Modeling fire whirl signatures associated with a wind shift

Erratic fire behavior including formation of fire whirls can occur when wind shifts such as associated with cold fronts or sea breezes intercept a fire line. A recursive coupled fire/atmosphere fire spread model (Rabbit Rules) simulated the FireFlux prairie grass fire. The model simulated fire with 1 m resolution and plume scale winds with 30 m resolution. Partway through the simulation, the winds aloft were shifted to blow from the opposite direction. As a result, the plume scale convergence zone ahead of the fire line collapsed back toward the fire line, then moved across the fire line to behind the fire line as the fire transitioned from a headfire to a backfire. During the transition, a vortex signature – a center of rotation collocated with a center of convergence formed 50 m ahead of the fire line and became collocated with the fire line during the transition to a backfire. The collocation of the vortex signature with the fire line is an indicator of fire whirl formation. The vortex signature moved behind the fire line and gradually dissipated.