Large eddy simulation of near-source diffusion into the convective boundary layer in weak winds

Most of diffusion models for continuous point source (e.g., Gaussian plume diffusion model) neglect the longitudinal diffusion in comparison with advection or transport by mean flows. However, measurement (Eberhard et al., 1988) and theory (Arya, 1995) indicate that in weak winds, longitudinal diffusion near the source becomes important and cannot be ignored. In this study, the behavior of longitudinal diffusion in the convective boundary layer (CBL) under weak winds is systematically investigated with varying nondimensional mean velocity using a validated large eddy simulation (LES) model, which will help develop a weak wind dispersion model.

The Lagrangian approach by Lamb (1982) is used for particle dispersion. Sufficient enough particles to determine the mean concentration of the passive material from a point source are released in a quasi-stationary CBL developed by LES under weak winds. Then, we track each of particles through the CBL using the LES-calculated Eulerian velocity field for Lagrangian velocities. Simulations are performed for varying nondimensional wind speed in weak wind (e.g., U/W*=0, 0.2, 0.5, 1.0, 1.5, 2.0, where U is the mean wind speed and W* is the convective velocity scale) and nondimensional source height. The mean concentration fields and the statistics of the concentration fluctuations from LES are analyzed as a function of nondimensional upwind and downwind distance (X=x/h), crosswind distance (Y=y/h), and height above the surface (Z=z/h), where h is the CBL height. The results are compared with those from the theory (Arya, 1995) and the Gaussian plume model.