Kara M. Yedinak

WRF-Fire: coupled weather-wildland fire modeling with the weather research and forecasting model

AbstractA wildland fire-behavior module, named WRF-Fire, was integrated into the Weather Research and Forecasting (WRF) public domain numerical weather prediction model. The fire module is a surface fire-behavior model that is two-way coupled with the atmospheric model. Near-surface winds from the atmospheric model are interpolated to a finer fire grid and are used, with fuel properties and local terrain gradients, to determine the fire’s spread rate and direction. Fuel consumption releases sensible and latent heat fluxes into the atmospheric model’s lowest layers, driving boundary layer circulations. The atmospheric model, configured in turbulence-resolving large-eddy-simulation mode, was used to explore the sensitivity of simulated fire characteristics such as perimeter shape, fire intensity, and spread rate to external factors known to influence fires, such as fuel characteristics and wind speed, and to explain how these external parameters affect the overall fire properties. Through the use of theoret...

The Science of Firescapes: Achieving Fire-Resilient Communities

Abstract Wildland fire management has reached a crossroads. Current perspectives are not capable of answering interdisciplinary adaptation and mitigation challenges posed by increases in wildfire risk to human populations and the need to reintegrate fire as a vital landscape process. Fire science has been, and continues to be, performed in isolated “silos,” including institutions (e.g., agencies versus universities), organizational structures (e.g., federal agency mandates versus local and state procedures for responding to fire), and research foci (e.g., physical science, natural science, and social science). These silos tend to promote research, management, and policy that focus only on targeted aspects of the “wicked” wildfire problem. In this article, we provide guiding principles to bridge diverse fire science efforts to advance an integrated agenda of wildfire research that can help overcome disciplinary silos and provide insight on how to build fire-resilient communities.

An examination of fire spread thresholds in discontinuous fuel beds

Many fuel beds, especially live vegetation canopies (conifer forests, shrub fields, bunch-grasses) contain gaps between vegetation clumps. Fires burning in these fuel types often display thresholds for spread that are observed to depend on environmental factors like wind, slope, and fuel moisture content. To investigate threshold spread behaviours, we conducted a set of laboratory burn experiments in artificial fuel beds where gap structure, depth, and slope were controlled. Results revealed that fire spread was limited by gap distance and that the threshold distance for spread was increased for deeper fuel beds and steeper slopes. The reasons for this behaviour were found using a high-speed thermal camera. Flame movements recorded by the camera at 120 Hz suggested fuel particles experience intermittent bathing of non-steady flames before ignition and that fuel particles across the gap ignited only after direct flame contact. The images also showed that the flame profile within the fuel bed expands with height, producing greater horizontal flame displacement in deeper beds. Slope, thus, enhances spread by increasing the effective depth in the uphill direction, which produces wider flames, and thereby increases the potential flame contact. This information suggests that fire spread across discontinuous fuel beds is dependent on the vertical flame profile geometry within the fuel bed and the statistical properties of flame characteristics.

Towards a new paradigm in fire severity research using dose–response experiments

Most landscape-scale fire severity research relies on correlations between field measures of fire effects and relatively simple spectral reflectance indices that are not direct measures of heat output or changes in plant physiology. Although many authors have highlighted limitations of this approach and called for improved assessments of severity, others have suggested that the operational utility of such a simple approach makes it acceptable. An alternative pathway to evaluate fire severity that bridges fire combustion dynamics and ecophysiology via dose–response experiments is presented. We provide an illustrative example from a controlled nursery combustion laboratory experiment. In this example, severity is defined through changes in the ability of the plant to assimilate carbon at the leaf level. We also explore changes in the Differenced Normalised Differenced Vegetation Index (dNDVI) and the Differenced Normalised Burn Ratio (dNBR) as intermediate spectral indices. We demonstrate the potential of this methodology and propose dose–response metrics for quantifying severity in terms of carbon cycle processes.

An examination of flame shape related to convection heat transfer in deep-fuel beds.

Fire spread through a fuel bed produces an observable curved combustion interface. This shape has been schematically represented largely without consideration for fire spread processes. The shape and dynamics of the flame profile within the fuel bed likely reflect the mechanisms of heat transfer necessary for the pre-heating and ignition of the fuel during fire spread. We developed a simple laminar flame model for examining convection heat transfer as a potentially significant fire spread process. The flame model produced a flame profile qualitatively comparable to experimental flames and similar to the combustion interface of spreading fires. The model comparison to flame experiments revealed that at increasing fuel depths (>0.7 m), lateral flame extension was increased through transition and turbulent flame behaviour. Given previous research indicating that radiation is not sufficient for fire spread, this research suggests that flame turbulence can produce the convection heat transfer (i.e. flame contact) necessary for fire spread particularly in vertically arranged, discontinuous fuels such as shrub and tree canopies.

Effects of fire radiative energy density dose on Pinus contorta and Larix occidentalis seedling physiology and mortality

Climate change is projected to exacerbate the intensity of heat waves and drought, leading to a greater incidence of large and high-intensity wildfires in forested ecosystems. Predicting responses of seedlings to such fires requires a process-based understanding of how the energy released during fires affects plant physiology and mortality. Understanding what fire ‘doses’ cause seedling mortality is important for maintaining grasslands or promoting establishment of desirable plant species. We conducted controlled laboratory combustion experiments on replicates of well-watered nursery-grown seedlings. We evaluated the growth, mortality and physiological response of Larix occidentalis and Pinus contorta seedlings to increasing fire radiative energy density (FRED) doses created using natural fuels with known combustion properties. We observed a general decline in the size and physiological performance of both species that scaled with increasing FRED dose, including decreases in leaf-level photosynthesis, seedling leaf area and diameter at root collar. Greater FRED dose increased the recovery time of chlorophyll fluorescence in the remaining needles. This study provides preliminary data on what level of FRED causes mortality in these two species, which can aid land managers in identifying strategies to maintain (or eliminate) woody seedlings of interest.

Impacts of fire radiative flux on mature Pinus ponderosa growth and vulnerability to secondary mortality agents

Recent studies have highlighted the potential of linking fire behaviour to plant ecophysiology as an improved route to characterising severity, but research to date has been limited to laboratory-scale investigations. Fine-scale fire behaviour during prescribed fires has been identified as a strong predictor of post-fire tree recovery and growth, but most studies report these metrics averaged over the entire fire. Previous research has found inconsistent effects of low-intensity fire on mature Pinus ponderosa growth. In this study, fire behaviour was quantified at the tree scale and compared with post-fire radial growth and axial resin duct defences. Results show a clear dose–response relationship between peak fire radiative power per unit area (W m–2) and post-fire Pinus ponderosa radial growth. Unlike in previous laboratory research on seedlings, there was no dose–response relationship observed between fire radiative energy per unit area (J m–2) and post-fire mature tree growth in the surviving trees. These results may suggest that post-fire impacts on growth of surviving seedlings and mature trees require other modes of heat transfer to impact plant canopies. This study demonstrates that increased resin duct defence is induced regardless of fire intensity, which may decrease Pinus ponderosa vulnerability to secondary mortality agents.

Toward an integrated system for fire, smoke and air quality simulations

In this study, WRF-Sfire is coupled with WRF-Chem to construct WRFSC, an integrated forecast system for wildfire behaviour and smoke prediction. WRF-Sfire directly predicts wildfire spread, plume and plume-top heights, providing comprehensive meteorology and fire emissions to chemical transport model WRF-Chem, eliminating the need for an external plume-rise model. Evaluation of WRFSC was based on comparisons between available observations of fire perimeter and fire intensity, smoke spread, PM2.5 (particulate matter less than 2.5 μm in diameter), NO and ozone concentrations, and plume-top heights with the results of two WRFSC simulations, a 48-h simulation of the 2007 Witch–Guejito Santa Ana fires and a 96-h WRF-Sfire simulation with passive tracers of the 2012 Barker Canyon fire. The study found overall good agreement between forecast and observed local- and long-range fire spread and smoke transport for the Witch–Guejito fire. However, ozone, PM2.5 and NO concentrations were generally underestimated and peaks mistimed in the simulations. This study found overall good agreement between simulated and observed plume-top heights, with slight underestimation by the simulations. Two promising results were the agreement between plume-top heights for the Barker Canyon fire and faster than real-time execution, making WRFSC a possible operational tool.

The ability of winter grazing to reduce wildfire size and fire-induced plant mortality was not demonstrated: a comment on Davies et al. (2015)

A recent study by Davies et al. sought to test whether winter grazing could reduce wildfire size, fire behaviour and intensity metrics, and fire-induced plant mortality in shrub–grasslands. The authors concluded that ungrazed rangelands may experience fire-induced mortality of native perennial bunchgrasses. The authors also presented several statements regarding the benefits of winter grazing on post-fire plant community responses. However, we contend that the study by Davies et al. has underlying methodological errors, lacks data necessary to support their conclusions, and does not provide a thorough discussion on the effect of grazing on rangeland ecosystems. Importantly, Davies et al. presented no data on the post-fire mortality of the perennial bunchgrasses or on the changes in plant community composition following their experimental fires. Rather, Davies et al. inferred these conclusions based on their observed fire behaviour metrics of maximum temperature and a term described as the ‘heat load’. However, we contend that neither metric is appropriate for describing the heat flux impacts on plants. This lack of post-fire data, several methodological errors and the use of inappropriate thermal metrics limit the authors’ ability to support their stated conclusions.

Vegetation effects on impulsive events in the acoustic signature of fires

Acoustic impulse events have long been used as diagnostics for discrete phenomena in the natural world, including the detection of meteor impacts and volcanic eruptions. Wildland fires display an array of such acoustic impulse events in the form of crackling noises. Exploratory research into the properties of these impulse events revealed information regarding the specific properties of plant material. Unique acoustic frequency bands in the upper end of the sonic spectrum correlated to changes in vegetation properties. The signature of acoustic impulse events as they relate to plant species and plant water stress, were investigated in controlled laboratory combustion experiments. Correlation in the frequency range of 6.0-15.0 kHz was found for both species and water stress, indicating the possibility that a digital filter may be capable of identifying vegetation properties during wildland fire events.