MMM - Individual Webpage for Janice Coen

Analysis of Infrared Imagery of Wildland Fire Dynamics

Sample infrared imagery captured with an Inframetrics (now FLIR) Thermacam. The imager produces high frequency (60 Hz) imagery of radiant energy in the 3-5 micrometer range. The color in the image corresponds to the radiant temperature shown in the color scale to the left.

Understanding crown fire dynamics requires knowledge of 3-D winds in & near fire.

During FROSTFIRE(July 1999, image above), an infrared imager detected high temperature regions & produced high frequency (60/sec), high resolution (0.375 m x 0.8 m) images of temperature in crown fires in the Alaskan boreal forest.

Image flow analysis procedure was used to derive winds in image plane.

Purpose: Calculate combustion-zone winds and examine mechanisms for rapid propagation of crown fires.

The infrared data during the pictured crown fire run. (animation)

The inferred airflow in the image plane (animation).

Results:

Maximum updrafts were inferred to be 32-60 m/s, with maximum downdrafts of 18-30 m/s. Strong inflow was observed into the base of updrafts, with recirculation of Particles of Iincomplete Combustion (PICs).

Repeated examples of narrow flaming fingers burst upslope along the ground for 10’s of meters at 28-48 m/s before turning upward. These were low enough to preheat, dry, & ignite canopy.

These bursts exceeded ambient winds by factor of 10 & result from nonlinear vortex interactions.

–Rapid spread was not a result of strong environmental winds.

–Unlikely it can be represented by simple empirical formulas.

This powerful, dynamic mechanism is likely behind reports of firefighters being overtaken by “fireballs”.

For more details, see:
Coen, J. L., S. Mahalingam, and J. W. Daily, 2004: Infrared imagery of crown-fire dynamics during FROSTFIRE. J. Appl. Meteor., 43,1241-1259. PDF