(narrator speaking) A vortex refers to something spinning around an axis.
In the wildland fire arena, vortices like fire whirls are almost
always observed along flaming fronts, whether at ground or landscape scale. To transfer knowledge from fire scientists
to boots on the ground about a lesser known,
but potentially dangerous fire behavior phenomenon
called the Counter Rotating Vortex Pair, this video will use the 2020
El Dorado wildfire as a backdrop to explore
historical knowledge, anatomy of, and conditions
in, which they form. Awareness of Counter Rotating Vortex Pairs may help with future tactical decisions. The Counter Rotating Vortex
Pair has been scientifically documented and written about by fire scientists
for over 50 years, but is not widely known or specifically taught to the firefighter
community. In the article entitled Review of Vortices
in Wildland Fire. Jason Forthofer of the U.S. Forest Service’s Missoula Fire Sciences
Lab, and Scott Goodrick of the Southern Research Station state
that, “the key features of this vortex type is the paired nature of the vortices
rotating in opposite directions. These vortices
often occur along the flanks of the fire and can also be observed in the main plume
at the head of the fire.” In 1976, a split column with two approximately 100 foot
diameter vortices rotating in opposite directions was observed on the New Miner
fire in central Wisconsin. Their rotation was relatively slow
compared to a fire whirl, but the columns
would occasionally collapse and spill over the flanks of the fire,
resulting in rapid lateral spread. As firefighters commonly
anchor, flank, and pinch to suppress fires, A Counter Rotating Vortex Pair
and the conditions that may lead to their formation
are important to understand. Let's take a look at the anatomy
of a Counter Rotating Vortex Pair. This three dimensional simulation
was created to illustrate what is believed to have occurred on the El Dorado Fire in southern
California on September 17th, 2020, during a burnout operation. A large column formed after dark
on the east side of the San Bernardino Mountain Range. Investigators determined
that a Counter Rotating Vortex Pair likely formed by analyzing needle
freeze directions and evaluating observed fire behavior
and atmospheric conditions. Note the anatomy of the Counter-rotating
vortex pair in this particular situation. Deep flaming zones from heavy fuel
accumulation and extremely dry fuels, resulting in a dense column
that acts to block the wind. Two columns rotating in opposite
directions counterclockwise on the left flank of the fire
and clockwise on the right flank. This area of the fire was protected
from the general southwest wind flow on the lee side of the San
Bernardino Mountain range. The column was tall enough to reach
the wind shear zone of the atmosphere. Charlie
Morton, a squad boss and the Big Bear Hotshots, was overtaken by fire
and perished as the result of fire behavior
caused by a Counter Rotating Vortex Pair. Recognizing conditions
for potential Counter Rotating Vortex Pair formation. It is important to understand
that predicting the formation of a counter-rotating vortex
pair is impossible. However, certain fire behavior, terrain
and atmospheric attributes may foreshadow
the development of a vortex pair in this time
lapse of the Roaring Lion fire from 2017. Mark Finney, research forester
at the Rocky Mountain Research Station, highlights
key characteristics of a Counter Rotating Vortex Pair. (Mark Finney speaking) This is of the Roaring Lion
fire in Montana and we’ll point out a few features of this. It’s probably one of the best
timelapse videos that I've seen of a couple of features
that are important to the El Dorado Fire. So you're going to start
to see some spinning of the plume, on the right hand side
and on the left hand side, you'll see that the plume is spinning
in opposite directions, ...opposite directions. And then on the right hand
side, you're going to see in just a minute an excursion. The smoke is going to kind of stream down
away from the fire and spread out into the valley floor
like it just did. And in a moment, you will see that
there are a number of spot fires down there that begin to grow
and get drawn back into the main plume. Fire behavior characteristics such as deep
flaming zones and long residence times in heavily accumulated fuels may create
a column and could result in conditions ripe for Counter Rotating Vortex Pair
formation. Watching for persistent plume
rotation and the column splitting into two will help
fire practitioners recognize formation
of a Counter Rotating Vortex Pair. (Mark Finney speaking) This is caused pretty much
just like a rock in the middle of a river. It's obstructing the wind flow
and the water around the rock has to create
eddies on the lee side. Now, the plume is not a rock. It's not impervious to the wind. But when you create a very strong plume,
most of the wind does not penetrate the plume
and disperse it. It has to flow around it. And as it flows around it, it starts
to catch the edges there and rotate them. And so you're seeing in the upper left
hand diagram there
that these are counter rotating pairs. They make the plume into the kidney
shaped structure rather than in a conical shape structure. Recognizing topography that blocks or channels
wind flow, such as the lead side of a ridge or
a mountain range is also a key indicator. Because of resulting atmospheric effects
like wind shear. Wind shear at ridge top
level has been shown to create vorticity. In conclusion,
we know that Counter Rotating Vortex Pairs can pose significant dangers
to operators in the fire environment. The conditions that may lead to
their formation are known, but exactly when and where is impossible
to predict. Understanding the fire behavior,
topographic and atmospheric conditions that lead to counter-rotating vortex power
generation are essential tools for fire practitioners
operating in the modern fire environment. (music playing)
