Analysis of Small-Scale Convective Dynamics in a Crown Fire Using Infrared Video Camera Imagery

Abstract

A good physical understanding of the initiation, propagation, and spread of crown fires remains an elusive goal for fire researchers. Although some data exist that describe the fire spread rate and some qualitative aspects of wildfire behavior, none have revealed the very small timescales and spatial scales in the convective processes that may play a key role in determining both the details and the rate of fire spread. Here such a dataset is derived using data from a prescribed burn during the International Crown Fire Modelling Experiment. A gradient-based image flow analysis scheme is presented and applied to a sequence of high-frequency (0.03 s), high-resolution (0.05–0.16 m) radiant temperature images obtained by an Inframetrics ThermaCAM instrument during an intense crown fire to derive wind fields and sensible heat flux. It was found that the motions during the crown fire had energy-containing scales on the order of meters with timescales of fractions of a second. Estimates of maximum vertical heat fluxes ranged between 0.6 and 3 MW m22 over the 4.5-min burn, with early time periods showing surprisingly large fluxes of 3 MW m22. Statistically determined velocity extremes, using five standard deviations from the mean, suggest that updrafts between 10 and 30 m s21, downdrafts between 210 and 220 m s21, and horizontal motions between 5 and 15 m s21 frequently occurred throughout the fire. The image flow analyses indicated a number of physical mechanisms that contribute to the fire spread rate,