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1. Introduction
Smoke dispersion is strongly affected by the structure and evolution of wildfire convective plumes. When wildfire plumes penetrate into the free troposphere they inject smoke aloft, causing regional- to global-scale impacts such as reduced insolation (Penner et al. 1992) and modified cloud microphysics (Andreae et al. 2004). On the other hand, when plumes remain confined within the atmospheric boundary layer, the smoke can more directly impact human populations, posing serious health hazards for affected communities (Delfino et al. 2009; Wegesser et al. 2009; Holstius et al. 2012; Johnston et al. 2012). Near-surface smoke can also cause persistent temperature inversions (Robock 1988, 1991), unexpected patterns of smoke transport (Lareau and Clements 2015), and travel hazards due to reduced visibility (Ashley et al. 2015). Satellite observations indicate that only a small fraction (4%-12%) of smoke plumes extend above the boundary layer (Kahn et al. 2008; Val Martin et al. 2010), but detailed observations of the plume-rise dynamics leading to variations in smoke injection height are lacking.
To date, most of our knowledge of convective plume-rise dynamics stems from laboratory tank experiments and theory. From experiments, simple formulas for plume rise in neutral and continuous stratification have been developed (Morton et al. 1956; Scorer 1957). Semiempirical formulas have also been established for more-complex cases with crosswinds and density-stratified interfaces (Richards 1961, 1963; Saunders 1962; Linden 1973; Briggs 1975; Manins 1979). Among these formulations, Briggs’s equation for buoyant plumes in a crosswind has gained widespread use and has been validated for a range of heat fluxes from industrial sources (Briggs 1975; Weil 1988). Benech et al. (1988), for example, found good agreement between observations of plume rise during the “Météotron” oil-burner experiments (Benech 1976; Church et al. 1980) and the Briggs equation.
The applicability of the Briggs plume-rise equation for wildfire plumes is, however, less clear. Raffuse et al. (2012) found systematic underprediction of smoke injection depth using the Briggs equation (embedded in a weather model) as compared with satellite lidar measurements. On the other hand, Cunningham and Goodrick (2013) found relatively good agreement between plume rise in a large-eddy simulation and the Briggs equation, at least in terms of plume centerline for an isolated plume. Plume isolation, however, is not necessarily observed during fires, and Achtemeier...