Ominous Clouds You Should Know About
In recent weeks we have witnessed historic wildfires that created their own weather and frequent lightning from a newly designated type of towering cloud called “pyrocumulus,” or recently abbreviated to simply “pyroCbs.” “Pyro” derives from Latin for “fire” and Cb as the common abbreviation for cumulonimbus. The threats from pyroCbs are considerable. According to the U.S. National Weather Service office in Boise, Idaho, the Bootleg Fire in south-central Oregon produced a pyroCb event almost every evening for several days in a row. By no means are these threats limited to the Western U.S. A pyroCb event not long ago in Portugal cost 60 lives, illustrating the power of this atmospheric phenomenon as well as its international scope. The emergency preparedness community is desperately seeking to understand and predict this powerful atmospheric phenomenon to help better prepare for evacuation and mitigation. The immediate concern for public safety focuses on the wildland fire management teams who work along the fire lines directly adjacent to this billowing monster. An aerial fire-fighting captain who has flown countless sorties along the edges of the Bootleg Fire queried, “Hey doc, we’re listening to these briefings from the incident command weather teams about these pyroCbs. The effects on the fire front are jaw-dropping. What are the effects on the aviation side?” This avid BCA reader also happens to be an airline pilot in the off-season, thus his query carried considerable concern that would potentially apply throughout many aviation sectors. At this reader’s suggestion, I reviewed the scientific research to date on pyroCbs. The research summarized in this article opens definite questions about the effects of pyroCbs on aircraft and their operations.
What Is A PyroCb? On July 21, the U.S. Naval Research Laboratory (NRL) conducted an online press conference about pyroCbs. It included Mike Fromm, a scientist at the U.S. NRL in Washington, D.C., who tracks pyroCb storms using satellite imagery, and David Peterson, a meteorologist with the Monterey office of the NRL, who specializes in earth and atmospheric sciences. According to these two respected researchers, pyroCbs are fire-induced thunderclouds that inject massive amounts of smoke particles into the upper atmosphere. They begin as a typical wildfire reaches its daily crescendo in the late afternoon. It is the hottest part of the day, with the lowest relative humidity, atmospheric instability and a situation often accompanied by erratic winds. These are conditions conducive to significant fire growth. When these events are hot enough, wildfires can trigger convective updrafts, the depths of which extend well into the lower stratosphere. Even though the recent news is focusing on pyroCbs coming from wildfires in the Western U.S., wildfires in Canada and Australia have spawned tremendous pyroCb events with global implications, and in recent weeks we are also seeing pyroCbs in other places across the globe.
How Does a PyroCb Form? According to Peterson, “These are thunderstorms driven directly by large wildfires. The smoke plume is fed directly into the developing cloud on top of the fire. Think of them as chimneys funneling smoke released by fire up into a thunderstorm, and then ejecting it at whatever altitude the thunderstorm tops out at.” A research report written by a team of atmospheric scientists led by Peterson and published in the scientific journal Climate and Atmospheric Science provided contrasts and comparisons with typical convective storms. “PyroCBs are characterized by relatively smaller cloud droplet and ice-particle size distributions, caused by an overabundance of smoke particles. What will this mean for the flight crew at high altitude? These particles typically provide a low radar signature.” “Smaller inherent size distributions within the inner storm core suppress precipitation, which inhibits scavenging and redistribution of smoke downward toward the ground. A considerable quantity of smoke particle mass is therefore exhausted from the top of pyroCBs through the high-altitude anvil outflow region, forming an efficient vertical smoke-transport pathway. PyroCb activity is often characterized by extreme updraft velocities (up to 6,900 fpm to 11,400 fpm!) that rapidly transport smoke particles directly from the surface to the lower stratosphere, similar to an explosive volcanic eruption.” (Source: Peterson David A., et al. “Australia’s Black Summer pyrocumulonimbus super outbreak reveals potential for increasingly extreme stratospheric smoke events,” Climate and Atmospheric Science, 13 July, 2021.)
DAVID A. PETERSON, U.S. NAVAL RESEARCH LABORATORY
According to Peterson’s observations, “It is an extremely dirty thunderstorm with a lot of smoke particles for water to condense on.” That creates differences in the properties of the cloud. It isn’t very efficient at generating precipitation. The droplets don’t get large enough to fall as rain. But it is a cloud that can produce a lot of lightning. By the way, I double-checked the references on the updraft velocities in these clouds. Consider the significance of an updraft that can project a particle from the ground to above 30,000 ft. in just 3 min.! From an aviation perspective, that is an air mass with strength far beyond the capability of any aircraft. Secondly, whenever there are rapid inflows of wind of this magnitude into a thunderstorm, there will be severe downdrafts. The atmospheric specialists on the incident command teams briefing the fire-fighting squad leaders are warning that the downdrafts, usually downwind, can produce erratic winds at the surface. This would be a risk to fire fighters on the ground as well as the airborne fire-fighting aircraft in the vicinity of this enormous cloud.
Australia Event Started NRL’s Interest Even though the recent news is focusing on towering clouds called pyrocumulonimbus (pyroCbs) coming from wildfires in the Western U.S., wildfires in Canada and Australia have also generated tremendous pyroCb events with global implications. For instance, an intense, multi-day outbreak of fire-induced and smoke-infused thunderstorms (known as pyrocumulonimbus or pyroCb) occurred during Dec. 29–31, 2019 and Jan. 4, 2020, in southeastern Australia. This Australian New Year Super Outbreak (ANYSO) of pyroCb activity resulted in roughly 1 teragram (equivalent to 1 million metric tons) of cumulative smoke particle mass being injected into the lower stratosphere, consistent in magnitude with the initial ash and sulfate plume of a moderate volcanic eruption. During the Australian pyroCb event, fires emitted an unprecedented amount of smoke to heights over 10 mi. One plume, estimated at 3 mi. thick and 621 mi. across--about the distance from Atlanta to Washington, D.C.--traveled east to South America by late January. Over the next several weeks, the smoke plumes dissipated and covered the Southern Hemisphere, circumnavigating the globe. This event showed the first evidence of smoke causing changes to winds in the stratosphere. “The stratosphere has traditionally been described as an impenetrable barrier to tropospheric aerosol, with the exception of volcanoes,” said George “Pat” Kablick III, a U.S. Naval Research Laboratory (NRL) atmospheric scientist. “This event showed for the first time that pyroCb plumes can cause rotation generated by solar heating of the smoke. We need to study how these pyroCb events change stratospheric composition and meteorology.” More than seven months after the initial brush fires in Australia, remnants of the plumes are still detectable, even at altitudes above 18 mi., said Kablick. “While they are no longer the tightly concentrated aerosol plumes they once were, certain satellite instruments can still detect faint signals,” he said. “However, they are more dispersed, and the rotation and self-lofting have probably ceased.”
Meteorologists call these “Absolutely Mind-Blowing” The single largest pyroCb observed to date in North America exploded on June 30 from the Sparks Lake fire in British Columbia. Mike Fromm, a scientist at the NRL who discovered pyroCbs about 20 years ago, called it a “monster pyroCb.” The wildfire grew to more than 62,000 acres and sent a smoke plume up to 10 mi., according to data collected by NASA’s Terra satellite. A GOES satellite observed the storm unleashing extraordinary bursts of lightning. The North American Lightning Detection Network detected nearly 113,000 cloud-to-ground lightning strokes during this event. This represents about 5% of Canada’s normal annual lightning. A meteorologist analyzing the satellite data and smoke spreading widely as updrafts punched into the stratosphere called it “absolutely mind-blowing.” Fromm observed that one of the aspects that distinguishes these from volcanic-originated clouds is that many pyroCbs can exist simultaneously in a region. A “PyroCb Outbreak” event occurred this July in Northern Manitoba and Saskatchewan. Satellite views showed intense activity, with 10 pyroCbs in this region. It was more pyroCbs than they had ever observed in North America on a single day since they started tracking pyroCb events with satellites in 2013. Images from the satellites have been posted on NASA’s Earth Observatory website and can been seen at this link: https://earthobservatory.nasa.gov/images/148630/a-summer-of-fire-breathing-smoke-storms?rand=6926
Upper Atmosphere Smoke Increasingly Coming from Wildfires Fromm pointed out that these storms are ejecting smoke to such high altitudes that it exceeds the altitudes of jet cruising aircraft. How high? In the words of Fromm, “We’re talking 50,000 to 60,000 ft.!” This means that the propagation of smoke in the upper atmosphere is increasingly driven by wildfires. It is raising the question of what this will do to the climate system. Two of the four largest smoke plumes in the stratosphere originated from wildfires. The other two originated from large volcanic events.
The second-largest plume occurred in Australia and produced 38 different pyroCbs. A pyroCB event that originated in British Columbia in 2017 created a plume that persisted for 10 months in the stratosphere.
According to David Peterson, a meteorologist with the Monterey office of the NRL, “The prediction of the effect of any aerosol plume in the atmosphere is a big research effort at NRL. We run a global aerosol model to account for any source…smoke, dust, pollution, whatever. In the case of pyroCb, this is a phenomenon that is pushing smoke up to high altitudes. Any forecast model, whether it is NRL, NOAA, etc., does not have a capability right now to account for that mechanism.”
Wildfires and Atmosphere PyroCbs require a synchronization of the wildfire condition on the ground and the meteorological conditions aloft. According to the journal study in Climate and Atmospheric Science, “These unusual storms are driven by a common meteorology, requiring dry air near the surface to nurture vigorous fire activity, and a complementary moisture source several kilometers aloft to facilitate condensation, latent heat release, and subsequent enhancement to the buoyancy of growing convective cells…. The synergy of favorable meteorology with broad and vigorous wildfire episodes generally limits pyroCB development to active summertime fire seasons in Australia, North America and northern Asia.”
JOHN HENDRICKS/OREGON OFFICE OF STATE FIRE MARSHAL VIA AP
Aircraft have collected airborne data samples. What are the results? The “Fire Influence on Regional to Global Environments and Air Quality” (commonly referred to as “FIREX-AQ”) was a comprehensive wildfire smoke investigation with field studies carried out during the wildfire season of 2019. Mission planning for FIREX-AQ was complex, involving hundreds of instruments and scientists from six federal agencies, 22 academic institutions, two foreign governments and numerous private-sector companies that collaborated in a complex logistical operation.
Specially instrumented aircraft were utilized to conduct air quality data sampling. The NASA DC-8, the world's largest flying chemistry laboratory, was the flagship of FIREX-AQ. This modified DC-8 carries a comprehensive suite of chemistry and aerosol instruments to analyze fire emissions in flight and study chemical reactions and transformations in smoke from both Western wildfires and agricultural/land-management burning emissions across the United States. A NASA ER-2 high-altitude research aircraft was also used. Correlating the inflight measurements of other aircraft with the ER-2's high-resolution observations will expand the ability of scientists to utilize lower-resolution data provided by satellites to understand what's happening on the ground. The ER-2 operated from its base in California during this project. A modified DHC-6 Twin Otter from NOAA deployed to Boise, Idaho and other locations across the northwestern U.S. in July and August 2019 to collect samples of airborne emissions, photochemistry and nighttime chemistry, in coordination with other research platforms. The NOAA Twin Otter focused on the variability of the emissions-measuring close to a wildfire for extended time periods; the fast evolution of smoke in the first few hours after emissions start; and the vertical distribution in the concentration, composition and optical properties of smoke in regionally impacted Western valleys. While the data-crunching from the volume of information collected in 2019 will continue for years, specialists at NOAA’s Chemical Sciences Division authored a report titled “The Impact of Wildfires on Climate and Air Quality.” They found that a wide variety of chemical compounds were released by wildland fire. The long list includes greenhouse gases [carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O)] and photochemically reactive compounds [e.g., carbon monoxide (CO), nonmethane volatile organic carbon (NMVOC) and nitrogen oxides (NOx)]. Obviously, there are threats to human health as well as effects on the climate from this mixture. For purposes of our study, it also certainly brings into question if we should be concerned about the effects of these compounds on the operation of aircraft.
Engines Are Like Vacuum Cleaners Turbine engines were designed to operate in “normal” air, which is composed of 78% nitrogen, 21% oxygen and 1% other gases. The proper ratio of oxygen is absolutely necessary in the combustion process. If the oxygen concentration in the engine inlet air drops below a certain ratio, then the combustion process may no longer be sustainable, resulting in an engine flame-out. In contrast, the composition of smoke plumes contrasts significantly from the “normal” airborne environment, both in chemical composition as well as particulate matter. The smaller-sized particulates remain suspended aloft in the atmosphere longer than coarse particles, and therefore disperse farther from the wildfire. The Australian Civil Aviation Safety Authority published an airworthiness bulletin (#72-002, dated Sept. 29, 2006) titled “Engines Operating in a Fire-Fighting Environment,” advising operators about the multiple effects of smoke particles on turbine engines. To begin with, particles ingested from the smoke can block small-diameter pressure-sensing and control lines, leading to erratic fuel metering to the combustion chamber, erratic engine operation and malfunctioning of accessories. Secondly, these particles can block fine cooling holes within turbine blades. This results in the loss of cooling air to the turbine blades, exposing blades to temperatures beyond their design parameters. Airborne particles will also cause deposition of contaminants inside heat exchangers (e.g., oil coolers) that may block free air flow and result in decreased heat transfer, which will then elevate fluid temperatures. Higher-than-permitted fluid temperatures result in rapid deterioration of internal engine components. At night crews won’t see this smoke plume, so it is entirely possible for a high-flying aircraft to fly through a region polluted by a smoke plume. Are the concentrations of smoke plumes at altitude sufficient to cause concern about the effects on turbine engines? Will the high-altitude ash particles and other compounds in the smoke plumes affect the cabin environmental system? Will the air filtration system capture these? How soon will these clog up the filters in the air system? These are valid questions that need validated studies to investigate.
Should We Be Concerned? As promised at the beginning of this article, recent scientific research from subject matter experts working within respected organizations was summarized to provide some of the early insights into the immensity of this newly designated weather event. David Peterson and his colleagues, found that detailed airborne sampling, in combination with ground and spaceborne observations, is therefore essential for improved understanding of pyroCb impacts. (Source: Peterson, David A., et al. “Wildfire-driven thunderstorms cause a volcano-like stratospheric injection of smoke.” Climate and Atmospheric Science, Aug. 20, 2018.) “An analysis of the smoke aerosol particles injected into the lower stratosphere from five near-simultaneous intense pyroCbs observed over western North America on Aug. 12, 2017, found that the physical particle properties and chemical evolution of pyroCb smoke in the stratosphere remains highly uncertain. Processing of pyroCb smoke during the lofting process into the stratosphere will change its composition and properties relative to surface or tropospheric smoke plumes. In addition, since pyroCb updrafts begin with strong surface inflow winds in a dry environment, additional aerosol particles such as mineral dust, may also be contributing factors.” Should we be concerned in aviation? The answer is a probable “yes,” but that is qualified with the caveat that, “ We really don’t know enough right now…and specifically, how the products of this cloud will affect an aircraft’s systems.” The wildland fire-fighting community is rightfully concerned about the substantial hazards created by this atmospheric monster. At this point the research on this threat is still in its infancy. Clearly much more needs to be discovered through the scientific process, and then properly translated into terms that are usable and relevant for the aviation industry.
Author’s Note: This is a special “thank you” to the several aerial fire-fighting captains who suggested the importance of bringing attention to this topic and who provided feedback as well as information exchange with their weather teams. At this very moment they are engaged in missions along the firelines of the Dixie and Bootleg Fires.
To see an ominous video of the immense growth of a PyroCb over the over Dixie Wildfire incident, click on this link:
https://twitter.com/i/events/1417456190393491463
