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Out of the wreckage

发布时间:2019-03-08 05:10:15来源:未知点击:

By Gabrielle Walker, Kurt Kleiner and Charles Seife EVEN by the standards of America’s “Tornado Alley”, the 76 twisters that ripped through Oklahoma, Nebraska, Kansas and Texas on Monday last week were extreme. The tornadoes were the most destructive for decades, killing 47 people, destroying 2000 homes and causing about $500 million worth of damage. At the same time, the tornadoes have provided an invaluable set of meteorological data that could at last allow researchers to begin to understand why some storms spawn devastating twisters while others produce nothing more damaging than showers of hailstones. “For 30 years, people have tried to look for differences between storms that make tornadoes and ones that don’t, and have come up empty-handed,” says Paul Markowski, a tornado researcher at the University of Oklahoma in Norman. Twister chasers hope that the data they gathered last week will be the key to realising their goal of being able to issue specific warnings when twisters are on their way. “In the next ten years I think we’ll have the answer,” says Howard Bluestein, also at the University of Oklahoma. Thunderstorms can form when a warm, damp mass of air at ground level runs underneath a cool, dry mass of air moving in the opposite direction. This situation is highly unstable: when the warm air starts rising through the cool dry air, its moisture condenses to form hail or rain. The latent heat released makes the warm air rise higher still. Such unstable conditions can lead to the formation of supercells—powerful thunder-storms with an extremely strong, rotating updraft. They commonly form across the plains states each spring, when the jet stream appears overhead and draws warm moist air over from the Gulf of Mexico. Last week’s storms were unusually numerous and violent—although not unprecedented. According to Harold Brooks, a researcher based in Norman with the National Oceanic and Atmospheric Administration (NOAA), a storm that occurred in Oklahoma and neighbouring states in 1991 produced a similar number of equally vicious twisters. But because most of them formed in rural areas, there were only two injuries and no fatalities. By contrast, the latest tornadoes hit densely populated areas, including Oklahoma City and the nearby town of Moore. Each spring, weather forecasters watch out for the characteristic anticlockwise spiralling clouds of supercells in Tornado Alley on the US National Weather Service’s radar images. But only 20 per cent of super-cells go on to produce tornadoes. So last week, it was impossible to issue definite warnings of impending destruction until tornadoes were actually spotted on the ground. What the forecasters need is a way to predict which supercells will yield tornadoes and where and when the twisters will appear. Theorists believe that thermal gradients in and around the storms are important. The latent heat of condensation warms clouds from within, while the air directly beneath is cooled by evaporation of falling rain. The air outside the storm, however, is warmer near the ground and cooler higher up. This difference in temperature gradients may cause air to barrel over the ground like a steamroller. If this rolling air is then tilted onto its side and dragged upwards by the supercell’s powerful updraft, it could provide the extra spin needed to spawn a tornado. Another possibility is that the key factor is the presence of downdrafts that pin tornado funnels to the ground. But until researchers can see these processes in action, they can’t be sure which are the most important. “We’re not exactly clueless, but we need more clues,” says Bluestein, who has been chasing tornadoes across the plains for more than 20 years. Such clues are hard to find. In 1994 and 1995, more than 75 researchers led by Erik Rasmussen of NOAA’s National Severe Storms Laboratory in Norman and Jerry Straka of the University of Oklahoma took part in VORTEX, the verification of the origins of rotation in tornadoes experiment. Using aircraft, mobile radars and trucks packed with meteorological instruments, they studied storms in Texas, Oklahoma, Kansas and New Mexico. Although they are still sifting through the data, it’s already clear that at the resolution at which they studied the storms—taking measurements every 5 to 10 kilometres—there are no obvious differences between those that produced tornadoes and those that didn’t. So last year, Rasmussen, Straka and a handful of other VORTEX researchers set out to obtain higher-resolution data by getting as close as 2 kilometres to the tornadoes. They followed storm after storm, yet not a single one yielded a twister. “We got great data for supercells, but we badly need some tornado data to compare those with,” says Markowski. Now at last tornado researchers may have that information. When last week’s tornadoes struck, Joshua Wurman of the University of Oklahoma and his colleagues took two trucks fitted with Doppler radars and intercepted the twister that hit Oklahoma City, measuring the highest wind speed ever recorded—512 kilometres an hour. They also obtained nearly 30 minutes of continuous data on a tornado measuring more than 1.5 kilometres across, with peak wind speeds of some 320 kilometres an hour. Meanwhile, Bluestein and his colleagues were using a new higher-frequency radar system that can probe winds on an even finer scale—15 metres or less. Their truck was just a kilometre or so from the Oklahoma City tornado, and Bluestein believes they have obtained spectacular data on the region where the tornado funnel touched the ground. Straka’s team followed three tornadoes, measuring pressures, temperatures and moisture levels. When they are pieced together, these data should help to pin down the factors that make relatively benign supercells spawn meteorological phenomena that still inspire awe even among those who have witnessed their power on hundreds of occasions. “When you go out there and watch a tornado, it’s this tremendous release of energy that you have no control over,