Fire Front Research Articles

Characterising the Chemical Composition of Bushfire Smoke and Implications for Firefighter Exposure in Western Australia

This study evaluates bushfire smoke as a workplace hazard for firefighters by characterising its chemical composition and potential health risks in Western Australia. Portable Fourier Transform Infrared (FTIR) spectrometry was used to measure airborne chemical concentrations at prescribed burns across five regions, including peat (acid sulphate) fire events. Samples were collected during both flaming and smouldering phases, as well as in perceived “clear” air resting zones. Results indicated that carbon monoxide (CO) was the dominant gas, reaching concentrations of 205 ppm at the fire front, followed by nitrogen monoxide (26 ppm) and methane (19 ppm). Peat fires produced distinct profiles, with ammonia (21.5 ppm) and sulphur dioxide (9.5 ppm) concentrations higher than those observed in typical bushfires. Smouldering phases emitted higher chemical concentrations than flaming phases 75% of the time. Even clear air zones contained measurable chemicals, with CO levels averaging 18 ppm, suggesting that firefighters are not free from exposure during rest periods. These findings highlight the need for fit-for-purpose respiratory protective equipment (RPE) and improved rest protocols to minimise exposure. The study underscores the importance of comprehensive health monitoring programs for firefighters to mitigate long-term health risks.

Infrared Thermographic Diagnostics of Wood Fire Resistance under Combined Thermal Effect Conditions from a Ground Fire Front and Firebrands

Experiments were conducted on certain types of combustible building materials and wood-based structures to determine the interaction between firebrands and forest fuel (FF). Using non-contact IR diagnostic methods in narrow spectral ranges of infrared wavelengths, the heat flux values generated by firebrands and the temperature field of the most heat-stressed sections of the structures under study were determined. Using a JADE J530SB thermal imager with an optical filter of 2,5—2,7 m, temperatures in the range 310—1500 K were measured in the infrared region. In order to interpret the recorded emission from the test sample, calibrations supplied by the narrowband optical filter manufacturer were used.

Smoke Emissions and Buoyant Plumes above Prescribed Burns in the Pinelands National Reserve, New Jersey

Prescribed burning is a cost-effective method for reducing hazardous fuels in pine- and oak-dominated forests, but smoke emissions contribute to atmospheric pollutant loads, and the potential exists for exceeding federal air quality standards designed to protect human health. Fire behavior during prescribed burns influences above-canopy sensible heat flux and turbulent kinetic energy (TKE) in buoyant plumes, affecting the lofting and dispersion of smoke. A more comprehensive understanding of how enhanced energy fluxes and turbulence are related during the passage of flame fronts could improve efforts to mitigate the impacts of smoke emissions. Pre- and post-fire fuel loading measurements taken during 48 operational prescribed burns were used to estimate the combustion completeness factors (CC) and emissions of fine particulates (PM2.5), carbon dioxide (CO2), and carbon monoxide (CO) in pine- and oak-dominated stands in the Pinelands National Reserve of southern New Jersey. During 11 of the prescribed burns, sensible heat flux and turbulence statistics were measured by tower networks above the forest canopy. Fire behavior when fire fronts passed the towers ranged from low-intensity backing fires to high-intensity head fires with some crown torching. Consumption of forest-floor and understory vegetation was a near-linear function of pre-burn loading, and combustion of fine litter on the forest floor was the predominant source of emissions, even during head fires with some crowning activity. Tower measurements indicated that above-canopy sensible heat flux and TKE calculated at 1 min intervals during the passage of fire fronts were strongly influenced by fire behavior. Low-intensity backing fires, regardless of forest type, had weaker enhancement of above-canopy air temperature, vertical and horizontal wind velocities, sensible heat fluxes, and TKE compared to higher-intensity head and flanking fires. Sensible heat flux and TKE in buoyant plumes were unrelated during low-intensity burns but more tightly coupled during higher-intensity burns. The weak coupling during low-intensity backing fires resulted in reduced rates of smoke transport and dispersion, and likely in more prolonged periods of elevated surface concentrations. This research facilitates more accurate estimates of PM2.5, CO, and CO2 emissions from prescribed burns in the Pinelands, and it provides a better understanding of the relationships among fire behavior, sensible heat fluxes and turbulence, and smoke dispersion in pine- and oak-dominated forests.

A Case Study of the Possible Meteorological Causes of Unexpected Fire Behavior in the Pantanal Wetland, Brazil

This study provides insights into large fires in the Pantanal by analyzing the atmospheric conditions that influenced the rapid fire evolution between 13 and 14 November 2023, when fire fronts spread rapidly, leading to critical situations for firefighters. The observation-based analysis helped us to identify some characteristics of the fire’s evolution and the meteorological conditions in the region. Furthermore, two simulations were run with the Meso-NH model, which was configured with horizontal resolutions of 2.5 km and 5 km. The fire behavior, characterized by satellite observations, revealed periods with a sudden increase in active fire numbers. High temperatures and low relative humidity in the region characterized the fire weather conditions. The simulations confirmed the critical fire condition, showing the benefits of increasing the resolution of numerical models for the Pantanal region. The convection-resolving simulation at 2.5 km showed the repeated development of gust fronts in the late afternoon and early evening. This study highlights this dynamic that, coupled with intense surface wind gusts, was crucial for the intensification of the fire spread and unexpected behavior. This study is a first step toward better understanding fire dynamics in the Pantanal through atmospheric modeling, and it can support strategies for firefighting in the region.

Anatomy of a post-wildfire recovery: Responses of mammals to a Black Summer wildfire in a fox-free landscape

Wildfires are predicted to increase in frequency and severity with climate change. Devising effective strategies to ensure wildfires do not hasten a decline in wildlife populations hinges on developing a deeper understanding of how species are affected by recent wildfires. I investigated the response of eight species of medium-sized mammals over a 4-year period (2020–2023) following a wildfire in the Australian Black Summer fire season. Pre-wildfire population levels were not known. These species differed in their ability to survive the passage of fire; from being able to flee the fire front, to remaining in protective shelters, to favouring unburnt forest types, to none of these strategies. Camera trapping was conducted from 5 months to 4 years after the fire across 62 sites in wet and dry sclerophyll forest (burnt and unburnt) and rainforest (unburnt) to characterise both initial survival and subsequent reproduction-led post-fire occupancy. There was consistency in the response of species in year 1. Those assumed to be able to flee the fires showed medium-high post-fire occupancy regardless of whether a site was burnt. Those that use protective shelters and unburnt forest also showed high occupancy. One species without these options showed greatly reduced occupancy in year 1 relative to years 3 and 4. Most species showed evidence of increased detection across years, which is inferred to reflect increased abundance, and was expected given substantially above average rainfall for three years following the fire. The feral cat was the only predator sufficiently widespread to be of concern in species’ recovery but was rarely detected in years 1 and 2, relative to years 3 and 4. This study provides evidence that many species, including three threatened species, have the ability to survive the passage of fire, and show high post-fire occupancy. The study landscape contained abundant habitat elements (unburnt rainforest, boulder fields) that promote survival through a wildfire and post-fire recovery, providing insights to guide future conservation actions.

Capturing and interpreting wildfire spread dynamics: attention-based spatiotemporal models using ConvLSTM networks

Predicting the trajectory of geographical events, such as wildfire spread, presents a formidable task due to the dynamic associations among influential biophysical factors. Geo-events like wildfires frequently display short and long-range spatial and temporal correlations. Short-range effects are the direct contact and near-contact spread of the fire front. Long-range effects are represented by processes such as spotting, where firebrands carried by the wind ignite fires distant from the flaming front, altering the shape and speed of an advancing fire front. This study addresses these modeling challenges by clearly defining and analyzing the scale-dependent spatiotemporal dynamics that influence wildfire spread, focusing on the interplay between biophysical factors and fire behavior. We propose two unique attention-based spatiotemporal models using Convolutional Long Short-Term Memory (ConvLSTM) networks. These models are designed to learn and capture a range of local to global and short and long-range spatiotemporal correlations. The proposed models were tested on two datasets: a high-resolution wildfire spread dataset produced with a semi-empirical percolation model and a satellite observed wildfire spread data in California 2012–2021. Results indicate that attention-based models accurately predict fire front movements that are consistent with known wildfire spread-biophysical dynamics. Our research suggests there is considerable potential for attention mechanisms to capture the spatiotemporal behavior of wildfire spread, with model transferability, that can guide rapid deployment of wildfire management operations. We also highlight directions for future studies that focus on how the self-attention mechanism could enhance model performance for a range of geospatial applications.

Experimental and numerical study on data-driven prediction for wildfire spread incorporating adaptive observation error adjustment

In recent wildfire prediction research, data assimilation (DA) methods like Ensemble Kalman filtering have gained traction for integrating observation data to enhance prediction accuracy. Most previous studies trusted that the observation data were accurate, and set a small observation error, which causes unreliable predicted results for scenarios with large observation error. To tackle this, our study introduced a method that iteratively adjusted the potential range of observation errors by comparing observation and simulation data over time. We conducted a 30-m experiment and kilometer-scale numerical simulations. Unlike prior research, we adopted larger error ranges (the similarity index with true data ranges from 0.6 to 1) for both real and synthetic observation data. In the experiment, to increase the complexity of fire spread, a heterogeneous fuel arrangement was employed. Irregular flame fronts appeared due to incomplete combustion and were difficult to replicate in simulations. Better accuracy was achieved using real observation data to revise predictions. Furthermore, to improve the applicability of the algorithm, numerical simulations were designed to consider observation error changing over time or not. The Root Mean Square Errors for the fire front prediction using the proposed method remained lower than that of the traditional DA approach.

The distributed strategy for asynchronous observations in data-driven wildland fire spread prediction

Background Asynchronous observations refer to observations that are obtained at multiple moments. The observation moments of fire fronts may differ throughout an entire wildfire area. Asynchronous observations include historical data, which hinders the effectiveness of data assimilation due to the lack of timely updates on changing fire fronts. Aims This paper proposed a distributed strategy combined with the Ensemble Transform Kalman filter (ETKF-distributed) for asynchronous observations. It can assimilate fire fronts immediately at any location by using new matching schemes between prediction and observation. Methods The ETKF-distributed undergoes testing using a wildland fire generated based on real terrain, vegetation, and historical weather data from the local area. In addition, the ETKF and ETKF-centralised proposed in our previous work were employed as comparisons. Observing System Simulation Experiments were conducted to generate asynchronous observation fire fronts. Key results The benefit of immediate assimilation enables the new method to maintain high accuracy predictions. Conclusions The allocation of observation resources can be focused in regions with high rates of speed when employing ETKF-distributed. Implications The ETKF-distributed has high efficiency and adaptability, making it highly promising for implementation in wildfire prediction.

Distribution characteristics of radiant heat flux on tank surfaces exposed to wildland fires

AbstractThe expansion of wildland‐urban interface tends to expose fuel tanks to wildland fires, the frequency and intensity of which have increased in recent years due to climate change and global warming. This study aims to examine the distribution of radiant heat flux on a tank surface exposed to a wildland fire front, with a particular focus on extreme wildland fires. A theoretical framework is proposed, which consists of a flame length correlation, a thermal radiation model, and a correlation of time to failure, to calculate the radiant heat flux received by the tank surface and the potential for tank failure. The horizontal and circumferential distribution characteristics of the radiant heat flux are intensively discussed, in terms of the heat flux profile, heating area, total heat, time to failure versus the fireline intensity, fireline width, spacing, and tank volume. In particular, extreme wildland fires, characterized by large fireline intensities, could potentially destroy the tank within the safety spacing mandated by the current regulations. Therefore, a comparison between the time to failure and the radiant heating duration is suggested to ascertain the necessary safety spacing. The uncertainties in model calculations are also addressed, which arise from the use of different flame length correlations and flame radiative fractions.

Reaction mechanism of catalytic gasification by calcium-based catalysts and oxidized coal residues in coalfield fire zones

Fires originating from coal spontaneous combustion within coalfields result not only in substantial coal resource depletion but also producing residual low-activity pyrolysis coal chars exhibiting varying degrees of oxidation. These chars develop progressively through successive heat penetration at the fire front and post-fire extinguishment phases. This paper focuses on the alkaline earth metal-activated catalytic gasification of residual oxidized coal in fire zones, constructs a carbon-based model of oxidized coal in fire zones. The results show that the reaction active sites of the oxidized coal carbon matrix model are mainly concentrated on the carbon atoms at the end of the aromatic ring. During catalytic gasification, the calcium-based catalyst engages with these active sites, forming a preliminary catalyst. The transformation of oxidized coal into CO primarily occurs through two distinct routes. Calcium attaches to the surface of the oxidized coal’s carbon-based structure, establishing active sites. Acting as a facilitator, it aids the movement of CO2 to the carbon-based surface, leading to its further breakdown into CO. The catalytic species containing calcium persistently amalgamates with active sites on coal coke surface, fostering the release of additional CO. Moreover, these catalytic species with calcium also bind CO2 and unite with active coal coke sites, generating carbon–oxygen complexes on the surface. These complexes are thermally unstable and decompose, yielding CO and initiating the formation of fresh active sites on the coal coke surface. Consequently, they interact further with calcium-based catalytic species, culminating in the creation of catalyst precursors, which drive a recurrent catalytic reaction process.

Exploring the atmospheric conditions increasing fire danger in the Iberian Peninsula

AbstractThe fire danger is particularly sensitive to meteorological conditions. The present study discusses the atmospheric conditions during three periods to evaluate how they can increase fire danger. A set of three convection‐permitting simulations was configured at 2.5 km resolution using the Meso‐NH model. In the first period, the intense surface heating induced by a heatwave favoured the development of the Iberian thermal low in July 2019, leading to the formation of precipitating systems and resulting in convective outflows that affected the central Iberian Peninsula. The outflows were shown to be an important feature in evaluating fire danger during the period; however, the simulation also highlighted the orographic effect as another phenomenon playing an important role in fire development and consequently enhancing the fire danger in some regions such as in the extreme southwest of Portugal. The orographic effect in this specific region was identified and analyzed in detail for two megafire events that occurred in Monchique in August 2003 and 2018. The Monchique Mountain's shape and orientation interacting with the airflow induced upslope and downslope winds that favoured the rapid propagation of the fire fronts in August 2018. The third experiment showed that the circulation of a sea breeze from the southern coast of Portugal may act as an enhancer for fire danger in the region when interacting with the regional mountain, such as verified in the 2003 megafire. The study shows that the fire danger over specific regions can be increased by different atmospheric phenomena and explored from atmospheric modelling. The convective outflows were an important factor enhancing fire danger; however, the orographic effect was confirmed as the main factor producing two megafires events in the extreme southwest of Portugal.

Critical Behavior of an Extreme Fire “Fire Junction”

In this paper the concept of extreme forest fire behavior denominated as Junction Fire is analyzed, this phenomenon consists of the interaction between two linear fire fronts which in their propagation intersect at a single point making a small angle between them. In their process of fusion, the phenomena of heat transfer by radiation and convection are extremely amplified due to the concentration of energy at the point of intersection. The concentration of energy and therefore the development of the powerful heat transfer mechanism induces an extremely high and sudden increase in the rate of spread until it reaches a maximum value after which begins to decrease. This behavior of the order parameter exhibits a non-universal phase transition. The analysis carried out in the ADIA-LEIF laboratories show that fires junction are a source of instability which leads to serious safety and management problems for those who are confronted with this type of forest fire. The results show that the fire junction spread exposed two main regimes; an initial acceleration phase characterized by a high rate of spread depends essentially on the initial angle between the fire fronts θ0 , which shifts during the evolution of the fire until the limit of creation of a single front of fire in a straight line explaining a decreasing phase. Nevertheless, the front fire dynamics depends both on the initial angle between the fire fronts θ0 and on the slope ground αJ exhibiting a non-universal phase transition known in the literature. Copyright © 2009 Praise Worthy Prize S.r.l. - All rights reserved.

Thermal Characteristics of Surface Fire Spreading Front Under Ambient Wind

Surface fire is the most common form of forest fire, primarily spreading along the forest land's surface and causing damage to nearby low-lying seedlings, shrubs, and the root systems of vegetation at its base. In actual forest fire scenarios, the occurrence of fires is often accompanied by environmental winds due to the typically complex terrain in forested areas. Forest vegetation is densely distributed, and when surface fires encounter obstacles such as tree trunks, the advancing fire front may interact with the tree trunks under the influence of environmental winds. This study focuses on understanding the thermal characteristics of the interaction between the advancing fire front and tree trunks in the presence of environmental winds. To investigate this interaction, temperature measurements are conducted on both sides of the obstacles and in the downstream region of the flame. Additionally, measurements of radiant heat flux are taken in the upper space above the experimental setup. By analyzing flame morphology parameters and experimental measurements, trends in thermal characteristic parameters with spatial position are identified. Regression formulas are developed to describe the temperature difference between the windward and leeward sides of the obstacles and the height above the experimental platform. Furthermore, the influence of flame climbing on radiant heat transfer in the upper space is discussed.

Enhancing wildfire propagation model predictions using aerial swarm-based real-time wind measurements: A conceptual framework

The dynamic behaviour of wildfires is mainly influenced by weather, fuel, and topography. Based on fundamental conservation laws involving numerous physical processes and large scales, atmospheric models require substantial computational resources. Therefore, coupling wildfire and atmospheric models is impractical for high resolutions. Instead, a static atmospheric wind field is typically input into the wildfire model, either as boundary conditions on the control surface or distributed over the control volume. Wildfire propagation models may be (i) data-driven; (ii)theoretical; or (iii)mechanistic surrogates. Data-driven models are beyond the scope of this paper. Theoretical models are based on conservation laws (species, energy, mass, momentum) and are, therefore, computationally intensive; e.g. the Fire Dynamics Simulator (FDS). Mechanistic surrogate models do not closely follow fire dynamics laws, but related laws observed to make predictions more efficiently with sufficient accuracy; e.g. FARSITE, and FDS with the Level Set model (FDS-LS). Whether theoretical or mechanistic surrogate, these wildfire models may be coupled with or decoupled from wind models (e.g. Navier-Stokes equations). Only coupled models account for the effect of the fire on the wind field. In this paper, a series of simulations of wildfire propagation on grassland are performed using FDS-LS to study the impact of the fire-induced wind on the fire propagation dynamics. Results show that coupling leads to higher Rates of Spread (RoS), closer to those reported from field experiments, with increasing wind speeds and higher terrain slopes strengthening this trend. Aiming to capture the fire–wind interaction without the hefty cost of solving Navier-Stokes equations, a conceptual framework is proposed: 1) a swarm of unmanned aerial vehicles measure wind velocities at flight height; 2) the wind field is constructed with the acquired data; 3) the high-altitude wind field is mapped to near-surface, and 4) the near-surface wind field is fed into the wildfire model periodically. A series of simulations are performed using an in-house decoupled physics-based reduced-order fire propagation model (FireProM-F) enhanced by wind field “measurements”. In this proof of concept, wind velocities are not measured but extracted from physics-based Large Eddy Simulations taken as ground truth. Unsurprisingly, higher measurement frequencies lead to more accurate and realistic predictions of the propagating fire front. An initial attempt is made to study the effect of wind measurement uncertainty on the model predictions by adding Gaussian noise. Preliminary results show that higher noise leads to the fire front displaying more irregular shapes and slower propagation.

Triggering Pyro-Convection in a High-Resolution Coupled Fire–Atmosphere Simulation

This study aimed to assess fire–atmosphere interactions using the fully coupled Meso-NH–ForeFire system. We focused on the Pedrógão Grande wildfire (28,914 ha), which occurred in June 2017 and was one of the deadliest and most damaging fires in Portugal’s history. Two simulations (control and fully coupled fire–atmosphere) were performed for three two-way nested domains configured with horizontal resolutions of 2 km, 0.4 km, and 0.08 km, respectively, in the atmospheric model Meso-NH. Fire propagation was modeled within the innermost domain with ForeFire, which solves the fire front with a 20 m resolution, producing the heat and vapor fluxes which are then injected into the atmospheric model. A simplified homogeneous fuel distribution was used in this case study. The fully coupled experiment helped us to characterize the smoke plume structure and identify two different regimes: (1) a wind-driven regime, with the smoke plume transported horizontally southward and in the lower troposphere, and (2) a plume-dominated regime, in which the simulated smoke plume extended vertically up to upper levels, favoring the formation of a pyro-cloud. The simulations were compared, and the results suggest that the change in the fire regime was caused by an outflow that affected the main fire front. Furthermore, the fully coupled simulation allowed us to explore the change in meteorology caused by an extreme fire, namely through the development of a pyro-cloud that also induced outflows that reached the surface. We show that the Meso-NH–ForeFire system may strongly contribute to an improved understanding of extreme wildfires events and associated weather phenomena.

Occupational exposure and health of wildland firefighters: protocol for a systematic review

The frequency of forest fires has increased in recent years. This phenomenon has led to an increase in the difficulty of fighting them and an increase in the occupational exposure of firefighters. This study aims to carry out a systematic review protocol to obtain information on firefighters' occupational exposure on the fire front. A protocol was carried out following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses: PRISMA Statement guidelines. There were no age, gender or publication date restrictions for including articles within the scope of firefighter samples. The study aimed to understand all the risks to which firefighters are exposed when fighting forest fires and their relationship with the onset of occupational diseases. Thus, all studies with laboratory or real-time monitoring, using samples of firefighters, and which carried out evaluations that made it possible to identify exposure risk factors and their relationship with occupational activity were included. This list makes it possible to verify all the studies carried out up to the date of the review on this subject and to guarantee a contribution to improving knowledge in this area of research.

Atmospheric turbulence observed during a fuel-bed-scale low-intensity surface fire

Abstract. The ambient atmospheric environment affects the growth and spread of wildland fires, whereas heat and moisture released from the fires and the reduction of the surface drag in the burned areas can significantly alter local atmospheric conditions. Observational studies on fire–atmosphere interactions have used instrumented towers to collect data during prescribed fires, but a few towers in an operational-scale burn plot (usually > 103 m2) have made it extremely challenging to capture the myriad of factors controlling fire–atmosphere interactions, many of which exhibit strong spatial variability. Here, we present analyses of atmospheric turbulence data collected using a 4 × 4 array of fast-response sonic anemometers during a fire experiment on a 10 m × 10 m burn plot. In addition to confirming some of the previous findings on atmospheric turbulence associated with low-intensity surface fires, our results revealed substantial heterogeneity in turbulent intensity and heat and momentum fluxes just above the combustion zone. Despite the small plot (100 m2), fire-induced atmospheric turbulence exhibited strong dependence on the downwind distance from the initial line fire and the relative position specific to the fire front as the surface fire spread through the burn plot. This result highlights the necessity for coupled atmosphere–fire behavior models to have 1–2 m grid spacing to resolve heterogeneities in fire–atmosphere interactions that operate on spatiotemporal scales relevant to atmospheric turbulence. The findings here have important implications for modeling smoke dispersion, as atmospheric dispersion characteristics in the vicinity of a wildland fire are directly affected by fire-induced turbulence.

Numerical investigation of the Pedrógão Grande pyrocumulonimbus using a fire to atmosphere coupled model

Understanding the development of fire-generated thunderstorms in mega fire events is important given their high impact on the evolution of the fire fronts, where the fire spread becomes highly unpredictable and difficult to suppress. This study aims to investigate numerically the influence of strong pyro-convective activity on the local atmospheric conditions by means of a numerical simulation based on the coupled Meso-NH/ForeFire code. To our knowledge, it is the first time that the effect of wildfire spread on the local atmospheric conditions is accounted explicitly in a high-resolution NWP model to investigate pyro-convection activity. More specifically, we study numerically the Portuguese Pedrógão Grande mega fire, which was one of the most destructive and deadliest wildfire hazards affecting the Mediterranean region in the recent years. The spatio-temporal propagation of the wildfire was assigned a priori on the basis of the official investigation's reports, while the impact of the forced fire evolution and of the ensuing heat and water vapour emissions on the local atmospheric conditions is accounted explicitly. The simulation, configured with very-high spatial and temporal resolutions, was capable of resolving the intense convective column reaching the upper troposphere and the fast development of the associated cloud system. The numerical fire produced intense updraughts with vertical velocities above 15 m/s, whereas the associated pyroCb cloud was composed by five different hydrometeor species along the main convective column and reached an altitude of 10 km. It is remarkable that the numerical experiment reproduced phenomena occurring at a fine scale related to cloud microphysics, such as very-localized outflows. This study, based on a coupled numerical simulation, was capable of illustrating in detail the development of a pyroCb cloud from strong pyro-convective activity.

Experimental and Numerical Study of the Trench Fire Spread Rule over a Sloped Uniform Fuel Bed: Rate of Fire Spread, Flame Morphology, and Heat Flux

Trench fires on sloped terrain are always complicated due to the corresponding flame dynamics and heat transfer mechanisms. Flame attachment may increase the rate of fire spread (ROS) by enlarging the heating area of unburned vegetation. In addition, variations in radiative and convective heat flux are of great importance to fire behavior characteristics. In this work, trench fire tests under different slopes (θ) and inclined sidewalls (A) were performed by numerical simulations based on the Lagrangian Particle Model (LPM) and Boundary Fuel Model (BFM) in the Fire Dynamics Simulator (FDS) and small-scale experiments, and the ROS, flame characteristics, and radiative/convective heat flux of the fire front are discussed in detail. The results indicate that the flame tends to adhere to the fuel bed with increasing slope angle and sidewall inclination. In particular, the flame becomes fully attached with a greater pressure difference than the buoyancy, which is caused by the unequal air entrainment between the front and behind the flame. When A = 90°, the critical slope angle of the flame adhesion (from slight tilt to full attachment) is identified as ~20°. The ROS (θ ≤ 15°) predicted by the BFM and LPM are closer to the small-scale experiments. The heat fluxes based on the experiments confirm the predominant mechanism of radiative heat transfer in trench fires at low slopes (θ ≤ 20°). Furthermore, convective heat transfer is more significant than radiative and becomes the main heating mechanism for θ ≥ 20°.

Analysis of methods for assimilating fire perimeters into a coupled fire-atmosphere model

Correctly initializing the fire within coupled fire-atmosphere models is critical for producing accurate forecasts of meteorology near the fire, as well as the fire growth, and plume evolution. Improperly initializing the fire in a coupled fire-atmosphere model can introduce forecast errors that can impact wind circulations surrounding the fire and updrafts along the fire front. A well-constructed fire initialization process must be integrated within coupled fire-atmosphere models to ensure that the atmospheric component of the model does not become numerically unstable due to excessive heat fluxes released during the ignition, and that realistic fire-induced atmospheric circulations are established at the model initialization time. The primary objective of this study is to establish an effective fire initialization method in a coupled fire-atmosphere model, based on the analysis of the impact of the initialization procedure on the model’s ability to resolve fire-atmosphere circulations and fire growth. Here, we test three different fire initialization approaches leveraging the FireFlux II experimental fire, which provides a comprehensive suite of observations of the pyroconvective column, local micrometeorology, and fire characteristics. The two most effective fire initialization methods identified using the FireFlux II case study are then tested on the 380,000-acre Creek Fire, which burned across the central Sierra Nevada mountains during the 2020 Western U.S. wildfire season. For this case study, simulated pyroconvection and fire progression are evaluated using plume top height observations from MISR and airborne fire perimeter data, to assess the effectiveness of different initialization methods in the context of establishing pyroconvection and resolving the fire growth. The analyses of both the experimental fire simulation and the wildfire simulation indicate that the spin-up initialization method based on historical fire progression that masks out inactive fire regions provides the best results in terms of resolving the fire-induced vertical circulation and fire progression.