Multi-ignition wildfires are not overly common. But when individual fires do converge, the consequences can be catastrophic. The largest fire on record in California, the 2020 August Complex fire, grew from the coalescence of 10 separate ignitions.
In a new study, published in Science Advances, researchers at Lawrence Livermore National Laboratory (LLNL), the University of California (UC), Irvine and collaborators examine multi-ignition fires, calculating their impact and modeling the mechanisms behind them by leveraging the Department of Energy's flagship Energy Exascale Earth System Model (E3SM). The work shows that when flames combine, they are disproportionately destructive: they spread faster, last longer, generate stronger atmospheric events and strain firefighting resources.
In California, the study found that multi-ignition fires make up only 7% of the total number of fires, but they contribute to 31% of the burned area in the state.
"Multi-ignition fires have a disproportionate effect on the burned area," said LLNL scientist and author Qi Tang. "Although they are quite rare, their influence is large compared to single-ignition fires."
Researchers at UC Irvine used remote sensing data to track multi-ignition fires and capture crucial data on their behavior. From there, the team at LLNL applied a simulation framework that captures fire-triggered thunderstorms (pyrocumulonimbus) and their downstream effects. The model connects the dots at the kilometer scale to create a big picture understanding of how wildfires ignite, move, merge, interact with regional atmospheric dynamics and thermal dynamics and trigger extreme thunderstorms.
"Pyrocumulonimbus events often occur when there's enormous wildfire event, but not all wildfires can trigger them," said Tang. "The distribution actually is very non-uniform around the world."
California, Canada and Siberia are some of the most likely locations for these fire-triggered thunderstorms. The extra heat from the fire at the surface creates a powerful updraft that lifts hot air and moisture into the sky. There, it condenses and can turn into a powerful storm. The same phenomenon often occurs on hot summer afternoons.
Pyrocumulonimbus create a higher chance of lightning strikes, and, depending on wind and environmental conditions, the clouds don't always coincide with the original fire location. This provides the perfect storm: an opportunity for a fire to spawn and later converge into a multi-ignition event.
Dealing with multiple fire fronts can be particularly difficult and dangerous for firefighters, who can get trapped if new flames spring up to surround them.
With their new modeling framework, LLNL scientists aim to predict - and possibly prevent - these events.
"We can help the community of firefighters know where the pyrocumulonimbus is more likely to occur, and that can lead to a prediction of where the fire triggers and the larger event would be," said Tang. "We might be able to do something to avoid multi-ignition events. That is one of our objectives."
More observational data, collected by a 2026 NASA field campaign, will further enable this modeling. The team at LLNL also aims to integrate their simulations with energy infrastructure planning.
"If we know there are fire events, we can simulate them and how that would influence the power grid," said Tang. "The results could potentially enhance U.S. energy security."
This research was funded by the LLNL Laboratory Directed Research and Development program. Additional support was provided by the Department of Energy Office of Science Biological and Environmental Research E3SM project.
