon downtown, Hong Kong, is decomposed into a few intrinsic mode functions (IMFs) through empirical mode decomposition (EMD). It is shown that four IMFs are sufficient to capture most of the turbulence kinetic energy (TKE) in real-urban ASL. The first and second IMFs, which are initiated by individual buildings, capture the small-scale vortex packets, populating within the irregular building clusters. On the contrary, the third and fourth IMFs capture the large-scale motions (LSMs) detached to the ground surface that largely govern the transport. LSMs are long, streaky structures that mainly consist of streamwise TKE components, enhancing the vertical transport processes. Besides, the streaky LSMs play crucial roles in near-field pollutant dilution while the small-scale vortex packets are more important to mid- and far-field processes.