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  Geotimes - May 2007 - Geophenomena
NEWS NOTES — GEOPHENOMENA
Volcanic eruptions spark dirty thunderstorms
A new eye on hurricanes

Smoke and ash plumes rise above Alaska’s St. Augustine volcano on Jan. 12, 2006, a day after an eruption began. The cloud above the volcano may behave like a “dirty” thunderstorm, producing bursts of electricity, or lightning. Photograph is by M.L. Coombs / U.S. Geological Survey.

In January 2006, Alaska’s St. Augustine volcano erupted. Along with its surges of smoke and ash, however, the volcano produced an electrical lightning show that scientists say may not be so different from more familiar thunderstorms.

Volcanic lightning is well-known to scientists, but there have been few direct scientific observations of the phenomenon. So when Augustine began to erupt on Jan. 11, 2006, a team of scientists quickly set up stations along Cook Inlet, about 100 kilometers east of the volcano, to measure radio waves emanating from the electrical storm. On Jan. 27 and 28, the volcano erupted again — against a background of strong radiation and several lightning-like bursts, the team reported Feb. 23 in Science. The team also noted flashes extending from the volcano’s summit up through the cloud, something never seen before.

Most often, the flashes occurred just after the eruption and high up in the ash cloud — supporting the idea that volcanoes, which release a lot of water and can have eruption clouds similar in size to ordinary thunderclouds, may produce storms that resemble “dirty” thunderstorms. In those thunderstorms, the team wrote, ice particles in the clouds collide and produce static charges; similarly, above an erupting volcano, rock fragments, ash particles and ice in the eruptive cloud may be interacting to create the bright bolts of light.

Carolyn Gramling

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As Hurricane Rita swirled over the Gulf of Mexico in September 2005, its innermost eyewall weakened, and an encircling band of winds moved in to take its place. Image is courtesy of U.S. Naval Research Laboratory.

Meteorologists have become more adept in recent years at forecasting a hurricane’s path, but anticipating a hurricane’s intensity is considerably trickier. Researchers are now gaining a better understanding of why some hurricanes intensify, however, thanks to airplane-based radar measurements of 2005’s Hurricane Rita. Those data show the dynamics of the “eyewall” that surrounds the center of a storm, findings that could help improve storm predictions, the authors say.

Scientists have been looking hurricanes in the eye for decades, says Bob Houze, an atmospheric scientist at the University of Washington. Specifically, scientists have focused on the powerful wall of winds, known as the eyewall, surrounding the calm core of a hurricane. As the hurricane swirls out over the ocean, the eyewall drags warm, moist air up from the surface of the water, prompting the winds over the warm air to weaken. Meanwhile, lines of convective clouds called rainbands that spiral inward toward the eyewall can combine and form a second eyewall, outside of and somewhat less powerful than the first. Between the two strong bands of wind and rain lies a region of dry air called a “moat.”

In some storms, Houze says, when the innermost eyewall loses energy and disappears, the outer eyewall can move in to take its place. That replacement can cause a storm to either intensify or weaken, but which path it takes has so far been poorly understood. To get a closer look at how eyewall dynamics can affect storm intensities, Houze’s team sent radar-equipped airplanes into the winds of Hurricane Rita, part of the RAINEX (Rainband and Intensity change Experiment) field program, which also investigated eyewall dynamics in hurricanes Katrina and Ophelia. Rita was particularly interesting, Houze says, because it had formed a second eyewall.

Data from the Rita missions showed that the moat between Rita’s two eyewalls was not merely a passive void: Instead, the air in the moat region was both dry and sinking, strongly resembling conditions within the storm’s calm eye, the team reported March 2 in Science. Such conditions within the moat could promote eyewall “replacement,” by weakening the conditions that created the primary eyewall, the authors say.

Air within a storm’s eye sinks in order to compensate for the air pulled in by the wildly circulating winds of the innermost eyewall, which help supply its strength. As it sinks, the air in the eye becomes warmer and drier. If a secondary eyewall exists, however, its own circulating rainbands may also pull in air from outside, causing the air within the moat to similarly sink. Over time, this robs the first eyewall of much of its energy source. Eventually, it can disintegrate, allowing the moat and the eye to join to form a new eye surrounded by the second eyewall.

The second eyewall may initially be less intense than the first, but it can quickly gain or lose energy, making the storm stronger or weaker than before. Such eyewall replacement may occur many times during the life of a hurricane — depending in part on the temperature of the ocean water in the storm’s path. In the case of Rita, eyewall replacement reduced the storm’s intensity from a category 5 to a 3 or 4, the team reported.

As researchers continue to better understand the dynamics of eyewall replacement and how they relate to intensity changes, scientists may be able to improve their predictions of a storm’s future intensity, the team reported. But eyewall replacement is only one factor, they note — some storms, such as Hurricane Katrina, never experienced eyewall replacement at all.

Carolyn Gramling

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