The Role of Advection and Diffusion in the Trajectory and Dispersion of Fire Emissions

In most existing models, the horizontal advection of smoke and its diffusion (lateral and vertical spread) are assumed to be controlled mainly by wind, and the formation and dissipation of atmospheric eddies. These elements are greatly simplified by assuming constant wind (at least for an hourly time step) in some cases (such as VSMOKE and SASEM), and a Gaussian dispersion is nearly always imposed. Perhaps the most critical issues are the constantly changing nature of the plume due to scavenging, chemical transformation, and changing convection dynamics that affect plume transport.

Many photochemical and dispersion models depend on gridded meteorological inputs. Unfortunately, numerical formulations of dynamic meteorological models (for example, MM5: Grell and others 1995; RAMS: Pielke and others 1992) do not adequately conserve several important scalar quantities (Byun 1999a, 1999b). Therefore, modelers often introduce mass-conserving interpolations. For example, Models-3/CMAQ (Byun and Ching 1999) uses the MCIP scheme (Byun and others 1999), Calpuff (Scire and others 2000a) employs CALMET (Scire and others 2000b), and TSARS+ (Hummel and Rafsnider 1995) is linked with NUATMOS (Ross and others 1988). Driving a photochemical or dispersion model without these mass-conserving schemes will produce inaccurate results, especially near the ground surface.