Typically, average wind speeds of 2 m s À1 or less are treated as light winds. Conventional modelling techniques, such as K-theory and Gaussian plume models, are inadequate at describing upwind diffusion and horizontal plume meander that occur in light winds. We apply the Lagrangian particle technique to model mean dispersion in light winds under both stable and unstable atmospheric stratifications. In stable conditions, we examine two formulations in the horizontal plane: one in which the Langrangian velocity autocorrelation is oscillatory (and which is known to lead to significant reduction of dispersion and spiralling particle trajectories) and, the other, a traditional one with an exponential autocorrelation. The former preforms slightly better for the same values of the Lagrangian time scales, turbulent kinetic energy (TKE) and its dissipation rate. Regardless of which formulation is used, the time scales and TKE must include the low-frequency meander in the horizontal. The use of conventional similarity parameterisation of horizontal turbulence leads to a very narrow plume with little upwind diffusion compared to the data. The Lagrangian modelling in unstable conditions indicates that the time elapsed since the start of the release is an important parameter, which implies that the assumption of steady state used in some models may pose problems. As winds become lighter, the maximum ground-level concentration decreases, but the ground area impacted by a given concentration is enlarged. Model application in a weakly convective surface layer shows that the neglect of turbulent velocity covariance leads to an overestimation of concentration by about 35%.
LAGRANGIAN PARTICLE MODELING OF DISPERSION