The Role of Plume Rise and Buoyancy in the Trajectory and Dispersion of Fire Emissions

Heat, particle, and gas emissions from fires vary in time and space, causing unique patterns of convection and resulting plume rise. This plume rise is a function of free convection in the atmosphere, which is caused by density differences within the fluid. As a fire heats and expands air near the ground, large density differences between the heated volume and the surrounding air mass are created, causing the heated parcel to rise. The potential height of the resulting plume depends on the heat energy of the source and rise velocity, which is affected by the exchange and conservation of mass, radiant heat loss, the buoyancy force, and turbulent mixing with the ambient air.

Hot, flaming fires can develop central convective columns with counter-rotating vortices that involve massive entrainment of the surrounding air mass (Clark and others 1996; Haines and Smith 1987; Haines and Updike 1971). This stage of fire can produce fast-rising plumes and turbulent downdrafts, carrying sparks that ignite new fires (see Fire and Plumes). Cumulonimbus clouds often develop with accompanying lightning and rain. Dynamic plume rise brings gas and particles high into the atmosphere where strong winds can disperse the smoke hundreds to thousands of kilometers. As high intensity fires cool, however, the central column often collapses, creating numerous small convective cells that are less dynamic but equally active in carrying smoke into the atmosphere. Smoldering fires often create plumes that are neutrally buoyant, limiting widespread dispersion but allowing surface winds to dominate smoke trajectories. This can lead to accumulations of smoke in valleys and basins at night.

Because plume rise can eventually result in wide-spread dispersion, plume rise calculations are essential for determining the height above ground from which plume dispersion is initiated. Uncertainties in such calculations can result in inaccurate predictions of plume transport and downwind smoke impacts. Given the pressing need to predict the impact of plumes from fires, the need for improved plume rise calculations is apparent.

Limitations of current plume-rise models

The basic mechanisms and algorithms used to describe plume rise and buoyancy were developed in the mid-1960s by Briggs (1969) for industrial, ducted emissions. These methods are still used today to estimate the plume rise and buoyancy of fires in spite of the significant differences in characteristics between ducted emissions and prescribed and wildland fires:

• Heat released from ducted sources is precisely known and usually emitted at relatively constant rates during a single phase of combustion. Heat released from fires is a function of fuel loading, fuel conditions, and ignition method through several phases of combustion (pre-ignition, flaming, smoldering, and residual), which create highly variable magnitudes and rates of heat release.
• Nearly all of the energy generated at the source of a ducted plume is transmitted to convection energy. In open burning, however, significant amounts of energy are lost by conduction and radiation, reducing the amount of available energy for convection.
• Plumes from ducted sources create single convective columns, but low intensity understory burning that occurs over broad areas does not develop a cohesive plume.

To improve plume rise predictions, emission production models need to do a better job of characterizing the spatial and temporal pattern of heat release from fires, and plume rise models need to be improved to account for the energy lost from the convective system through radiation and turbulent mixing. While models such as EPM and Burnup (described in Heat Release) simulate variable rates of heat release from fires, both models use general estimates of spatial distributions of fuel, including structure, composition, and moisture content. Also, significant elements of fires that influence convective energy -- such as the distribution of naturally piled fuel ("jackpots"), amount and density of rotten fuel and duff, and release of water vapor -- are not adequately captured.

Rough approximations on the proportion of energy available for convection were made more than 40 years ago (Brown and Davis 1959). Despite efforts to improve plume rise calculations by removing the density difference assumption (Scire and others 2000a), they still are in use today.

Low intensity fires that typically do not have a cohesive convective column must be treated, from a modeling perspective, as an area source in Eulerian grid models. In Lagrangian dispersion models, there is currently no valid means of calculating plume rise from unconsolidated convection. Eulerian coordinates (used by box and grid models) are coordinate systems that are fixed in space and time, and there is no attempt to identify individual particles or parcels from one time to the next. Lagrangian models (bell-shape or Gaussian distribution pattern, often applied to plume and puff models) are used to show concentrations crosswind of the plume.

Another complication for modeling is that once plumes from fires enter the atmosphere, their fluctuating convection dynamics make them more susceptible to erratic behavior than well-mannered industrial stacks. For example, different parts of a plume can be carried to different heights in the atmosphere at the same time. This causes unusual splitting patterns if there is a notable wind shear between lofted elevations, causing different portions of the plume to be transported in different directions. Therefore, predictions of the plumes impact on visibility and air quality under these conditions become highly uncertain (Walcek 2002). Even when the behavior of plumes from fires resembles that of stack plumes, the varying and widely distributed locations of wildland sources prevent consistent study. For example, downwash of plumes has been observed from ducted (stack) emissions after an inversion breaks up-  conditions that are common at the end of an onshore breeze if the plume is above the inversion at its source (de Nevers 2000; Venkatram 1988) or if horizontal stratification in the lower atmosphere is disrupted by mountains (de Nevers 2000).

These characteristics of plumes from fire are strikingly different than those of ducted industrial emissions yet little research has been done on this topic in the past several decades.