Ash Clouds: Severe storms can lift smoke into stratosphere

New field observations, satellite images, and computer models are steering some scientists toward a surprising conclusion: A severe thunderstorm, enhanced by the heat from a huge forest fire, can boost soot, smoke, and other particles as high as the lower stratosphere. The newly suspected transfer of aerosols to high altitudes could require significant changes in computer models of atmospheric circulation and climate.

ASH FOUNTAIN. A computer model of a fire-enhanced thunderstorm shows a plume of ash and soot (arrow) rising above cloud tops and into the stratosphere. Wang

Most of Earth’s weather–and most of its air pollution–resides in the troposphere, the layer of atmosphere that extends from the planet’s surface to altitudes between about 8 and 13 kilometers. Previous studies suggested that most particles floating in the stratosphere, the next-highest atmospheric layer, come from volcanic eruptions or are generated on the spot by high-flying aircraft, says Pao K. Wang of the University of Wisconsin–Madison.

Scientists generally hadn’t suspected that thunderstorms can transport particles across the troposphere’s well-defined upper boundary, but a wealth of observations is challenging that view, Wang told an audience last week in San Francisco at the fall meeting of the America Geophysical Union.

Consider the flurry of carbon-bearing particles encountered by a high-flying NASA research jet early this year. On flights near Kiruna, Sweden, instruments detected such aerosols at concentrations of up to 1 microgram per cubic meter. That’s more than 30 times the amount that can be accounted for by commercial aircraft, says Darrel Baumgardner of the National Autonomous University of Mexico in Mexico City. Elevated concentrations of carbon monoxide and other gases in air samples bolster the notion that the particles came from forest fires, he says. Baumgardner presented his findings at last week’s San Francisco meeting.

Also, satellite observations last summer showed that hundreds of fires across the Northern Hemisphere sometimes strengthened nearby thunderstorms, which then apparently pumped immense plumes of smoke and soot into the stratosphere. Some of those long-lasting plumes could be traced intact for distances exceeding 5,000 km, which suggests that many of the particles were in the stratosphere, riding high above weather systems that could have brought them back to the ground, says Michael D. Fromm, a meteorologist with the research firm Computational Physics in Springfield, Va. Similar plumes from intense fires near Canberra, Australia, reached the stratosphere in January 2003, he says.

Computer models of ash columns spouting from a large forest fire show that the extra heat can invigorate storm clouds that pass over or form nearby, says Wang. Strong updrafts in the storms can then carry plumes of smoke and soot that burst into the stratosphere, the simulations indicate.

Even small numbers of particles in the stratosphere can have significant effects on climate, says Wang. For instance, Baumgardner’s analyses suggest that the concentrations of sooty aerosols found over Sweden could boost absorption of incoming solar radiation in the lower stratosphere in winter months by as much as 15 percent.

No current models of atmospheric circulation or climate incorporate the effects of fire-enhanced thunderstorms injecting particles or gases into the stratosphere, says Wang.


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