Wildfire modeling is concerned with
numerical simulation of
wildfires
A wildfire, forest fire, or a bushfire is an unplanned and uncontrolled fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identified as a ...
to comprehend and predict fire behavior. Wildfire modeling aims to aid wildfire suppression, increase the safety of firefighters and the public, and minimize damage. Wildfire modeling can also aid in protecting
ecosystem
An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...
s,
watersheds, and
air quality
Air pollution is the presence of substances in the Atmosphere of Earth, air that are harmful to humans, other living beings or the environment. Pollutants can be Gas, gases like Ground-level ozone, ozone or nitrogen oxides or small particles li ...
.
Using
computational science
Computational science, also known as scientific computing, technical computing or scientific computation (SC), is a division of science, and more specifically the Computer Sciences, which uses advanced computing capabilities to understand and s ...
, wildfire modeling involves the statistical analysis of past fire events to predict spotting risks and front behavior. Various wildfire propagation models have been proposed in the past, including simple ellipses and egg- and fan-shaped models. Early attempts to determine wildfire behavior assumed terrain and vegetation uniformity. However, the exact behavior of a wildfire's front is dependent on a variety of factors, including wind speed and slope steepness. Modern growth models utilize a combination of past ellipsoidal descriptions and
Huygens' Principle to simulate fire growth as a continuously expanding polygon.
Extreme value theory
Extreme value theory or extreme value analysis (EVA) is the study of extremes in statistical distributions.
It is widely used in many disciplines, such as structural engineering, finance, economics, earth sciences, traffic prediction, and Engin ...
may also be used to predict the size of large wildfires. However, large fires that exceed suppression capabilities are often regarded as statistical outliers in standard analyses, even though fire policies are more influenced by large wildfires than by small fires.
[Alvarado, ''et al''., 66–68]
Objectives
Wildfire modeling attempts to reproduce fire behavior, such as how quickly the fire spreads, in which direction, how much heat it generates. A key input to behavior modeling is the
Fuel Model, or type of fuel, through which the fire is burning. Behavior modeling can also include whether the fire transitions from the surface (a "surface fire") to the tree crowns (a "crown fire"), as well as extreme fire behavior including rapid rates of spread,
fire whirl
A fire whirl, fire devil or fire tornado is a whirlwind induced by a fire and often (at least partially) composed of flame or ash. These start with a whirl of wind, often made visible by smoke, and may occur when intense rising heat and turb ...
s, and tall well-developed convection columns. Fire modeling also attempts to estimate fire effects, such as the
ecological
Ecology () is the natural science of the relationships among living organisms and their environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere levels. Ecology overlaps with the closely re ...
and
hydrological
Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydro ...
effects of the fire, fuel consumption, tree mortality, and amount and rate of smoke produced.
Environmental factors
Wildland fire behavior is affected by
weather
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmo ...
,
fuel
A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work (physics), work. The concept was originally applied solely to those materials capable of releasing chem ...
characteristics, and
topography
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary sci ...
.
Weather influences fire through
wind
Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...
and
moisture
Moisture is the presence of a liquid, especially water, often in trace amounts. Moisture is defined as water in the adsorbed or absorbed phase. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some comme ...
.
Wind
Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...
increases the fire spread in the wind direction, higher
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
makes the fire burn faster, while higher
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...
, and
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...
(rain or snow) may slow it down or extinguish it altogether. Weather involving fast wind changes can be particularly dangerous, since they can suddenly change the fire direction and behavior. Such weather includes
cold front
A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface Trough (meteorology), trough of Low-pressure area, low pressure. It often forms behind an extratropica ...
s,
foehn wind
A Foehn, or Föhn (, , , ), is a type of dry, relatively warm downslope wind in the lee of a mountain range. It is a rain shadow wind that results from the subsequent adiabatic warming of air that has dropped most of its moisture on windw ...
s,
thunderstorm
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustics, acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorm ...
downdrafts,
sea and land breeze, and
diurnal slope winds.
Wildfire fuel includes grass, wood, and anything else that can burn. Small dry twigs burn faster while large logs burn slower; dry fuel ignites more easily and burns faster than wet fuel.
Topography factors that influence wildfires include the orientation toward the sun, which influences the amount of energy received from the sun, and the slope (fire spreads faster uphill). Fire can accelerate in narrow canyons and it can be slowed down or stopped by barriers such as creeks and roads.
These factors act in combination. Rain or snow increases the fuel moisture, high
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...
slows the drying of the fuel, while winds can make fuel dry faster. Wind can change the fire-accelerating effect of slopes to effects such as downslope windstorms (called
Santa Anas, foehn winds, East winds, depending on the geographic location). Fuel properties may vary with topography as plant density varies with elevation or aspect with respect to the sun.
It has long been recognized that "fires create their own weather." That is, the heat and moisture created by the fire feed back into the atmosphere, creating intense winds that drive the fire behavior. The heat produced by the wildfire changes the temperature of the atmosphere and creates strong updrafts, which can change the direction of surface winds. The water vapor released by the fire changes the moisture balance of the atmosphere. The water vapor can be carried away, where the
latent heat
Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process—usually a first-order phase transition, like melting or condensation. ...
stored in the vapor is released through
condensation
Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...
.
Approaches
Like all models in computational science, fire models need to strike a balance between fidelity, availability of data, and fast execution. Wildland fire models span a vast range of complexity, from simple cause and effect principles to the most physically complex presenting a difficult supercomputing challenge that cannot hope to be solved faster than real time.
Forest-fire models have been developed since 1940 to the present, but a lot of chemical and thermodynamic questions related to fire behaviour are still to be resolved. Scientists and their forest fire models from 1940 till 2003 are listed in article. Models can be divided into three groups: Empirical, Semi-empirical, and Physically based.
Empirical models
Conceptual models from experience and intuition from past fires can be used to anticipate the future. Many semi-empirical fire spread equations, as in those published by the USDA Forest Service,
[Richard C. Rothermel. A mathematical model for predicting fire spread in wildland fires. USDA Forest Service Research Paper INT-115, 1972.] Forestry Canada,
[Forestry Canada Fire Danger Group. Development and structure of the Canadian forest fire behavior prediction system. Forestry Canada, Science and Sustainable Development Directorate, Ottawa, ON, Information Report ST-X-3, 1992.] Nobel, Bary, and Gill,
and Cheney, Gould, and Catchpole
for Australasian fuel complexes have been developed for quick estimation of fundamental parameters of interest such as fire spread rate, flame length, and fireline intensity of surface fires at a point for specific fuel complexes, assuming a representative point-location wind and terrain slope. Based on the work by Fons's in 1946,
[W. L. Fons. Analysis of fire spread in light fuels. ''Journal of Agricultural Research'', 72:93--121, 1946.] and Emmons in 1963,
[H. W. Emmons. Fire in the forest. ''Fire Research Abstracts and Reviews'', 5:163, 1963.] the quasi-steady equilibrium spread rate calculated for a surface fire on flat ground in no-wind conditions was calibrated using data of piles of sticks burned in a flame chamber/wind tunnel to represent other wind and slope conditions for the fuel complexes tested.
Two-dimensional fire growth models such as
FARSITE[Mark A. Finney. FARSITE: Fire area simulator-model development and evaluation. Res. Pap. RMRS-RP-4, Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 47 p., http://www.farsite.org , 1998.] and Prometheus, the Canadian wildland fire growth model designed to work in Canadian fuel complexes, have been developed that apply such semi-empirical relationships and others regarding ground-to-crown transitions to calculate fire spread and other parameters along the surface. Certain assumptions must be made in models such as FARSITE and Prometheus to shape the fire growth. For example, Prometheus and FARSITE use the Huygens principle of wave propagation. A set of equations that can be used to propagate (shape and direction) a fire front using an elliptical shape was developed by Richards in 1990.
Although more sophisticated applications use a three-dimensional numerical weather prediction system to provide inputs such as wind velocity to one of the fire growth models listed above, the input was passive and the feedback of the fire upon the atmospheric wind and humidity are not accounted for.
Physically based models and coupling with the atmosphere
A simplified physically based two-dimensional fire spread models based upon conservation laws that use radiation as the dominant heat transfer mechanism and convection, which represents the effect of wind and slope, lead to
reaction–diffusion system
Reaction–diffusion systems are mathematical models that correspond to several physical phenomena. The most common is the change in space and time of the concentration of one or more chemical substances: local chemical reactions in which the su ...
s of
partial differential equation
In mathematics, a partial differential equation (PDE) is an equation which involves a multivariable function and one or more of its partial derivatives.
The function is often thought of as an "unknown" that solves the equation, similar to ho ...
s.
More complex physical models join
computational fluid dynamics
Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid dynamics, fluid flows. Computers are used to perform the calculations required ...
models with a wildland fire component and allow the fire to feed back upon the atmosphere. These models include
NCAR
The US National Center for Atmospheric Research (NCAR ) is a US federally funded research and development center (FFRDC) managed by the nonprofit University Corporation for Atmospheric Research (UCAR) and funded by the National Science Foundat ...
's Coupled Atmosphere-Wildland Fire-Environment (CAWFE) model developed in 2005,
WRF-Fire at NCAR and
University of Colorado Denver
The University of Colorado Denver (CU Denver) is a Public university, public research university located in downtown Denver, Colorado. It is part of the University of Colorado system. Established in 1912 as an extension of the University of C ...
which combines the
Weather Research and Forecasting Model with a spread model by the
level-set method,
University of Utah
The University of Utah (the U, U of U, or simply Utah) is a public university, public research university in Salt Lake City, Utah, United States. It was established in 1850 as the University of Deseret (Book of Mormon), Deseret by the General A ...
's Coupled Atmosphere-Wildland Fire Large Eddy Simulation developed in 2009,
Los Alamos National Laboratory's
FIRETEC developed in,
the WUI (
wildland–urban interface
The wildland–urban interface (WUI) is a zone of transition between wilderness (unoccupied land) and land development, land developed by human impact on the environment, human activity – an area where a built environment meets or intermingles ...
)
Fire Dynamics Simulator (WFDS) developed in 2007,
and, to some degree, the two-dimensional model FIRESTAR.
[B. Porterie, D. Morvan, J.C. Loraud, and M. Larini. A multiphase model for predicting line fire propagation. In Domingos Xavier Viegas, editor, ''Forest Fire Research: Proceedings 3rd International Conference on Forest Fire Research and 14th Conference on Fire and Forest Meteorology, Louso, Coimbra, Portugal, 16--18 November 1998'', volume 1, pages 343--360. Associa\cc\ ao para o Desenvolvimento da Aerodinamica Industrial, 1998.] These tools have different emphases and have been applied to better understand the fundamental aspects of fire behavior, such as fuel inhomogeneities on fire behavior,
feedbacks between the fire and the atmospheric environment as the basis for the universal fire shape,
[J. L. Coen, T. L. Clark, and D. Latham. Coupled atmosphere-fire model simulations in various fuel types in complex terrain. In ''4th. Symp. Fire and Forest Meteor. Amer. Meteor. Soc., Reno, Nov. 13-15'', pages 39--42, 2001.] and are beginning to be applied to wildland urban interface house-to-house fire spread at the community-scale.
The cost of added physical complexity is a corresponding increase in computational cost, so much so that a full three-dimensional explicit treatment of
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
in wildland fuels by
direct numerical simulation
A direct numerical simulation (DNS)https://eprints.soton.ac.uk/66182/1/A_primer_on_DNS.pdf "A Primer on Direct Numerical Simulation of Turbulence –
Methods, Procedures and Guidelines", Coleman and Sandberg, 2010 is a simulation in computational ...
(DNS) at scales relevant for atmospheric modeling does not exist, is beyond current supercomputers, and does not currently make sense to do because of the limited skill of weather models at spatial resolution under 1 km. Consequently, even these more complex models parameterize the fire in some way, for example, papers by Clark
use equations developed by Rothermel for the USDA forest service
to calculate local fire spread rates using fire-modified local winds. And, although FIRETEC and WFDS carry prognostic conservation equations for the reacting fuel and oxygen concentrations, the computational grid cannot be fine enough to resolve the reaction rate-limiting mixing of fuel and oxygen, so approximations must be made concerning the subgrid-scale temperature distribution or the combustion reaction rates themselves. These models also are too small-scale to interact with a weather model, so the fluid motions use a computational fluid dynamics model confined in a box much smaller than the typical wildfire.
Attempts to create the most complete theoretical model were made by Albini F.A. in USA and Grishin A.M. in Russia. Grishin's work is based on the fundamental laws of physics, conservation and theoretical justifications are provided. The simplified two-dimensional model of running crown forest fire was developed in
Belarusian State University by Barovik D.V.
and Taranchuk V.B.
Data assimilation
Data assimilation
Data assimilation refers to a large group of methods that update information from numerical computer models with information from observations. Data assimilation is used to update model states, model trajectories over time, model parameters, and ...
periodically adjusts the model state to incorporate new data using statistical methods. Because fire is highly nonlinear and irreversible, data assimilation for fire models poses special challenges, and standard methods, such as the
ensemble Kalman filter (EnKF) do not work well. Statistical variability of corrections and especially large corrections may result in nonphysical states, which tend to be preceded or accompanied by large spatial
gradient
In vector calculus, the gradient of a scalar-valued differentiable function f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The g ...
s. In order to ease this problem, the
regularized EnKF
penalizes large changes of spatial gradients in the Bayesian update in EnKF. The regularization technique has a stabilizing effect on the simulations in the ensemble but it does not improve much the ability of the EnKF to track the data: The posterior ensemble is made out of
linear combination
In mathematics, a linear combination or superposition is an Expression (mathematics), expression constructed from a Set (mathematics), set of terms by multiplying each term by a constant and adding the results (e.g. a linear combination of ''x'' a ...
s of the prior ensemble, and if a reasonably close location and shape of the fire cannot be found between the linear combinations, the data assimilation is simply out of luck, and the ensemble cannot approach the data. From that point on, the ensemble evolves essentially without regard to the data. This is called filter divergence. So, there is clearly a need to adjust the simulation state by a position change rather than an additive correction only. The ''morphing EnKF''
combines the ideas of data assimilation with
image registration
Image registration is the process of transforming different sets of data into one coordinate system. Data may be multiple photographs, data from different sensors, times, depths, or viewpoints. It is used in computer vision, medical imaging, mil ...
and
morphing
Morphing is a special effect in motion pictures and animations that changes (or morphs) one image or shape into another through a seamless transition. Traditionally such a depiction would be achieved through dissolving techniques on film. Si ...
to provide both additive and position correction in a natural manner, and can be used to change a model state reliably in response to data.
Limitations and practical use
The limitations on fire modeling are not entirely computational. At this level, the models encounter limits in knowledge about the composition of
pyrolysis
Pyrolysis is a process involving the Bond cleavage, separation of covalent bonds in organic matter by thermal decomposition within an Chemically inert, inert environment without oxygen. Etymology
The word ''pyrolysis'' is coined from the Gree ...
products and
reaction pathways, in addition to gaps in basic understanding about some aspects of fire behavior such as fire spread in live fuels and surface-to-crown fire transition.
Thus, while more complex models have value in studying fire behavior and testing fire spread in a range of scenarios, from the application point of view, FARSITE and
Palm
Palm most commonly refers to:
* Palm of the hand, the central region of the front of the hand
* Palm plants, of family Arecaceae
** List of Arecaceae genera
**Palm oil
* Several other plants known as "palm"
Palm or Palms may also refer to:
Music ...
-based applications of BEHAVE have shown great utility as practical in-the-field tools because of their ability to provide estimates of fire behavior in real time. While the coupled fire-atmosphere models have the ability to incorporate the ability of the fire to affect its own local weather, and model many aspects of the explosive, unsteady nature of fires that cannot be incorporated in current tools, it remains a challenge to apply these more complex models in a faster-than-real-time operational environment. Also, although they have reached a certain degree of realism when simulating specific natural fires, they must yet address issues such as identifying what specific, relevant operational information they could provide beyond current tools, how the simulation time could fit the operational time frame for decisions (therefore, the simulation must run substantially faster than real time), what temporal and spatial resolution must be used by the model, and how they estimate the inherent uncertainty in numerical weather prediction in their forecast. These operational constraints must be used to steer model development.
See also
*
Catastrophe modeling
Catastrophe modeling (also known as cat modeling) is the process of using computer-assisted calculations to estimate the losses that could be sustained due to a catastrophic event such as a hurricane or earthquake. Cat modeling is especially appli ...
*
Extreme value theory
Extreme value theory or extreme value analysis (EVA) is the study of extremes in statistical distributions.
It is widely used in many disciplines, such as structural engineering, finance, economics, earth sciences, traffic prediction, and Engin ...
*
Fuel model
References
External links
PROMETHEUS fire growth simulatorWRF-FireWildfire simulations on YoutubeWildfire visualizations at NCAR
*
ttp://www.mmm.ucar.edu/people/coen/files/newpage_m.html Coupled Weather-Wildfire Modeling - Wildfire Case Studies
Fire research links*
''Why are wildfires defying long-standing computer models?''September 2012
{{Atmospheric, Oceanographic and Climate Models
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