Multi-scale Analyses of Wildland Fire Combustion Processes in Open-canopied Forests using Coupled and Iteratively Informed Laboratory-, Field-, and Model-based Approaches
Dr. Nicholas Skowronski | USDA Forest Service, Northern Research Station
Many Department of Defense (DoD) facilities throughout the eastern United States use low-intensity prescribed fire to manage hazardous fuels, restore ecological function and historic fire regimes, and encourage the recovery of threatened and endangered species in the forests they manage. Current predictive models used to simulate fire behavior during low-intensity prescribed fires (and wildfires) are empirically based, simplistic, and fail to adequately predict fire outcomes because they do not account for variability in fuel characteristics and interactions with important meteorological variables. This project proposes to use a suite of measurements at the fuel particle, fuel bed, field plot, and stand scales to quantify how variability in fuel characteristics and key meteorological factors interact to drive fire behavior during low-intensity prescribed burns. Experiments have been designed to inform the development and evaluation of mechanistic, physics-based models that explicitly account for combustion, turbulent transfer, and energy exchange by coupling and scaling individual component processes. Outcomes from these experiments will improve the understanding of, and ability to accurately predict, fire behavior under a wide range of management scenarios.
The specific objectives of proposed research are as follows:
- Improve understanding of the processes driving heat transfer, ignition, thermal degradation, flaming and smoldering combustion, mass consumption, and fire propagation at the scale of individual fuel particles and fuel layers in low-intensity surface fires.
- Develop an understanding of how fuel consumption is affected by spatial variability in fuel particle type, fuel moisture status, bulk density, and horizontal and vertical arrangement of fuel components in low-intensity surface fires.
- Increase understanding of the effects of multi-scale atmospheric dynamics, including ambient and fire- and forest overstory-induced turbulence, on fire spread and convective heat transfer in low-intensity surface fires.
- Ensure that the measurements undertaken support the development and validation of physics-based fire behavior models using an iterative approach consisting of laboratory, field, and model simulations.
Proposed research will measure and simulate key factors controlling fire behavior across multiple spatial and temporal scales, enable necessary replication in both laboratory experiments and field activities, and account for relevant variation in fuel characteristics and meteorological factors. Research efforts will focus on open-canopy, pitch pine-dominated stands, with oaks, shrubs and herbaceous vegetation in the understory. These forests: (1) have a relatively simple forest structure, (2) are directly relevant to DoD installations throughout the Northeast (NE) United States (U.S.), (3) are adjacent to large population centers in the NE U.S., and (4) are an appropriate analog for other open-canopy, pine-dominated forests found on DoD lands on the Atlantic and Gulf Coastal Plains.
The project’s specific technical approach is to:
- Use a combination of integrated laboratory and field measurements to quantify particle-scale and fuel-bed-scale based combustion and turbulent transfer processes to support the development and testing of a mechanistic description of surface and ground fire behavior.
- Conduct a series of small-scale, highly instrumented prescribed burn experiments in the field to quantify the interactions among fuel-bed structure, moisture content, and meteorological factors (e.g. wind, humidity, temperature) driving variability in fire behavior and fuel consumption.
- Conduct two management-scale field experiments during operational prescribed burns to quantify how larger-scale atmospheric dynamics, including ambient, fire-induced, and forest canopy-induced turbulence regimes within and near the fire environment affect fire propagation, energy exchange, and fuel consumption.
- Augment the management-scale field experiments with numerical model simulations of coupled fire-atmosphere dynamics under differing environmental conditions to further understanding of how those dynamics affect management-scale fire propagation and heat transfer.
The approach spans scales with an ensemble of interactive and complementary studies: particle-based combustion experiments in the laboratory, fuel-bed scale combustion and wind-field experiments in the laboratory and the field, a series of small-scale, highly instrumented prescribed burn experiments in the field during dormant and growing season conditions, and management-scale field experiments conducted during two operational prescribed burns. Physics-based combustion, fire behavior, and atmospheric exchange models will be used to simulate a number of plausible scenarios to help guide management decisions.
The primary benefit of proposed research is that it will support the further development of physics-based fire behavior and coupled fire-atmospheric dynamics models, as well as support the assumptions needed to develop simpler models that would be appropriate for operational fire management decision-making. The proposed approach enable a detailed description of the fundamental phenomena that are driving the combustion of different fuels and to identify and quantify important fire-spread model parameters and evaluate their relative importance in driving fire behavior. This research also will result in data that will be invaluable to the fire behavior modeling community, because the gap between the highly controlled experiments in the laboratory and open-field, or in situ, conditions has never been adequately addressed in the field under well-characterized wind conditions. Proposed wind tunnel field experiments will bridge this gap and generate extensive data that can be used for physics-based model testing and improvement. These studies will result in vital data for simulating multi-scale atmospheric dynamics, including the interactive effects of ambient-, fire-, and forest canopy-induced turbulence, on fire spread and convective heat transfer during low-intensity surface fires. Finally, and most importantly, additional sensitivity simulations will be carried out to assess whether fire propagation and heat flux processes identified during the small-scale field measurement component of the study affect fire propagation on management-scale type fires. This assessment will help determine whether new tools that diagnose and predict those processes should be made available to fire managers and/or fire-weather forecasters for helping them make decisions when planning and executing low-intensity prescribed fires. Overall, the proposed research will enhance our understanding of combustion processes in wildland fuels and significantly improve our ability to accurately predict fire behavior under a wide range of management scenarios.