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from inside hurricanes and tornadoes
Interference makes for 3D images
Several research teams are developing instruments that allow them to make three-dimensional maps of lightning structure within a storm. One method is to use multiple receivers to measure differences in the arrival times of the radio impulses. Zen Kawasaki of Osaka University in Japan has developed a method that uses differences in the phases of the radio waves as they arrive.
Kawasaki employs two wide-band interferometers. These are radio receivers with four antennas and electronics arranged so the arriving radio waves interfere with each other. This allows the receiver to calculate the direction of the radio pulse coming from a lightning stroke. With two stations set up about 23 km (13.8 mi) from each other, a researcher can determine the origin of a radio pulse.
The two methods are complementary. Paul Khreibel at New Mexico Institute of Mining Technology said his time-of-arrival system is better for three-dimensional and total activity studies, while interferometers like Kawasaki's are better for studying electrification channels within storms.
Left: A 3D plot of lightning during a storm, Nov. 20, 1996, over northwest Australia. Links to. Credit: Osaka University.
"We can clearly get three-dimensional images of the lightning leader," Kawasaki said.
Kawasaki and his researchers tested the system in two field campaigns during the monsoon season in late summer 1996, 1997, and 1998 off Darwin, Australia.
Kawasaki said his team is working on a more sensitive narrow-band interferometer, too. The current narrow-band system has a sensitivity of 1 microsecond (1 million per second). The narrow-band model being developed will take even faster samples, once every 10 nanoseconds (100 million per second).
"With the narrow-band interferometer, we can see positively charged cloud-to-ground flashes," Kawasaki said. "But, they are the negative leader activity."
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A hurricane research campaign in 1998 showed that hurricanes typically don't produce much lightning, but sometimes they can.
"What we want to know is why," said Monte Bateman of the Universities Space Research Association. "Some longtime hurricane pilots have reported that when a hurricane does produce lightning, intensification often follows."
Left: Doppler radar on the ER-2 showed an intense updraft as hurricane Georges moved over the island of Hispaniola. Credit: NASA. Links to.
The third Convection and Moisture Experiment (CAMEX 3) in August and September 1998 focused a number of ground, airborne, and satellite instruments on hurricane activities in the western Atlantic Ocean, the Gulf of Mexico and on thunderstorms over Florida. One of the tools used in the campaign was the high-altitude ER-2 reconnaissance aircraft, equipped with eight lightning detectors among other instruments.
The ER-2 recorded only a few lightning flashes as it flew over the eye of hurricane Bonnie on Aug. 26.
Hurricane Georges was a different matter when it waded ashore and battered the island of Hispaniola on Sept. 22. Georges showed "nearly continuous lightning, in and around the eye," Bateman said. In addition, the ER-2 pilot reported seeing about one blue jet every minute for 30 minutes when he was flying over the hurricane. Blue jets are fascinating electrical events that rise from the cloud tops into the upper atmosphere. Their cause is not yet known.
Bateman said that the increased lightning around Georges' eye probably was due to air forced upward - called orographic forcing - when the hurricane hit the mountains. He added that the pilot reported the eye of the hurricane was stacked like a wedding cake over the mountain, rather than being the depressed structure normally seen over the oceans.
As impressive as the difference was, it is muted compared to a normal Florida thunderstorm. Bateman also showed a graph of an Aug. 15 storm that the ER-2 studied. Georges' lightning activity was about 10 times less than the lightning activity in a typical thunderstorm.
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Putting instruments in a tornado's path is far more exciting than what movie viewers saw depicted in the hit movie "Twister."
"We were only a few minutes ahead of it," explained Steve Hunyady, an instrumentation engineer at New Mexico Institute of Mining Technology, as he described dropping eight "turtles" outside Allison, Texas, as an F4 tornado approached on June 8, 1995.
The turtles are less attractive than the gold ornaments the actors released into a tornado in the movie. That level of miniaturization is a few years away. But Hunyady said that the turtles were able to provide valuable data about the electric fields around a tornado. Scientists would like to know what happens inside, since observers have reported some lightning and frequent electrical glows inside the vortex.
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"We built them strong so they could be lofted," which wind tunnel tests predicted would happen, but "we've never seen one move yet."
Each turtle contains an electric field meter, a pressure sensor in a small snorkel, a thermometer, and a clock, plus a small computer to store eight hours worth of data.
Fortune was with the researchers that day and the F4 moved almost exactly between two of the turtles. To the researchers surprise, they saw the electric field dipped, and no lightning as recorded.
Hunyady said that lack of electrical activity may be due to lofted debris reducing the electric field, or possibly to increased conductivity inside the tornado itself.
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