Wildfires can sometimes suddenly escalate into an unpredictable and life-threatening inferno. The cause of such behaviour is occurring increasingly often – even in the Netherlands. How can researchers and firefighters gain better control over this phenomenon, known as pyrocumulus?
On Thursday 3 April, heathland to the northeast of Ede caught fire. Soil physics and land management researcher Cathelijne Stoof saw the massive plumes of smoke from the sixth floor of the Wageningen University & Research campus. In a photo she took, the smoke plume rises first and then bends sideways. At the kink, a small bulge points upward.
"If that bulge had been just a little higher, we would have had a pyrocumulus. The fire then spreads extremely fast and unpredictably. That creates dangerous situations," explains meteorologist Chiel van Heerwaarden.
What is a pyrocumulus?
'Pyro' means fire, and 'cumulus' is a type of puffy cloud. This type of cloud forms when the atmosphere is unstable: warm and humid air in the lower layers is overlaid by much colder air. Smoke and water vapour then rise quickly and condense into a cloud. That cloud begins to draw in air at high speed - like a giant vacuum cleaner. This rapidly intensifies the fire, causing it to spread quickly and erratically.
To the naked eye, a pyrocumulus is nearly impossible for firefighters to identify. "When you're that close to a fire, you're underneath the cloud and can't see how big it is," says Marc Castellnou from Spain. He is a PhD researcher in fire meteorology at WUR and a fire commander in Catalonia. Whether such a cloud forms depends not only on the size and intensity of the fire but also on the atmospheric conditions. Castellnou: "There's a tipping point. That's when the fire begins to influence the weather. And that's a self-reinforcing effect." He refers to sudden temperature spikes, unexpected strong winds from a different direction, or even lightning and rain. "A fire can accelerate from half a kilometre per hour to five, or even ten kilometres per hour."
Rare - yet on the rise
It's a rare phenomenon, but with devastating consequences. In 2017, 66 people died and hundreds were injured in a single wildfire in Portugal. Similar deadly fires have occurred elsewhere in the world. According to researchers, the Netherlands has seen this phenomenon twice in recent years. A pyrocumulus formed in both cases, though a major disaster with many casualties was avoided. Castellnou has observed a sharp rise in risk in Catalonia over the past seven years. "Previously, conditions for the formation of a pyrocumulus were present in 10% of wildfires. Now that's increased to 18%."
Climate change appears to be increasing the likelihood of pyrocumulus formation, due to rising temperatures and extreme drought. Just like in Southern Europe, there is also reason for concern in Central and Northern Europe. That's why Van Heerwaarden, Stoof, Castellnou, and Professor of Meteorology and Air Quality Jordi Vila are researching the phenomenon. "The chance of a wildfire day is lower in your region - it's less hot and dry," says Castellnou. "But if it happens, the risk of pyrocumulus is actually greater, because there's more combustible material in nature, precisely because it doesn't burn as often."
Launching weather balloons
Van Heerwaarden: "The potential for this chaotic fire behaviour is still hardly included in the models firefighters have been using for decades." "No, they focus on fuel, surface weather, and the landscape itself," adds Stoof. "The latest scientific model also incorporates the atmospheric conditions that can lead to pyrocumulus. This is a fundamentally different way of analysing fire behaviour." The researchers are training this new model with wildfire data from around the world.
During a fire, atmospheric conditions can be measured by a weather balloon launched by a fire service analyst. Within seconds, the data feed into the model. This helps emergency services to choose the best strategy on site - for example, evacuating residents immediately if the risk of pyrocumulus is high. On Saturday 12 April, a weather balloon was launched for the first time in the Netherlands during a wildfire in the Loonse and Drunense Dunes. The risk of a pyrocumulus proved to be low in that case.
Fundamental research to prevent disaster
The research by Wageningen's wildfire experts is a close collaboration between fundamental science and real-world firefighting practice. And that's urgently needed, according to Stoof, who advocates for better fire prevention. "We need to prepare not only for today's fires, but also for those of the future. Soon, fire meteorologists will monitor the conditions throughout the entire fire season. It will become just as important as producing regular weather forecasts."
