Simulating the characteristic patterns of the dispersion during sunset PBL

A diffusion equation limit derived from the Langevin stochastic particle model is used to study the dispersion of scalars caused by an evolving turbulence in the transition process occurring during the sunset period. Therefore, the random displacement equation is employed to simulate the cross-wind concentrations of pollutants released from low and high point sources. Turbulence inputs are parameterized to describe in a continuous manner the dispersion effects produced by a decaying convective elevated and a shear-dominated stable surface turbulence. The simulation results show that for the initial stage of the sunset transition phenomenon, the pollutants are transported rapidly to the surface. On the other hand, for the sunset evolution advanced stages, the dispersion process happens in a deep stable boundary layer (SBL), in which the pollutants can travel long distances practically without reaching the surface. The major progress shown in this analysis is the description of the transport properties associated to decaying convective eddies in the residual layer (RL). The study shows that the diffusion effects associated to these decaying convective eddies strongly influence the dispersion of scalars during the sunset transition period.