The role of advection of fluxes in modelling dispersion in convective boundary layers

摘要

A Eulerian three-dimensional higher-order closure dispersion model is presented. The model uses mean wind and turbulence quantities from a second-order atmospheric boundary-layer model. The dispersion model is validated against results from tank andfield experiments and compared to results from Lagrangian dispersion models. The results show quite good agreement with experiment and Lagrangian modelling results for point source dispersion in a convective boundary layer (CBL). Sensitivity studies with the model help to identify the role played by advection and horizontal transport terms in the equations for the fluxes in simulating the essential features of pollutant dispersion. Results from the sensitivity tests show that the characteristic features of dispersion from point sources in the CBL--with an ascending plume during ground-level release and a descending plume from a lifted point source--are caused by an imbalance between the advection and diffusion terms in the equation for the vertical flux. Furthermore, it is shown that there is also a tendency for the plume to split horizontally, which is similarly caused by an imbalance between the advection and diffusion terms in horizontal fluxes. The simulated lateral standard deviations of distance x from a point source, for the vertically integrated concentration, are proportional to x close to the source and to x~(1/2) far away from the source, if and only if the advection and diffusion terms are included in the equations for the turbulent fluxes of concentration.