Infrared imagery of crown-fire dynamics during FROSTFIRE

A thorough understanding of crown-fire dynamics requires a clear picture of the three-dimensional 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 20° slope under light winds of 3 m s^-1 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 × 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^-1 accompany maximum downdrafts of 18-30 m s^-1. Horizontal wind speeds of 12-28 m s^-1 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^-1 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^-1, with a peak 10-s spread rate of 1.26 m s^-1. This powerful, dynamic mechanism of fire spread could explain firefighter reports of being overtaken by "fireballs."