CTDMPLUS: A Dispersion Model for Sources near Complex Topography. Part I: Technical Formulations

Abstract
The Complex Terrain Dispersion Model (CTDMPLUS), a point-source, steady-state model for complex-terrain applications, is described. The model simulates the flow and plume distortion near user-selected, three-dimensional terrain features, yet retains simplicity by applying flow-distortion corrections to flat-terrain, Gaussian, and bi-Gaussian pollutant distributions. The algorithms for stable and near-neutral conditions are based on the demonstrated concept of a dividing streamline. These algorithms were developed using data from three major plume-impaction field studies and a number of fluid-modeling studies. The algorithms for plumes released into convective layers are based on recent understanding of the convective boundary layer through fluid modeling, numerical modeling, and field studies. The non-Gaussian nature of vertical dispersion is accounted for; lateral dispersion is modeled with the aid of convective scaling concepts. A terrain preprocessor and a meteorological preprocessor, which provide input specifically for the CTDMPLUS model, are described. The model requires a fully three-dimensional description of individual terrain features in order to estimate flow (and plume) distortions. Estimates of surface-layer parameters (friction velocity and Monin-Obukhov length) and depth of the mixed layer are required to define the state of the boundary layer.

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