LAGRANGIAN PARTICLE MODELING OF BUOYANT POINT SOURCES - PLUME RISE AND ENTRAPMENT UNDER CONVECTIVE CONDITIONS

This paper describes several aspects of a Lagrangian particle model capable of simulating dispersion from buoyant point sources. The equations of Briggs (American Meteorological Society, 1975) are used to calculate the plume final rise heights, and a skewed homogeneous turbulence parameterization is used within the convective boundary layer. The homogeneous assumption enables an order of magnitude greater time step to be used than is normally the case, and was shown to have minimal effect on hourly averaged ground level concentrations by Hurley and Physick (Atmospheric Environment 25A, 1313-1325, 1991; 27A, 619-624, 1993). By including statistics from both ambient and source-induced (plume) turbulence in the probability density function (pdf) of the Langevin equation, we are able to apply this equation to particles in the plume as it rises from the stack to the final rise height. The model is used here to simulate various plume rise and entrapment laboratory experiments of Willis and Deardorff (Atmospheric Environment 17, 2435-2447, 1983; 21, 1725-1735, 1987) under convective conditions with a capping stable atmosphere. The simulations show that the model can reproduce the results of the laboratory experiments when a 15% enhancement to the entrainment parameter in the mean plume rise equations is used. Justification for this modification can be related to neglect of the effect of ambient turbulence upon entrainment in the plume rise equations, which in free convective turbulence may be significant.