Rodman R. Linn

STUDYING WILDFIRE BEHAVIOR USING FIRETEC

A coupled atmospheric/wildfire behavior model is described that utilizes physics-based process models to represent wildfire behavior. Five simulations are presented, four of which are highly idealized situations that are meant to illustrate some of the dependencies of the model on environmental conditions. The fifth simulation consists of a fire burning in complex terrain with non-homogeneous vegetation and realistic meteorological conditions. The simulated fire behavior develops out of the coupling of a set of very complex processes and not from prescribed rules based on empirical data. This represents a new direction in wildfire modeling that we believe will eventually help decision makers and land managers do their jobs more effectively.

Modeling interactions between fire and atmosphere in discrete element fuel beds

In this text we describe an initial attempt to incorporate discrete porous element fuel beds into the coupled atmosphere–wildfire behavior model HIGRAD/FIRETEC. First we develop conceptual models for use in translating measured tree data (in this case a ponderosa pine forest) into discrete fuel elements. Then data collected at experimental sites near Flagstaff, Arizona are used to create a discontinuous canopy fuel representation in HIGRAD/FIRETEC. Four simulations are presented with different canopy and understory configurations as described in the text. The results are discussed in terms of the same two discrete locations within the canopy for each simulation. The canopy structure had significant effects on the balance between radiative and convective heating in driving the fire and indeed sometimes determined whether a specific tree burned or not. In our simulations the ground fuel density was the determining factor in the overall spread rate of the fire, even when the overstory was involved in the fire. This behavior is well known in the fire meteorology community. In the future, simulations of this type could help land managers to better understand the role of canopy and understory structure in determining fire behavior, and thus help them decide between the different thinning and fuel treatment strategies available to them.

Coupled influences of topography and wind on wildland fire behaviour

Ten simulations were performed with the HIGRAD/FIRETEC wildfire behaviour model in order to explore its utility in studying wildfire behaviour in inhomogeneous topography. The goal of these simulations is to explore the potential extent of the coupling between the fire, atmosphere, and topography. The ten simulations described in this paper include five different topographies, each run with two different ambient wind speeds of 6 and 12 m s–1. The five topologies explored are: an idealised hill (which serves as the base centerline for the other topographies), two variations of the hill with lateral gradients downwind from the ignition line (one sloping up from the ‘hill’ at the centerline to form an upward sloping canyon parallel to the ambient wind, and the other sloping down from the centerline to form a ridge parallel to the ambient flow), one with a second hill upwind of the ignition line such that the fire is ignited in the bottom of a canyon that runs perpendicular to the ambient wind, and finally a flat terrain. The four non-trivial topographies have the same profile along the centerline downwind of the ignition line to help assess the impacts of topographic gradients that are perpendicular to the ambient wind. It is hoped that analysis of these simulations will help reveal where point-functional models are sufficient, where topographically modified wind fields are needed, and where fully coupled fire and transport models are necessary to properly describe wildfire behaviour.

Validation of FIRETEC wind-flows over a canopy and a fuel-break.

The wildfire model FIRETEC simulates the large coherent eddies of the wind-flows induced by the canopy. It has been qualitatively validated in its ability to simulate fire behavior, but there is still a need to validate physical submodels separately. In the present study, the dynamics and turbulence of the flow simulated by FIRETEC are validated in a manner similar to other air-flow models without fire, through comparison with measurements associated with flows within continuous and discontinuous forests captured through in situ and wind-tunnel experiments with neutral thermal stratification. The model is shown to be able to reproduce accurately all essential features of turbulent flow over both forests. Moreover, a short sensitivity study shows that the model is not very sensitive to uncertain parameters such as vegetation drag coefficient. Finally, it is shown in the discontinuous forest case that wind gusts on fuel-breaks can be very strong and significantly higher than in surrounding canopies, even if their directions are more stable. These results and others briefly reviewed in the present paper allow better understanding of wind-flow perturbations induced by fuel-breaks. This new validation added to previous ones confirms the ability of FIRETEC for investigating effects of fuel-break design on fire propagation.

Coherent vertical structures in numerical simulations of buoyant plumes from wildland fires

The structure and dynamics of buoyant plumes arising from surface-based heat sources in a vertically sheared ambient atmospheric flow are examined via simulations of a three-dimensional, compressible numerical model. Simple circular heat sources and asymmetric elliptical ring heat sources that are representative of wildland fires of moderate intensity are considered. Several different coherent vortical structures that dominate the plume structure and evolution are evident in the simulations, and these structures correspond well with those observed in plumes from wildland fires. For the circular source, these structures include: (i) a counter-rotating vortex pair aligned with the plume trajectory that is associated with a bifurcation of the plume, (ii) transverse shear-layer vortices on the upstream face of the plume, and (iii) vertically oriented wake vortices that form periodically with alternating sign on either side of the downstream edge of the plume base. For the elliptical ring source, a streamwise counter-rotating vortex pair is apparent on each flank, and a transverse horizontal vortex is observed above the head of the source. In all simulations the plume cross section is represented poorly by a self-similar Gaussian distribution.

Impacts of tree canopy structure on wind flows and fire propagation simulated with FIRETEC

Introduction Forest fuel management in the context of fire prevention generally induces heterogeneous spatial patterns of vegetation. However, the impact of the canopy structure on both wind flows and fire behavior is not well understood.Material and methods Here, a coupled atmosphere wildfire behavior model, HIGRAD/FIRETEC, was used to investigate the effects of canopy treatment on wind field and fire behavior in a typical Mediterranean pine ecosystem.Discussion First, the treatment-induced winds were simulated with the model. We observed that with decreasing cover fraction the wind velocity increased within the treated zone. The wind spatial variability increased when the vegetation was aggregated into larger clumps. Fire simulations indicated that a decrease of fire intensity occurred after several meters of propagation in the treated zone. This intensity decrease was significant with a cover fraction below 25%, but negligible with a cover fraction greater than 50%. The treatment also induced a more significant inclination of the plume away from vertical. The size of the tree clumps did not show significant effects on fire behavior.Conclusion This study was a preliminary investigation of wind/fire interaction over various canopy treatments, by using a physically based model. It gives some practical considerations for discerning the appropriate cover fraction and open perspectives for further investigations.

Coupled Atmospheric–Fire Modeling Employing the Method of Averages

A conservative forward-in-time numerical technique to improve the efficiency of a fully compressible wildfire model is presented. The technique is based on a method of averaging (MOA), which allows the costly advective terms based on a synchronous advection algorithm (e.g., the monotonicity of scalar fields is preserved) to be computed on a time step several times larger than would be dictated by the speed of the fastest waves. The MOA technique is explicit and does not require the use of either direct or iterative solvers to invert a matrix; instead the governing equations are solved to first order within an inner loop in which time-averaged quantities are obtained for use in a more costly outer loop. A linearized stability analysis of the entire scheme, including the interaction of gravity wave propagation and material motion, is presented and numerical stability for a wide range of physical and numerical parameters is demonstrated. Convergence studies are used to verify that the overall method maintains second-order accuracy. A model to simulate the propagation of the firefront in wildfires is described, and several calculations are provided to illustrate the application and advantages of the MOA in a problem that includes many complex physical processes.

The potential and promise of physics-based wildfire simulation

Abstract Wildfire — that is the unmanaged, uncontrolled burning of forests, brushlands, or grasslands with or without the presence of structures — is an increasing threat to society. We briefly review the extent of this threat, particularly its relationship to changes in the systems it threatens, and discuss its management. Recent developments in computer models of wildfire are reviewed and their application to several aspects of the wildfire threat are proposed in the context of a national resource. The requirements of an operational wildfire prediction center are discussed as a method to leverage existing and near-term capabilities into new tools to help mitigate the potential threat of this natural process.

Fuel bulk density and fuel moisture content effects on fire rate of spread: a comparison between FIRETEC model predictions and experimental results in shrub fuels

Fuel bulk density and fuel moisture content effects on fire rate of spread were assessed in shrub fuels, comparing experimental data observed in outdoor wind tunnel burns and predictions from the physically-based model FIRETEC. Statistical models for the combined effects of bulk density and fuel moisture content were fitted to both the experimental and the simulated rate of spread values using non-linear regression techniques. Results confirmed a significant decreasing effect of bulk density on rate of spread in a power law in both laboratory burns and simulations. However, experimental data showed a lesser effect than simulations, suggesting a difference in the effective drag. Fuel moisture content effect was highly consistent, showing a similar exponential relationship with rate of spread in laboratory and in simulations. FIRETEC simulations showed similar orders of magnitude with predictions of two field-based empirical models, finding a significant correlation between rate of spread values. The study confirms the efficacy of the combined approach through experimental data and simulations to study fire behaviour.

Exploring three-dimensional coupled fire–atmosphere interactions downwind of wind-driven surface fires and their influence on backfires using the HIGRAD-FIRETEC model

The obstruction of ambient winds and the possible existence of indrafts downwind of a wildfire are aspects of coupled fire–atmosphere interaction influencing the effectiveness of a backfiring operation. The fire-influenced winds behind a headfire as well as their influences on backfire spread are explored using the three-dimensional HIGRAD-FIRETEC model. Fires are simulated under weak to strong wind speeds and in shrubland and grassland fuel types. The importance of three-dimensionality in the simulation of such phenomena is demonstrated. Results suggest that when fire–atmosphere interaction is constrained to two-dimensions, the limitations of air moving through the head fire could lead to overestimation of downwind indrafts and effectiveness of backfiring. Three-dimensional simulations in surface fuels suggest that backfires benefit from the obstruction of ambient winds and potentially the existence of an indraft flow in only a limited range of environmental conditions. Simulations show that flows are most favourable when the wildfire is driven downslope by a weak wind and the backfire is ignited at bottom of the slope. Model simulations are compared with backfiring experiments conducted in a dense shrubland. Although this exercise encountered significant difficulties linked to the ambient winds data and their incorporation into the simulation, predictions and observations are in reasonable agreement.