What causes fire tornadoes?

Reference: Dowdy, A. and Pepler, A. (2018).  “Pyroconvection Risk in Australia: Climatological Changes in Atmospheric Stability and Surface Fire Weather Conditions.”  Geophys. Res. Let., 45, 2005-2013. https://doi.org/10.1002/2017GL076654

Title Image Source: Wikimedia Commons.

Since a single natural disaster isn’t scary enough, wildfires are often accompanied by what is known as pyroconvection, or the development of severe weather such as hail, lightning, or rather apocalyptic-looking fire whirls and even fire tornadoes.  Pyroconvection can cause a wildfire to grow larger and more erratic, increasing the danger it poses to homes, ecosystems, and firefighters.  A study from 2018 looked at how the risk of pyroconvection varies by geography, finding southeast Australia to be particularly susceptible.

How can wildfires fuel a thunderstorm?

Thunderstorms are the result of rising air.  (Their billowing ascent may be easier to see in a timelapse.)  This rising air is actually the source of energy for thunderstorms.  More specifically, the energy comes from gravitational potential energy, which is released when warm, humid air near the surface rises above colder, denser air in its surroundings.

Vegetable oil, water, dish soap, corn syrup, and honey form distinct layers based on their density in a plastic cup.
Dense fluids like to sink, and less dense fluids like to rise. The same is true in our atmosphere, except density is mostly determined by temperature. Warm air tends to rise, and cold air tends to sink. Image credit: Ross Fairbairn.

There are two major components to the potential energy that acts as fuel for a thunderstorm.  The first is having a major source of heat or humidity near the surface (the hotter the air, the less dense it is, and the more likely to rise).  The second, known as atmospheric stability, measures how the temperature and humidity of the atmosphere change as you travel upward.  A parcel of air can keep rising as long as it is less dense than its environment, and atmospheric stability is important because it tells us how dense the environment is.  Both of these components – surface conditions and atmospheric stability – have been used by Andrew Dowdy and Acacia Pepler, as well as many other scientists, to evaluate the risk of thunderstorms and how, for instance, thunderstorms might change with climate.

A layer of smoke underlies a towering cumulonimbus cloud above a beach in Australia.
Pyrocumulonimbus cloud in Tasmania (2013). Image credit: Australian Government Bureau of Meteorology, Janice James.

As you may now have guessed, severe wildfires spawn thunderstorms because they provide an extreme source of heat on the ground.  In a process known as pyroconvection, wildfires heat up the air, which then rises, condenses, and forms what are known as pyrocumulus, or in extreme cases like the one pictured above, pyrocumulonimbus, clouds.

Why does pyroconvection matter?

Pyroconvection can exacerbate the situation on the ground by introducing hazardous winds, which increase the size of the fire by helping it leap from one tree to the next.  Pyrocumulus clouds can also result in the formation of new fires several miles away, either by blowing burning embers around or through lightning strikes (known as pyrogenic lightning).  Pyrogenic lightning is thought to have initiated new wildfires during the Black Saturday bushfires, which were a series of fires that killed 173 people in 2009.

A silhouette of a charred tree is visible in front of fires.
Bunyip State Forest (2009) near Melbourne, Australia. Image credit: Andrew Brownbill/EPA, The Guardian.

In addition to causing wildfires to grow or propagate, pyroconvection can result in extreme temperatures and erratic winds that are dangerous for firefighters, particularly when fire whirls or, more rarely, massive fire tornadoes form.  A firefighter named Jeremy Stoke was killed by the Carr fire tornado linked to in the first paragraph.

Andrew Dowdy and Acacia Pepler examined how climate change might have an effect on pyroconvection by combining two different indices that assess wildfire risk.  The first of these indices, the McArthur Forest Fire Danger Index (FFDI), measures conditions on the ground and accounts for factors like increased drought, heat, and wind speed.  The second index, the Continuous Haines index (CH), includes factors like atmospheric stability and dryness.  Dowdy and Pepler found that both indices are needed to assess the risk of pyroconvection.  First the FFDI can be used to identify conditions on the ground that make it susceptible to wildfire formation; then the CH can be used to identify conditions in the atmosphere, like atmospheric stability, that make pyroconvection more likely.

Will climate change result in more pyroconvection?

The authors found that the environmental conditions, as measured by the FFDI and CH indices, had grown considerably more favorable for pyroconvection in southeastern Australia from the years 1979-2016.  They noticed that there isn’t a one-size-fits-all threshold on the CH that governs wildfire risk, but instead wildfires occur at different values of the CH in different regions.  By looking instead for values that were anomalous in a given region (ie higher than the 95th percentile), they found that compound events with both very high FFDI and very high CH were particularly enhanced in southeast Australia, where the bushfires in January generated a lot of pyroconvection as well as unprecedented destruction.

In a followup study in 2019, the authors used a suite of climate models to simulate how the climate in Australia will respond to anthropogenic climate change.  They found that much of Australia is likely to become even more susceptible to wildfires as a result of rising temperatures and dryness.

The “Black Summer”

The full effects of the 2019-20 bushfires, which are now colloquially known as the “Black Summer,” are yet to be seen, but the disaster is thought to have killed nearly 500 people and, by some estimates, an unfathomable one billion animals.  At the Royal Commission hearings held last week in Canberra, Australia’s Threatened Species Commissioner warned that some of the wildlife may never recover, and a fire historian warned that Australia could be on the brink of entering a new era of enhanced bushfires.

The outlook is sobering, but there are steps that conservationists can take to mitigate the crisis, and some of them are outlined here.  In particular, tunnels can be made underground to help surviving animals evade predators like cats, controlled burns can be undertaken to reduce dry material for fires, and conservationists can try to identify and protect fire refugia, which are small pockets of animals who survived the fire.  (A few months ago, fire refugia were the subject of an Envirobites article!) 

In the end, though, symptomatic solutions can’t prevent fires like this from growing more and more common.  Only dramatic reductions in our fossil fuel emissions can do that.

A cuddly baby koala sits sits on its mom's back.
Koala mom and joey. Image Credit: Wikimedia Commons.

On a Personal Note…

I actually wrote most of this post a few months ago (bet you can guess when), but in light of recent events, I wanted to mention that I see scientists as belonging inextricably to the communities we live in, and to highlight the fact that science has to be understood in the context of these communities. Here’s an example of how that contextualizing looks for wildfires: all around the world, from the Amazon to California to Australia, indigenous communities are uniquely at risk from wildfire devastation, due to a combination of living in vulnerable areas, limited access to the resources required for recovery, and structural oppression from those in power. Indigenous communities are also uniquely positioned to protect their land, through political engagement as well as vast cultural knowledge that Western conservationists stand to learn a lot from.

So in that spirit, I just wanted to say that Black Lives Matter, and I, as a scientist and human, support the protests against white supremacy and police violence. 

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Rohini Shivamoggi

I'm a PhD student studying atmospheric sciences at MIT. I study the formation of secondary eyewalls in hurricanes, which hopefully will help us improve our forecasts of hurricane intensity. Before I got to MIT, I grew up in Florida and studied Chemistry and Physics at Harvard University. My other interests include weather forecasting, photography, and encouraging diversity in STEM! You can find me on Twitter @RShivamoggi.

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