Storms brewing over volcanoes

by Carolyn Gramling
Thursday, January 5, 2012

Lightning illuminates the night sky over Redoubt Volcano in March 2009. Photo courtesy of geologist and photographer Bretwood Higman

Two hundred years ago, a sea captain was sailing in the Azores, a Portuguese archipelago in the middle of the Atlantic Ocean, when he observed “an immense body of smoke rising from the sea.” As he watched, the smoke, from a volcanic vent offshore of the island of St. Michael’s, began to rotate on the water “like a horizontal wheel,” the captain wrote in his 1811 account of the event. The rotating smoke and ash grew into a dark column and ascended high into the sky, spawning waterspouts and flashes of lightning. “To give you an adequate idea of the scene by description is far beyond my powers,” he wrote.

But Pinaki Chakraborty, a fluid dynamicist at the University of Illinois in Urbana-Champaign, disagrees: The captain’s rare eyewitness account of a tornadic storm brewing above a volcanic eruption was more than adequate, Chakraborty says. It provides an important clue to a previously unsuspected meteorological link between volcanic plumes and supercell thunderstorms.

During a powerful eruption, a volcano can produce an umbrella-shaped plume of ash, with a vertical column at the base and a spreading cloud at the top. Over the years, observers of volcanic eruptions have noted these distinctive clouds; in some cases, they have also observed waterspouts or dust devils, or powerful flashes of lightning in the column, Chakraborty says. But how these disparate phenomena were connected has been unclear. Now, in a new study published in Nature, he and his colleagues suggest a kind of “unified explanation” for the atmospheric phenomena observed over erupting volcanoes: More than just plumes of ash, volcanic columns strongly resemble mesocyclones, the spinning columns of air at the heart of tornadic thunderstorms.

Chakraborty became interested in the question while studying satellite images of the plume emerging from Mount Pinatubo’s 1991 eruption in the Philippines. “The thing that piqued my interest was the [plume’s] umbrella,” he says. A time-series of satellite images, taken every hour for four hours, showed that as the umbrella spread out, its edges contorted from a circular shape into a wavy series of lobes. Over time, the lobes also seemed to shift in a counterclockwise direction — suggesting there was some kind of rotation going on, he says.

Coriolis forces related to Earth’s spin were one initial explanation for the rotation, Chakraborty says, but the rotation was in the wrong direction, and was developing too rapidly — within an hour, as opposed to a day. But there was another possibility: The forces behind the ash cloud’s rotation might be similar to the meteorological forces driving cloud rotation in a powerful thunderstorm. If so, the column beneath the umbrella is also likely rotating. The column itself is obscured to satellites by the overlying umbrella cloud, but eyewitness accounts — although rare — suggest similarities between thunderstorms and volcanic storms.

The sea captain’s story is what made things click, Chakraborty says. The unusual account contained a trinity of volcanic phenomena rarely reported together: a rotating column, spawned tornadoes and lightning. “When we saw these three things together,” Chakraborty says, “that’s when it struck us that maybe we should be looking to thunderstorms.”

The recipe for a mesocyclone within a tornadic thunderstorm is a combination of strong wind shear creating horizontal vortices in the atmosphere and a powerful updraft — caused by a plume of air lighter than the surrounding air — that rises swiftly, engulfs surrounding air and tilts the rotating vortex into a vertical column as it spins. “We propose that a very similar phenomenon happens” above an erupting volcano, Chakraborty says.

Over a particularly powerful eruption, volcanic mesocyclones may spawn waterspouts and dust devils, or produce lightning. Volcanic lightning generally appears to form a “sheath” around the mesocyclone (such a phenomenon was observed during the 2008 eruption of Chile’s Chaitén volcano). That shape could be explained by the rotation, Chakraborty says — a mesocyclone sucks electrical charges away from the center of rotation, and concentrates them on the periphery of the spinning clouds.

“This idea is interesting, in that it explains phenomena that have been witnessed but not really explained by volcanologists,” says Ken Wohletz, a volcanologist at Los Alamos National Laboratory in New Mexico who was not connected with the study. Dozens of observations of such volcanic phenomena exist, although many are hard to locate, Wohletz says. But “there has not been a really coherent explanation for them.”

Getting volcanologists to accept this explanation might take some time, he says, but “it opens the door for a new appreciation and understanding of the complexity of volcanic columns.” The concept of volcanic mesocyclones also highlights an increasing need for interdisciplinary crossover between the fields of volcanology and meteorology, Wohletz says: Already, volcanologists increasingly are investigating ways to predict the fate of hazardous volcanic ash clouds, as well as volcanoes' impact on climate.

And to really understand what’s happening inside a rotating volcanic plume, volcanologists will also want to use the tools of meteorology, Chakraborty says. Generally, in a volcanic plume, the ash is so thick that the column is obscured; the sea captain “was very lucky that he got to see it,” he says. To study similarly obscured storms, meteorologists use Doppler radar, which takes a cross-section through a rotating column to observe what’s going on inside. “I’m hoping that people from the thunderstorm [community] will become interested in this.”

Scientists got a chance to watch volcanic lightning develop for the first time during the eruption of Alaska's Mount Redoubt in March. Alerted by Redoubt's rumblings in January, a team of researchers from New Mexico Tech in Socorro, N.M., rushed to the region to set up a system of radio sensors, called a Lightning Mapping Array, that captures a time series of three-dimensional images of lightning flashes over the volcano — data that they say will help scientists better understand the evolution of electrical charges within a volcanic plume.

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