Diurnal wind-structure variations and dispersion of pollutants in the boundary layer

Abstract

A coupled boundary-layer model and Lagrangian particle model are used to investigate the role of boundary-layer shear especially that produced by inertial oscillations in affecting the horizontal dispersion of pollutants on time-scales of 24–36 h. The coupled models show that the amplitude and the effective periods of the inertial oscillations are the main cause of nocturnal accelerating dispersion. The effective width of the plume in the morning is determined by whether the morning daytime mixing coincides with the phase of the inertial oscillation being at a maximum or minimum value. The phase of the oscillation is determined by latitude. Thus, latitude is shown to be an extremely important parameter in determining horizontal dispersion. An analytical model is introduced to investigate the role of external parameters such as latitude in influencing the horizontal dispersion. The analytical model is based on a simple Ekman-type model for the daytime and nighttime boundary layer. The Ekman model is used to provide initial conditions to an inertial oscillation regime between the nighttime boundary layer and the old daytime boundary layer. The analytical model was able to reproduce the magnitude and phase of the inertial oscillations reasonably well. However, the Ekman model overestimates the shear in the boundary layer causing the inertial oscillation to be too large. A semi-empirical method was used to provide more reasonable estimates of the daytime boundary-layer structure. This semi-empirical approach gave rates of the horizontal dispersion which were in general agreement with the numerical results.