With 600 square miles burned so far, the Park Fire is already one of California’s biggest wildfires ever – and her still far from contained. Driven by strong winds, the flames chewed through parched vegetation and spewed smoke high into the atmosphere. So much smoke and rising hot air, in fact, that it was create fire tornadoes and one of the strangest natural phenomena on earth: the pyrocumulonimbus cloud, or pyroCb.
It’s a smoke cloud that makes a dangerous wildfire like the Park Fire, burning in the northern part of the state, even more unpredictable. PyroCbs can produce lightning that continues to spark more fires around the very flame that made the clouds. And as the planet warms, pyroCbs appear to be becoming more common, as they start from the biggest, fiercest wildfires, which are themselves getting worse. “PyroCbs are such massive, almost volcanic eruptions,” said Rajan Chakrabarty, an aerosol scientist who studies the clouds at Washington University in St. Louis. Louis studied, said. “These pyroCbs create their own fire department.”
The Park Fire grew massively on a diet of extra dry fuel. This part of California didn’t burn for decades, so much plant life built up under the summer sun and dried out. Very low humidity helped soak up what little moisture there was in the vegetation, turning the landscape into a pile of gorse.
Such a large and intense fire is a breeding ground for pyroCbs – wonders of fire physics. As a blaze like the Park Fire burns — and burns more fiercely thanks to climate change causing higher temperatures and drier fuels — the heat from the flames rises, driving smoke particles tens of thousands of feet into the atmosphere. As the air rises, it cools and expands. Water then condenses on the smoke particles, and the cloud forms.
The masses of rising air in a pyroCb form a kind of void at ground level, which draws in more air, generating winds that encourage the spread of the flames. The most intense wildfires consume so much oxygen that they can somewhat suffocate themselves, but pyroCb winds spray more of the gas into the firestorm. It’s “a self-perpetuating process,” said Daniel Swain, a climate scientist at UCLA and the National Center for Atmospheric Research. “Because the more intense it is, the more oxygen rushes in, which means the more intense it is, which means the more oxygen rushes in. So you can see how it goes.”
At the same time, a high pyroCb can create a downdraft, making surface winds even more unpredictable. “The convection creates a lot of chaos, so it becomes very difficult to predict where air is coming from and where it’s going,” said Payton Beeler, an atmospheric scientist who studies pyroCbs at the Pacific Northwest National Laboratory in Richland, Washington. This in turn leads to chaotic fire behavior as those winds push the flames across the landscape at different speeds and in different directions.
The smoke from a pyroCb travels far beyond the flame that caused it. “Some of the aerosols that are injected into the upper troposphere or lower stratosphere tend to stick around for up to six to eight months,” Beeler said. “And they can basically be transported across hemispheres.”
The black carbon from a pyroCb cloud doesn’t behave exactly up there either. In a paper published last week in the journal Nature Communications, Beeler found that the black carbon from a pyroCb absorbs up to twice as much visible sunlight as black carbon from smaller fires or from urban sources, such as burning coal. “Particles in the pyroCb plume tend to have very, very thick coatings of organic matter,” Beeler said, “and that’s different from black carbon from other sources.”
This increases light absorption and increases temperatures in the atmosphere. “It’s like a black sweater — it absorbs all the sun and heats the environment around it,” said Chakrabarty, who co-authored the paper with Beeler.
However, why this happens in pyroCb clouds, scientists still do not fully understand. It could be that there is something distinct about the way a pyroCb spawning wildfire burns, or that there is a secondary process going on within the cloud to coat the particles with more organic matter. (Organics in this case come from the fire’s burning of vegetation.)
Another unanswered question is whether pyroCb clouds are already more common due to climate change fueling wildfires, or whether scientists are getting better at detecting them with satellites, or a combination of the two. PryoCb plumes have appeared all over the world, from Australia to Siberia, as fuels become drier and temperatures rise. The monster is on fire in Canada this summer bred them, too. “It seems to be happening more often,” Beeler said. “Whether it’s a function of warm climate and better identification, I think it’s probably both. But the impact appears to be very long-lasting and long-lasting.”
