Scalar Dispersion over Surface Discontinuities

Gaussian dispersion model is commonly used to parameterize the transport of air pollutants in the atmospheric boundary layer (ABL). One key parameter of the model is the dispersion coefficient which is usually obtained empirically and only valid for the cases of fully developed ABL over homogeneous terrains. However, land surfaces are seldom homogeneous (for example, rural-to-urban surface transition), the use of a homogenous dispersion coefficient is questionable. As such, an improved parameterization of the dispersion coefficient is needed in order to accurately model the scalar dispersion patterns above surface discontinuities. In this study, we use computational fluid dynamics (CFD) large-eddy simulation (LES) method to simulate the flow and scalar dispersion over surface discontinuities. Sinusoidal surfaces with different wave lengths and magnitudes are used to represent generic rough surfaces. The development of internal boundary layer including the adjustment of mean wind and turbulence are investigated. The changes in the characteristic velocity and turbulent length scales over the surface discontinuities are found to directly influence the scalar dispersion patterns. Based on the LES results, a simple parameterization of the scalar dispersion coefficient is proposed. The accuracy of the parametrization is justified with simulation results of scalar dispersion over different rough terrains.