Dry lightning study could aid wildfire forecasting

Researchers from the School of the Environment at Washington State University, Vancouver, have developed the first long-term climatology of dry lightning — lightning which occurs with less than 2.5mm of rainfall — in central and northern California,

“Unlike human-caused fires that originate in a single location, lightning outbreaks can strike multiple locations and start numerous simultaneous wildfires, creating a substantial challenge for fire response,” says Dmitri Kalashnikov, a doctoral student in natural sciences and lead author of a paper published in the journal Environmental Research: Climate.

“Understanding the meteorology of dry lightning across this region can inform forecasting of possible wildfire ignitions, helping to better constrain future risk of wildfire ignition in California and can aid fire suppression efforts, as firefighting resources can be strategically pre-positioned in at-risk areas,” explains Deepti Singh, WSU assistant professor and co-author of the paper.

The team utilized daily lightning counts from the National Lightning Detection Network and precipitation observations from 1987–2020 in combination with atmospheric reanalyses to characterize the climatology of dry lightning and the associated meteorological conditions during the warm season (May–October) when wildfire risk is highest.

Tallying the impact

Dry lightning outbreaks are the leading cause of some of the largest wildfire outbreaks in modern California history.

“In 2020… multiple lightning-caused fires burned nearly one million hectares across this region,” Kalashnikov said, “and other prominent widespread lightning-caused wildfire outbreaks also occurred in 1987 and 2008.”

The researchers found that moisture and instability high in the atmosphere, particularly above a hot, dry, lower atmosphere, were key drivers of dry lightning across all regions in central and northern California. Furthermore, widespread dry lightning outbreaks can occur anytime in the warm season (between May–October), even in “quiet” years for lightning activity. By categorizing the widespread dry lightning days, they were able to identify four distinct large-scale atmospheric patterns associated with the outbreaks that feature different configurations of weather systems.

Moreover, they found that nearly half (46%) of lightning that struck the ground in this time period was under dry lighting conditions. While dry lightning was more likely to occur at higher elevations (>2000m) with activity more concentrated in July–August, the pattern reverses from summer to autumn: lower elevations are struck more frequently in September–October. During this time of the year, natural fuels are drier which increasing the risk of wildfire outbreaks.