(Coupled Atmosphere-Wildland Fire Environment)

Simulation and visualization of the 2012 High Park
                wildfireCAWFE sim of King fire
 CAWFE model simulations of the 2012 High Park fire (left) and the 2014 King Fire (right).

The CAWFE modeling system combines a numerical weather prediction (NWP) model that predicts how weather varies in time and space even in complex terrain with wildland fire behavior modules. These components are connected in two directions such that the evolving wind, along with fuel properties and terrain slope, directs where the fire grows and how fast, while heat released by the fire modifies its atmospheric environment thereby creating its own weather (e.g., fire-induced winds). The model is described in Clark et al. (2004) and Coen 2005a.  Coen (2013) documents the model equations.

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CAWFE was developed recognizing that fires interact with the atmosphere surrounding them and that this produces many fundamental fire behaviors. Research applying CAWFE showed that fire-atmosphere interactions produce numerous wildland fire phenomena, including the commonly-observed bowed shape (below); the heading, flanks, and backing regions; fire whirls; horizontal roll vortices.

Universal fire shape simulation

(left) Heat produced by the fire (more intense colors are hotter), smoke (misty purple field), and surface winds (longer arrows indicate stronger winds, the arrow indicates direction).  In this simulation, a fire began as a line in winds that were all coming at 3 m/s from the left, but which created a fire with a head, flanks, and backing region, and shaped the winds in the fire vicinity to be moving rapidly forward at the fire head, parallel to the flanks, and weak in the backing region. (right) The Onion fire, Owens Valley, CA (courtesy of Charles George, USDA Forest Service).

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CAWFE has been applied to many landscape-scale wildland fire events in varying fuel, terrain, and weather conditions. Provided that a coupled model can reproduce fine-scale (100s of meters) circulations and include fire feedbacks on atmospheric motions, models such as CAWFE can simulate overall rate and direction of spread, distinguishing characteristics of fire events, and transitions in fire behavior. CAWFE has reproduced other fire phenomena and illuminated the conditions in which they form, such as distinctive shapes of the fire perimeter, pyrocumulus, firenadoes, horizontal roll vortices, flank runs, rotating plumes, and the splitting (example below) or merging of fire lines.

 

 (left) CAWFE simulation of the 2006 Esperanza wildfire. Heat produced by the fire (more intense colors are hotter), smoke (misty purple field), and surface winds (longer arrows indicate stronger winds, the arrow indicates direction). (right) FireMapper infrared imagery of the Esperanza wildfire. (courtesy of Philip J. Riggan, USDA Forest Service).


Just how strong is the effect of the fire on its surrounding environment and why is it important?

The modification of the winds by the fire is the cause of virtually all phenomena that create the individual character of large event -  splitting of fire fronts, draws of flanks up canyon perpendicular to overall fire spread, how fires drawn themselves together (ex.: deliberately set fires may either be drawn into large wildfires or turn into wildfires themselves), and in the extreme, the generation of fire whirls and blowups, where the firestorm-like connection/grip/bond between the increasing fire intensity and the atmosphere tightens, such that the fire creates its own weather.  

This simulation reproduced several hours in the early period of the Big Elk Fire (below), a 2200 ha Colorado wildfire.  This case represents a relatively simple scenario, with no large-scale weather features - winds were driven primarily by solar heating of mountain slopes, producing weak afternoon upslope conditions during the active fire periods, and by the fire-induced winds themselves. 

 Big Elk simulationBig Elk
                fire (photo K. Close)

(left) CAWFE simulation of the Big Elk Fire climbing Kenney Mountain, in the center.  The red field shows where the air was warmed at least 10 degrees by the heat released from the fire.  The misty white field represent smoke, with denser areas representing higher concentrations.  The wind speed is shown by the length of the arrows (longer arrows indicate stronger winds) and direction near the surface.  (right) Photo of the Big elk fire during this period (photo courtesy of K. Close).

 

The magnitude of the effect of the fire on the winds is shown below.

difference
                between simulation with and without fire feedbacks to
                atmosphere

CAWFE simulation of the Big Elk Fire. The effect of the fire on the winds, shown by the difference between a simulation and one in which the fire could not modify the winds. The perspective is the same as the figure above, but looking straight down. The strength and direction of the change of the wind by the fire is shown both in the arrows and in the contours.  The effect of the fire-atmosphere interactions is that the fire draws air into the base of the plume at its leading edge, creating strong winds over the leading edge of the fire that increase the spread of the fire. 

 

Note that:
(1) The magnitude of the fire effects are to change the winds by 10-12 m s-1 near the fire, but they may also changes the winds in the fire environment by several m s-1 even 5 km from the fire.  This is how one fire may alter the winds affecting a nearby fire. 
(2) The fire effects on the winds make them unsteady, which may cause wind shifts that are a further hazard.  Again, these effects are only seen in models that capture the ability of the fire to alter the winds in its environment.

 

EXAMPLES OF USE:


ACKNOWLEDGEMENTS:

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This work was supported by the National Aeronautics and Space Administration under awards NNX12AQ87G, NNH11AS03, and NNN12AA01C, and the Federal Emergency Management Agency under award EMW-2011-FP-01124. The National Center for Atmospheric Research is sponsored by the National Science Foundation (NSF). Any opinions, findings, and conclusions or recommendations expressed in this material are the authors’ and do not reflect the views of NSF.

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REFERENCES


Clark, T. L., J. Coen, D. Latham: Description of a coupled atmosphere-fire model, 2004: Intl. J. Wildland Fire, 13, 49-64.

Coen, J. L., 2013: Modeling Wildland Fires: A Description of the Coupled Atmosphere-Wildland Fire Environment Model (CAWFE). NCAR Technical Note NCAR/TN-500+STR. 38 pp. http://nldr.library.ucar.edu/repository/collections/TECH-NOTE-000-000-000-866.

Coen, J. L., 2011. Some new basics of fire behavior.  Fire Management Today.  71(1), 37-42.  

Coen, J. L. 2005 Simulation of the Big Elk Fire using coupled atmosphere-fire modeling. Intl. J. Wildland Fire, 14, 49-59.

Coen, J. L. and P. J. Riggan, 2014: Simulation and thermal imaging of the 2006 Esperanza wildfire in southern California: Application of a coupled weather-wildland fire model. International Journal of Wildland Fire 23, 755-770. 

Coen, J. L. and W. Schroeder, 2013: Use of spatially refined remote sensing fire detection data to initialize and evaluate coupled weather-wildfire growth model simulations. Geophys. Res. Lett. 40:1-6. (doi:10.1002/2013GL057868)

Coen, J. L. and W. Schroeder, 2015: The High Park Fire: Coupled weather-wildland fire model simulation of a windstorm-driven wildfire in Colorado's Front Range. J. Geophys. Res. Atmos. 120:131-146

Coen, J. L. and W. Schroeder, 2017: Coupled Weather-Fire Modeling: from Research to Operational Forecasting. Fire Management Today. 75:39-45.

Coen, J. L., W. Schroeder, B. Quayle, 2018: The generation and forecast of extreme winds during the origin and progression of the 2017 Tubbs Fire. Atmosphere. Submitted.


Coen, J. L., E. N. Stavros, and J. A. Fites-Kaufman 2018: Deconstructing the King megafire. Ecological Applications. doi:10.1002/eap.1752.

Keywords:  wildfire model, fire behavior, forest fire, fire model, wildland fire model