Researchers at the University of California, Irvine have identified that clusters of fires started by lightning, known as fire complexes, are a primary factor behind California’s most destructive wildfire seasons. The findings, published in Science Advances, suggest that these fire complexes can help agencies improve how they manage such incidents.
“Nobody has ever looked into these kinds of fires before,” said Rebecca Scholten, a postdoctoral fellow in Earth system science and lead author of the study. “We theorized that when two or more fires in a fire complex merge, they would just burn themselves out. But we found the opposite – the fires grow worse.”
The research team analyzed fire complexes ignited between 2012 and 2023 across California and Arctic-boreal regions. In California, although multi-ignition fires are less common than single-ignition ones, they were responsible for 31 percent of total burned area. In Arctic-boreal regions, this figure was 59 percent.
One significant example is the August Complex fire in 2020, which burned approximately one million acres over seven counties after lightning sparked 38 separate blazes that merged into larger fires. The KNP Complex Fire also caused extensive damage to Sequoia and Kings Canyon National Parks.
To understand how these events unfold, Scholten’s team examined the growth patterns of individual fires within a complex and their tendency to merge. They discovered that merging fires can create atmospheric conditions conducive to extreme behavior such as pyrocumulonimbus events—fire-induced thunderstorms capable of igniting new blazes and complicating firefighting efforts. The study noted that most pyrocumulonimbus events in Russia and Canada during 2023 were triggered by fire complexes.
“The concentrated energy released from two fire fronts getting close to one another can allow fires to punch into the upper atmosphere,” said James Randerson, professor of Earth system science and co-author of the study. “When this happens, winds near the surface can become erratic, hampering fire suppression.”
Fire complexes often grow rapidly and last longer than other types of wildfires, exhausting available resources for emergency responders. Better understanding how merging fires worsen situations may assist agencies in prioritizing their response efforts.
“It might help you decide which ones you want to put out first,” Scholten said.
The researchers also highlighted a need for weather agencies to improve forecasts for dry lightning—a key trigger for these large-scale fire complexes—as it is not currently standard practice among weather services. “Dry lightning is not a standard event that weather services try to predict,” Scholten added. “But it’s something that could be useful as fire complexes become more likely to form as the climate continues to warm and create conditions ideal for fires to ignite.”
The project received funding from several sources including a Dutch Research Council Rubicon Grant awarded to Scholten (019.241EN), NASA, and the U.S. Department of Energy. Collaborators included departments within UC Irvine as well as NASA, Naval Postgraduate School, Vrije University in the Netherlands, and Lawrence Livermore National Lab.
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