LARGE-EDDY SIMULATION OF DISPERSION IN CONVECTIVE BOUNDARY-LAYERS WITH WIND SHEAR

A series of large-eddy simulations of the planetary boundary layer with various values of imposed surface heat flux and geostrophic wind speed have been conducted. These simulations consider conditions varying from free convection to a neutral static stability shear flow. The dispersion characteristics of these boundary layers have been investigated by numerically calculating flow trajectories within the time-varying velocity fields. Noting that only large-scale motions are explicitly described in these simulations, attention is confined to dispersion occurring on scales comparable with the boundary-layer depth. The dispersion characteristics of the free convection limit shows good agreement with past results and for values of u*/w* less-than-or-equal-to 0.3 (where u* is the friction velocity and w* the convective scaling velocity) the dispersion properties are close to free convection, With larger values of u* (for fixed w*) the turbulence energy increases and so must short-time dispersion rates. In contrast, the vertical dispersion at greater time is reduced and takes on a simple diffusive character as the characteristic features of the convective boundary layer decline. This marked change in vertical dispersion is seen to be linked to the reduced length scales of shear-driven turbulence compared with convective turbulence.