A new study shows for the first time that wildfires burning in West Coast states can amplify storms in downwind states. Heat and small airborne particles produced by western wildfires intensify severe storms in the distance, in some cases with baseball-sized hail, heavier rain and flash flooding to states such as Colorado, Wyoming, Nebraska, Kansas, Oklahoma and the Dakotas.
Typically, western wildfires and storms in the central US are separated by seasons. Because fires start earlier each year, the two events are now moving closer together.
Earth scientist Jiwen Fan, a lab associate at the Department of Energy’s Pacific Northwest National Laboratory, began investigating a relationship between the two phenomena when she noticed that the 2018 wildfires in the west overlap with storms in the central US. She found that both events happened simultaneously for a week. Looking further, Fan found that it was the first time these storms and wildfires had converged in 20 years, with the storms lasting more than four days.
“I thought, maybe there’s a connection there,” said Fan, who led the new study. Her team used data describing the storms’ hail and rain levels, as well as the fires and plumes of smoke, to investigate a possible mechanism behind the connection. The group used weather models that track heat and smoke particles to investigate how the fires could affect the weather remotely.
“We have to be careful and informed,” Fan said. “The more we understand about the factors that contribute to storms like this, which cause massive loss of property, the better we can prepare for them. And if we look at the future climate, we know that wildfires will increase, especially in the west. .”
“Severe storms are also expected to increase in the center of the US,” Fan added. “Therefore, it is reasonable to expect that these concurrent events will become more frequent, and the impact of western wildfires on central storms may become increasingly important in the future.”
The article was published in the magazine on Monday 17 October Proceedings of the National Academy of Sciences.
Smoky skies, stronger storms
What’s behind the relationship? Let’s start with fires raging in the western US. As they burn, these fires give off incredible amounts of heat. For example, some fires heated the fire area 10-40 times hotter than normal July background temperatures. They also release billowing smoke particles called aerosols.
That heat creates a strong difference in air pressure. The air pressure is high during the fires. In the stormy center of the US, the air pressure is generally lower. As the high pressure builds near the fire, the surrounding air flows to air of lower pressure, amplifying the winds already flowing from west to east.
Those stronger westerly winds then carry smoke aerosols from western to central states. During their journey, the wind also picks up atmospheric moisture and carries it along. Now transported to storms brewing over the central US, the greater concentration of moisture and aerosols triggers a series of storm-strengthening reactions.
Like water droplets that collect on the needles of a redwood tree, the aerosols provide additional surface area on which water vapor can condense. When the water condenses, heat is released. This added heat provides energy that amplifies storms. When a storm is strong enough, the condensed water droplets freeze and begin to form hailstones.
In the storm, strong updrafts repeatedly lift the hailstones. Every second a hailstone spends in the storm is another time when it can collect more cooled water droplets, creating an ever-larger hailstone, like string after string added to a ball of string. Once the rocks become too heavy to be lifted by the storm’s updraft, they fall to the ground, causing damage to crops, buildings, cars, and sometimes people.
Local wildfires in the center of the US also intensified the same storms, according to the study, but to a lesser extent. Those wildfires are much weaker than their western counterparts. Fan’s group plans to seek similar connections in other regions.
The findings could help inform future severe weather forecasts, said Yuwei Zhang, lead author of the new study and a postdoctoral fellow in Fan’s research team.
“The cost of the storms we studied was more than $100 million in damage,” Zhang said. “If we know that distant wildfires contribute to stronger storms, that information could provide better projections, which could help prevent some degree of destruction.”
The Carr Fire, which claimed a quarter of a million acres in California, and the Mendocino Complex Fire, which caused $257 million in damage and burned 280 buildings in the same state, were among the fires investigated.
Hail in a changing climate
Many areas in the United States will see more wildfires — that means more aerosols from wildfires will be lifted into Earth’s atmosphere and affect the climate in ways scientists are trying to understand. In addition to amplifying severe storms from wildfires, how will the warming climate directly affect severe weather, especially storms that produce hail?
In a separate study published in the journal Earth’s Future, Fan explored how climate change could alter hailstorms across the central US. Fan found that some storms are sensitive to climate change, resulting in large hail more often, while other storms don’t have that sensitivity. Those sensitive storms are associated with a large-scale weather pattern, which is different from that of the storms that are insensitive to a changing climate.
“By linking the effects of climate change on hail storms with the easier to model large-scale weather patterns, this study improves our understanding of hail storm predictability with important implications for risk management,” Fan said.
PNNL authors of the study on western wildfires affecting central US storms include Fan, Zhang and Manish Shrivastava. Cameron Homeyer of the University of Oklahoma, as well as Yuan Wang and John Seinfeld of the California Institute of Technology, are also authors.
PNNL authors of the study on hail storm response to anthropogenic climate change include Fan, Zhang, Jingyu Wang, Jong-Hoon Jeong, Xiadong Chen, Shixuan Zhang, Yun Lin and Zhe Feng. Rebecca Adams-Selin of Verisk Atmospheric and Environmental Research in Lexington, Massachusetts, is also a contributing author.
This work was supported by the Department of Energy’s Office of Science and the National Science Foundation, and made possible through resources from the National Energy Research Scientific Computing Center, a user facility of the DOE Office of Science.