White Paper

White Paper: Application Of Infrared Imagery For Understanding Wildfire Dynamics

Source: FLIR Systems, Inc - Research & Science

Janice Coen, John Daily, and Shankar Mahalingam

A thorough understanding of wildfire dynamics requires a clear three-dimensional picture of the winds in and near the fire, including the flaming combustion zone and the convective updrafts produced by the fire. These observations and analyses present a unique high-spatial-resolution and high-temporal-resolution perspective of the motions within crown fires propagating up a forested slope under light winds during the FROSTFIRE experiment in interior Alaska. The purpose of this work is to calculate combustion-zone winds and examine mechanisms for the rapid propagation of crown fires. An infrared imager was used to detect high-temperature regions produced by incandescent soot particles in and near the fire and to produce a sequence of high-frequency (60 Hz), high-resolution (0.375 m x 0.8 m) two-dimensional images of temperature. An image-flow analysis technique was applied to these data to derive wind fields in the image plane. Maximum updrafts of 32–60 m/s accompany maximum downdrafts of 18–30 m/s. Horizontal wind speeds of 12–28 m/s show strong inflow into the base of the convective updrafts and imply recirculation of air and incomplete combustion products from the fire. Motions were more complex than a single large convective plume or many buoyant tree, scale plumes rising separately. Instead, repeated examples of narrow flaming fingers, representing a scale larger than individual trees, initially burst upslope along the ground for tens of meters at speeds up to 28–48 m/s before turning upward. These bursts exceeded ambient environmental winds—those considered to be driving the fire — by a factor of 10 and were low enough to propagate the crown fire actively by both igniting and preheating/drying canopy fuel ahead of the fire. Average spread rates were 0.75–1.11 m/s, with a peak 10-s spread rate of 1.26 m/s. This powerful, dynamic mechanism of fire spread could explain firefighter reports of being overtaken by "fireballs.‘‘

FLIR Systems, Inc - Research & Science