Concept for treating dense-gas dispersion under realistic conditions of terrain and variable winds.

摘要

A method is presented for including dense-gas effects in an existing advection-diffusion (particle-in-cell) type model capable of dispersion simulations over terrain and with time-varying synoptic winds. The physical processes associated with dense-gas dispersion affect both the windfield and the turbulent diffusivity. We propose to include these effects by perturbing the ambient windfield and diffusivity within the local region of the dense-gas cloud. The perturbed local windfield will be calculated by using a vertical or layer averaging approach and will be nested between the windfield and dispersion calculations. The ambient diffusivity will be replaced by an adapted form of a dense-gas, K-theory diffusivity that has the property that it approaches the ambient diffusivity level as the cloud density approaches the ambient value. For numerical models, the low-lying nature of dense-gas clouds often presents a problem with resolution in the vertical direction. This is largely overcome in the proposed approach by: (1) the use of vertical averaging and assumed vertical cloud profiles to calculate the dense-gas perturbations on the flow field and diffusivity, and (2) the use of the particle-in-cell technique with a Monte Carlo (stochostic) displacement equation that does not rely on the concentration gradient to calculate the trajectories of the concentration marker particles. These dense-gas dispersion modifications attempt to preserve the main features of the advection-diffusion, particle-in-cell model and, thereby, minimize the impact on the existing code. All of the changes can be implemented by either modifying an existing subroutine or adding a new one. 17 refs.