8.1 BACKGROUND

        The trajectory and diffusion of an atmospheric plume are known to be influ-enced by complex terrain.  A number of models have been developed to emulate special processes in complex terrain (i.e., Burt 1977; Hovind et al. 1979; Strimaitis et al. 1983; Allwine and Whiteman 1988; Bader and Whiteman 1989).  The effects of the underlying surfaces can modify the transport, dis-per-sion, and deposition processes.  Areas with high ground-level concen-tra-tions can occur as the result of two different complex terrain effects.  First, under moderate or high wind conditions, an elevated plume may impinge on local topographical features resulting in localized high-level concentra-tions.  Second, under low wind or stable atmospheric conditions, a channeling of releases in a single direction can maximize ground-level concentrations.
 
        MEPAS accounts for the effects of underlying surfaces on the vertical wind structure and the deposition rate of airborne pollutants onto these surfaces.  These formulations are designed to improve the computations of climatological transport, dispersion, and deposition for a given site by using site-specific input data.

       A straight-line Gaussian atmospheric model, which assumes the plume tra-jec-tories are in straight lines from a central release point, cannot account for the processes in complex terrain where the winds tend to follow local topo-graphical features (valleys, gorges, slopes, etc.).  A modified model has been developed that incorporates the major influences of local channeling for wind channeling in the immediate vicinity of the release.
 
        An elevated release which is lower than surrounding terrain may inter-sect local topographical features.  Studies have shown that the tendency for the plume to intersect a hill is a function of the ambient meteorological condi-tions.  The techniques described by Hanna et al. (1982) for accounting for topographical interactions in complex terrain have been adapted for appli-ca-tion in the MEPAS climatological dispersion component.  The formula-tions given below account for the tendency for winds to intersect or flow over terrain features depending on ambient atmospheric conditions.