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.
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.
(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).
(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.
(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.
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:
REFERENCES