When studying turbulent dispersion two driving mechanisms have to be taken in account, namely the thermal and the mechanical forcing. Although dispersion of a tracer in a Convective Boundary Layer (CBL) is a relatively well-known process investigated over the last decades by means of theoretical, numerical and field/laboratory works, numerical studies of dispersion that combine buoyancy and shear-driven boundary layer are less frequent. In this study, the dispersion of an inert tracer line source in a boundary layer with different combinations of buoyancy force (surface heat flux) and geostrophic wind is analyzed by means of a Large Eddy Simulation (LES).

The first experiment studies tracer dispersion in a pure buoyancy case (free convection). The dispersion parameters are analyzed as function of the diffusion time and source height. In our analysis a distinction is made between the various contributions to the diffusion process. In particular, the total dispersion parameter is divided in two components: the meandering part describing the contribution of the large-scale turbulent eddy motion and the relative dispersions related to the increasing size of the plume due to small-scale mixing.

This study is compared to several experiments of dispersion in a shear driven boundary layer prescribing different values of the geostrophic wind and surface heat flux. The dispersion of the plume in terms of the shear-buoyancy ratio (u_*/w_*, where u_* is the friction velocity and w_* is the convective scaling velocity) is also considered. This value plays a role in the formation of 2-dimensional rolls and might influence tracer dispersion. The results show that increasing the wind speed limits the vertical mixing, and the tracer is mainly advected horizontally. Consequently, the ground concentrations as well the dispersion parameters are influenced. In short, a tracer released near the ground is transported horizontally by the increasing wind for a longer time before lifting off, whereas a tracer emitted at higher elevations reaches the ground more slowly than in the pure buoyancy case. Similarly the vertical dispersion parameter is reduced both for near ground and higher releases. The horizontal dispersion is enhanced because of the contribution to the dispersion of the v-component of the wind.