Dispersion of a passive plume in an idealised urban convective boundary layer: A large-eddy simulation

This large-eddy simulation study investigates the effects of an idealised surface inhomogeneity of sensible heat flux on dispersion of a passive plume emitted from elevated sources into an urban convective boundary layer (UCBL). The results show that when Deardorff's translation is made to introduce a wind with its direction aligned with the centreline of park blocks, dispersion of such a passive plume is strongly affected by turbulent structure associated with the patchy pattern of surface heat flux. In comparison with a case with a homogeneous surface, when the point source is located above the central line across the park areas, the mean plume height Z(c) is lower, the vertical dispersal parameter S(z) is smaller, the surface concentration C(o) is generally higher, and pollutant is less dispersed in the vertical direction. When the point source is aligned with the built-up area, however, the opposite situation occurs. The difference in surface concentration at a same downwind distance can be as large as 100% among the cases over a homogeneous surface, the location above built-up surface and the location above park surface in the idealised urban area. The extent to which the patchy pattern of surface heat flux influences the plume dispersion depends on two parameters: d/Z(i) and b/p, where d is the distance between the centres of two adjacent parks, b is the width of built-up area, p is the size of park, and Z(i) is the UCBL height. This study suggests that dispersion of a passive plume in a UCBL has different behaviour from that for a homogeneous surface in the previous studies. Estimate of pollutant concentration by the existing methods or models that are based on observations over a homogeneous surface cannot distinguish such differences revealed by the results in this paper.