A Numerical Study of Smoke Dispersion Sensitivity to Forest Canopy Structure and Fire Sensible Heat Source Magnitude

Low-intensity prescribed fire is a tool frequently used for fuels and habitat management on public and private lands. Smoke from such fires can have a direct effect on public health and safety in nearby communities, as well as the health and safety of operational fire management personnel, mainly through air quality and visibility degradation. Predicting the local dispersion of smoke from low-intensity prescribed fires is challenging because dispersion is sensitive to environmental factors such as near-surface meteorological conditions, local topography, forest overstory vegetation, and atmospheric turbulence within and above vegetation layers, in addition to fuels, the time of day when the fire is ignited, and the pattern of ignition. Fine-scale simulations of smoke dispersion and fire behavior have the potential to yield knowledge that ultimately helps fire managers make go/no-go decisions, assess the potential for a given fire to meet management goals, and better anticipate local-to-regional-scale smoke impacts. Specifically, there is a general need to better understand the vertical distribution of smoke concentration above low-intensity fires and its relationships to fuels, fire dynamics (e.g., fire intensity), and conditions in the lower atmosphere. There is also a need for studies of how low-intensity fires influence mean and turbulent flows within a forest canopy and how these interactions affect smoke transport and dispersion.

This study aims to address two primary research questions: (1) What is the dependence of the smoke vertical distribution on fire sensible heat release? (2) What is the relationship between the smoke vertical distribution and characteristics of the underlying forest canopy? To help answer these research questions, a Lagrangian particle dispersion model (FLEXPART-WRF) is used to simulate particulate matter dispersion using meteorological output from a full-physics atmospheric model with a forest canopy sub-model (ARPS-CANOPY). Simulations are run with 25 combinations of fire sensible heat source magnitude and forest canopy vertical structure. Analyses presented include vertical cross-sections and profiles of particulate matter concentration, two- and three-dimensional plots of smoke particle positions, and summary statistics relating smoke vertical distribution, fire heat source magnitude, and canopy vertical structure. Results show that near-surface smoke vertical distribution is sensitive to both fire heat source magnitude and canopy vertical structure, with the greatest differences in smoke distribution occurring between cases with and without the fire sensible heat source, and between cases with overstory- and understory-concentrated forest canopy profiles.