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New research may upend what we know about how tornadoes form新的研究可能会颠覆我们所知道的龙卷风是如何形成的
As the climate changes, twister behavior on the ground is changing too随着气候的变化，地面上的龙卷风行为也在发生变化
BY CAROLYN GRAMLING 2:06PM, DECEMBER 14, 2018
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BIRTH OF A TWISTER Radar data combined with visual observations of this tornado that struck El Reno, Okla., on May 31, 2013, are offering new clues into how such twisters are born.雷达数据与这次袭击俄克拉荷马州里诺的龙卷风的目视观测相结合。2013年5月31日，美国马萨诸塞州的一项研究为此类龙卷风的形成提供了新的线索
NICK NOLTE/WIKIMEDIA COMMONS (CC BY -SA 3.0)
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WASHINGTON — Tornadoes may form from the ground up, rather than the top down. 龙卷风可能从地面向上形成，而不是自上而下形成
That could sound counterintuitive. Many people may picture a funnel cloud emerging from the bottom of a dark mass of thunderstorms and then extending to the ground, atmospheric scientist Jana Houser said December 13 in a news conference at the American Geophysical Union meeting.大气科学家Jana Houser说，许多人可能会想象一个漏斗云从一团黑色雷暴的底部升起，然后延伸到地面
Scientists have long debated where the wind rotations that lead to twisters in these thunderstorms begin.科学家们长期以来一直在争论在这些雷暴中导致龙卷风的风的旋转从哪里开始。 Now Houser, of Ohio University in Athens, and her colleagues have new data that upend this “top-down” idea of tornadogenesis.现在，雅典俄亥俄大学的豪瑟和她的同事们有了新的数据，推翻了“自上而下”的龙卷风形成的观点。
The supercell thunderstorms that can spawn tornadoes form where a powerful updraft of warm, moist air — such as air moving northward from the Gulf of Mexico — gets trapped beneath a layer of colder, drier air — such as air moving southward from Canada.超级单体雷暴可以在温暖潮湿的空气(如从墨西哥湾向北移动的空气)的强大上升气流被一层更冷、更干燥的空气(如从加拿大向南移动的空气)所困的地方形成龙卷风。 That’s an inherently unstable condition, but the other necessary ingredient for tornadoes to form is wind shear: fast-moving winds that move the air masses, causing them to rotate horizontally. 这是一种固有的不稳定状态，但龙卷风形成的另一个必要因素是风切变:快速移动的风移动气团，导致它们水平旋转。Air then rising through the supercell can tip the rotation from horizontal to vertical — creating conditions ripe for a tornado.
But the very moment of twister birth remains largely elusive. Tornadoes can form within just 30 to 90 seconds, so research on how they start is often limited by not quite getting to the right place at the right time to watch a twister actually being born — and her team was no exception, Houser said.龙卷风可以在30到90秒内形成，所以对龙卷风如何形成的研究往往受到限制，因为人们不能在正确的时间到达正确的地点，观看龙卷风真正形成的过程——她的团队也不例外，豪瑟说。
But using a rapid-scanning Doppler radar mounted at the back of a truck, the team managed to capture the full evolution of four tornadoes, including two powerful twisters that struck near El Reno, Okla., one on May 24, 2011, and another on May 31, 2013.但是，通过安装在一辆卡车后面的快速扫描多普勒雷达，研究小组成功地捕捉到了四场龙卷风的全部演变过程，其中包括袭击俄克拉荷马州埃尔里诺(El Reno)附近的两场威力强大的龙卷风。分别是2011年5月24日和2013年5月31日。
The radar, which collects data every 16 seconds, showed that rotating winds for the 2011 storm appeared to start at multiple levels in the atmosphere at once, rather than starting high up and moving downward.每16秒收集一次数据的雷达显示，2011年风暴的旋转风似乎同时从大气中的多个层面开始，而不是从高空开始向下移动。 For the 2013 storm, tornado-strength rotation actually began at the lowest measured elevation, about 20 meters above the ground. By 60 seconds later, the rotation was at a height of about 3.5 kilometers.2013年的风暴，龙卷风强度的旋转实际上开始于最低的测量高度，大约离地面20米。60秒后，旋转高度约为3.5公里。 Crowdsourced still and video images of the El Reno storm also revealed a funnel near the ground — even before the radar was able to capture it.众包的El Reno风暴的静止图像和视频图像也显示了靠近地面的漏斗——甚至在雷达能够捕捉到它之前。
Houser team’s discovery that the 2013 storm had a funnel long before the radar saw rotation is “a pretty compelling finding,” says Leigh Orf, an atmospheric scientist at the University of Wisconsin–Madison. But, he says, that twister was such a particularly “unusual, rare, weird, bizarre storm” that he would hesitate to use it as an indicator of typical tornado behavior.但是，他说，那个龙卷风是如此特别的“不寻常、罕见、怪异、奇异的风暴”，以至于他会犹豫是否要把它作为典型龙卷风行为的指标
“In my scientific view, there are likely different modes of tornadogenesis,” Orf says. He notes that his own simulations of the 2011 storm match Houser’s finding that that particular tornado seemed to develop rapidly spinning winds at multiple altitudes at once.他指出，他自己对2011年风暴的模拟与豪瑟的发现相符，豪瑟发现，那场龙卷风似乎同时在多个高度迅速形成旋转的风。
LENGTHY TRAIL On April 27, 2011, a “long-track” tornado cut a destructive swath through Alabama, striking Tuscaloosa and Birmingham before lifting off the ground.2011年4月27日，一股“长道”龙卷风横扫阿拉巴马州，在脱离地面之前袭击了塔斯卡卢萨和伯明翰 New research suggests that, on average, tornado paths have become longer by about 130 kilometers since 1994.
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Forecasters typically scan supercell thunderstorms, looking for telltale signs that a tornado might be imminent.气象预报员通常扫描超级单体雷暴，寻找龙卷风即将来临的迹象。 Because the deadly funnel clouds form so quickly, understanding how tornadoes form is important — and increasingly so in the United States where tornadoes appear to be becoming more powerful.由于致命的漏斗云形成的速度如此之快，了解龙卷风是如何形成的非常重要——在美国，了解龙卷风似乎变得越来越强大，这一点也越来越重要。
These twisters are also staying on the ground, on average, for more time than they did just a few decades ago. That gives tornadoes more time to do damage, James Elsner, an atmospheric scientist at Florida State University in Tallahassee, reported in a separate study at the meeting.
In that study, also published November 10 at EarthArXiv.org, Elsner calculated that the overall power in gigawatts released by tornadoes in the United States has increased, at a rate of about 5.5 percent per year.埃尔斯纳估计，美国龙卷风释放出的千兆瓦电力总量已经增加，每年增长5.5%。 Using a statistical model for tornado power, based on reports of nearly 28,000 tornadoes from 1994 to 2016, he found that although the overall number of tornadoes hasn’t increased, the twisters are forming in bigger bunches. In these clusters, there is a higher proportion of tornadoes rated at the highest intensities, such as EF4 or EF5 tornadoes.在这些风暴云团中，具有最高强度龙卷风的比例较高，如EF4或EF5龙卷风。
That shift, Elsner says, is linked to rising ocean temperatures in the Gulf of Mexico and the western Caribbean Sea. With more heat and moisture available in the atmosphere due to global warming, tornadoes, perhaps not surprisingly, can pack a more powerful punch, he says.他说，由于全球变暖，大气中有了更多的热量和水分，龙卷风可能会带来更大的威力，这并不奇怪。
Citations
J. Houser et al. Rapid-scan mobile radar observations of tornadogenesis. American Geophysical Union fall meeting, Washington, D.C., December 14, 2018.
J. Elsner. Increasingly powerful tornadoes. American Geophysical Union fall meeting, Washington, D.C., December 12, 2018.
