USA — A series of large wildfires occurring in California, Oregon and Washington are providing enterprising scientists with an opportunity to study one of natures most unusual, dirtiest and spectacular clouds, known as pyrocumulus, or pyrocu (pronounced pyroQ) for short.
These clouds, comprised of ash and soot particles lifted skyward by the upwardly moving current of exhaust air above a fire, are both the result of a wildfire and a contributor to that fires behavior. Therefore, understanding these clouds is key to saving firefighters lives as well as the lives and property that may lie in the path of a capricious blaze.
As of Tuesday, about 490,000 acres were burning across the U.S., with 12 large fires burning in California, 11 in Oregon and four in Washington, according to the National Fire Information Center in Boise, Idaho. Some of these fires have launched plumes of smoke up into the jet stream, where it has been carried eastward, obscuring the sky across Minnesota and Wisconsin.
The clouds associated with these fires can resemble volcanic eruptions and sometimes they even generate their own lightning and thunder, in which case they are renamed pyrocumulonimbus. (Pyro is the Latin word meaning fire.)
On July 31, at the Oregon Gulch fire along the state border with California southeast of Medford, Oregon, such clouds were caught on camera by a flight of Oregon National Guard F-15 fighter jets. Next to the towering clouds, the weapons of war appear as tiny, powerless specks.
At another fire over the weekend, the Bald Fire in California, scientists obtained high-resolution data on pyrocumulus clouds. One of those scientists, Neil Lareau of the Fire Weather Research Laboratory at San Jose State University, told Mashable that scientists dont actually know a lot about these clouds, although they do have a general understanding of how they form.
For very large fires, and with the right atmospheric conditions, he said, the upwardly moving air above the fire can reach a level where the air cools enough to condense into clouds.
Once the cumulus cloud forms there is an additional release of heat as the liquid water condenses. The addition of more heat provides renewed vigor for the updraft, which then ascends deep into the troposphere and is marked by crisp bright white clouds, Lareau says.
The appearance of these clouds can be quite stunning, as seen in many recent photos of the large fires in northern California, Oregon, and Washington. The ones we saw had a marble-like look in the cloud due to the mixing of grey ash with bright white liquid water droplets,” he adds.
Lareau and his colleagues are traveling to fires with specialized gear, like a truck-mounted automated weather station complete with a Doppler Lidar system designed to reveal hidden interactions between fires and the atmosphere. Viewed using this tool, the clouds resemble infants on a sonogram, except with a lot more movement.
Very little is known about dynamics of large convective plumes from wildfires, including those that produce pyrocumulus clouds. Some fires develop these large, upright columns of rapidly ascending air. Some don’t, he says. The ones with the large concentrated updrafts are referred to as ‘plume dominated’ fires and tend to exhibit extreme fire behavior that can threaten life and property. They generate their own winds that can propel and accelerate the fire in very unpredictable ways. In other words, they take on a life of their own.
Instead of becoming a tornado chaser, Lareau and his colleagues are chasing fires. We’re gathering observations of phenomena that have really never been quantitatively documented, and that is exciting. We’re making some interesting discoveries along the way, Lareau says.
The researchers are red card certified, which means they have been trained in fire safety. This allows them to gain access to large wildfire events where other scientists are not able to go. Lareau himself has extensive experience in dangerous wilderness settings, having earned a graduate degree in mountain meteorology and worked as a weather observer at the Mount Washington Observatory in New Hampshire, where the worlds strongest wind gust 231 miles per hour was recorded.
Already, the researchers have used the Doppler Lidar, which uses light to judge distance, to observe large wildfires that featured rotating currents of rapidly rising air, a phenomenon more frequently seen in tornado-producing thunderstorms. This phenomenon was seen at the Stoney Fire in California in July, according to the research group’s blog.
These rotating updrafts can lead to firenadoes, which have lit up the Internet this year. Sometimes strong updrafts generated above the fire acquire rotation, Lareau told Mashable. The vigorous convergence and ascent in the plume concentrates and stretches some spin from the environment. The result is a vortex that can reach the strength of small tornadoes. This can lead to rapid and unpredictable fire spread and threaten firefighter safety.
On June 30 of last year, the Yarnell Hill fire in Arizona rapidly changed direction and overcame the position of 20 Granite Mountain Hotshot firefighters. All but one, who had held a lookout position, were killed. This kind of research may be helpful in future situations like that, by improving predictions of fire behavior.
The researchers have also documented cases where a fires upwardly moving column of air, like the exhaust from a cars engine, broke through a barrier and into a stable layer of air aloft, such as an area of ocean-dominated air known as the marine layer. Marine layers are common in southern California, where wildfires are notoriously fickle.
Once the updraft from the fire is free of this “lid” it can explode upward, Lareau says. The rapid ascent can change the dynamics of the fire by rapidly drawing more air into the area that is burning. The fire may intensify and move in a new direction.
Lareau says the research team has not yet analyzed all the data it gathered from the Oregon Gulch event on Saturday, but the pictures speak for themselves.
Lidar view of a convective plume of smoke from a wildfire growing rapidly after penetrating a stable layer of air above it. This was taken at the Butts Fire in California in July. Source: San Jose State University
Pyrocumulus clouds arent just important for firefighters to keep a wary eye on. They are also important players in the Earths climate. Studies have shown that so-called plume-dominated fires that feature pyrocumulus clouds are able to loft enough soot and other particles up to the height where jet aircraft typically fly that they can influence the amount of solar energy that reaches Earths surface. In addition, large fires can transport dark particles such as soot to highly reflective surfaces, such as the Greenland ice sheet.
Once soot is deposited onto ice, it causes more solar energy to be absorbed, thereby speeding up the melt process. In late July and early August, satellites spotted smoke from Canadian wildfires drifting across Hudson Bay toward Greenland.
Although this has not been an unusually active wildfire season so far, California in particular is facing a unique combination of combustible ingredients, with a record drought combined with record heat and a rare uptick in monsoon-related thunderstorms that have struck everywhere from Los Angeles to the high-elevation forests in the northern part of the state. Thousands of lightning strikes associated with thunderstorms across the tinderbox-dry state have ignited fires while also unleashing deadly mudslides.
More fires are likely to erupt in the coming months, with the peak of the California season looming in the fall.
Pyrocumulus cloud observed by Oregon National Guard F-15 fighter jets. Source: Oregon National Guard