Review of Vortices in Wildland Fire

Table 1

Common vorticity forms associated with wildland fires and their hazards.

Phenomenon	Causal factor(s)	Potential danger
Fire whirl formation on the lee side of a plume	Shear-generated vorticity near the ground is concentrated and reoriented to the vertical on the lee side of the plume.	Increased energy release rate, spread rate, and spotting. The whirl could travel downwind from the fire and overtake firefighters.
Fire whirl formation near an L-shaped fire in a cross flow wind	Shear-generated vorticity near the ground is concentrated and reoriented to the vertical on the lee side of the L, as shown in Figure 4.	Increased energy release rate, spread rate, and spotting. The whirl could suddenly form in the “interior” area of L.
Fire whirl formation near a cold front	Vorticity along the frontal boundary is concentrated in to a fire whirl.	Increased energy release rate, spread rate, and spotting.
Fire whirl formation due to multiple interacting fire plumes	The drafting and blocking effects of multiple interacting fire plumes concentrate vorticity that was likely shear generated near the ground.	Increased energy release rate, spread rate, and spotting. Whirl could build into a fire storm.
Fire whirl formation on the lee side of a hill/mountain	Vorticity associated with the wake region of a terrain obstruction such as a hill or mountain is concentrated into a fire whirl.	Increased energy release rate, spread rate, and spotting. The fire could quickly switch from a sheltered, backing fire with low fire behavior to more extreme fire behavior. The whirl could travel downwind from the fire and overtake firefighters.
Transverse vortex on upwind side of smoke column	Horizontal vorticity is produced through buoyancy.	Not a source of erratic fire behavior, but rather an indicator of a potential increase in the rate of combustion and an associated change in fire behavior.
Single longitudinal vortex	Unstable atmosphere and strong winds generate horizontal vortices with axis parallel to the wind direction. Vortex formation is not tied to the fire.	Slight variations in wind direction can destabilized the vortex, causing the vortex to fall outward across the flank of the fire, providing a mechanism for lateral bursts in fire spread.
Counter-rotating longitudinal vortex pair	Transverse ambient vorticity due to surface wind shear is altered by the fire as it is tilted into the vertical and reoriented to the longitudinal direction. Evident as a bifurcated smoke plume.	Can produce concentrated wind bursts at the head of the fire that lead to strong fingering of the fire front. The vortices are not always stable as variations in wind direction can cause one of the vortices to collapse and bring hot gases and fire brands into contact with the unburned fuel.