Supercells are long-lasting, fierce thunderstorms that spin out the most destructive tornadoes, those with winds faster than 111 mph, which cause most tornado deaths. But scientists would like to know why only a few supercell thunderstorms produce tornadoes. A better understanding could help forecasters issue tornado warnings farther in advance with fewer false alarms.
Analysis of data collected from a tornado that hit Goshen County, Wyo., on June 5, 2009 is bringing scientists closer to the answer. The researchers have linked an on-and-off downdraft from high in the supercell with the tornado’s formation, weakening and reformation.
For years, scientists have been focusing on downdraft winds from high in supercells that hit the ground to send winds out from storms, called gust fronts, as triggers for tornado development. In 1991 scientists hypothesized that air coming as rear-flank gust fronts help create tornadoes, says Karen Kosiba of the Center for Severe Weather Research in Boulder, Colo.
The Goshen data confirms that a second rear-flank downdraft created a secondary rear-flank gust front that strengthened, weakened and strengthened again as the tornado formed, died out and then reformed, she says.
Supercells also produce front-flank downdrafts that spread out as front-flank gust fronts racing out ahead of the storm as it moves across the countryside. Many supercells, especially tornadic ones, have rear-flank gust fronts, but it is not known how many have the recently discovered secondary rear-flank gust fronts, says Josh Wurman, founder of the Center for Severe Weather Research.
He says the Goshen tornado data increases researchers’ confidence that secondary rear-flank gust fronts are important to tornado formation.
Nevertheless, he adds: “I do not expect there will be an eureka, where all of a sudden there’s a moment when tornado forecasts go from 13 minute warning to 40 minutes warning (ahead of time) in one year.”
Kosiba, Wurman and Paul Robinson of the Center for Severe Weather Research, with Yvette Richardson, Paul Markowski and James Marquis of Penn State University, describe and discuss the Goshen tornado findings in the April issue of the “Monthly Weather Review.”
They used data from Doppler weather radars on trucks, weather instruments mounted on car roofs called “mobile mesonets,” a weather balloon and photos. They were part of the VORTEX2 tornado research project. During the project’s field phases in May and June 2009 and 2010, researchers collected data on more than 30 supercell thunderstorms that produced 20 weak or short-lived tornadoes.
Wurman says roughly $10 million of the $13 million VORTEX 2 funding from the National Oceanic and Atmospheric Administration is for analysis, which could last 10 years. The Goshen tornado, which was recorded from before it formed until it died, supplied them with more data than any other tornado ever has.
Tornadoes form in the warm, humid air that’s flowing up into a thunderstorm, usually near the boundary between the rising air and cold air that falling rain or hail is dragging down to the ground. A big question is what supplies the twisting motion that turns air rising straight into a whirling tornado.
A general, simplified picture, including data from the Goshen tornado, is:
- The forward flank downdraft creates a spinning motion that’s parallel to the ground. This can be seen sometimes in spinning, horizontal roll clouds that detach from a thunderstorm and move away from it;
- Air rising into the thunderstorm tilts the spinning air that stretches upward as a vertical column of spinning air;
- A secondary gust front wraps around the developing tornado and strengthens it.
“With the Goshen tornado we had such a good look at all of the tornado from birth until death we were able to look at the evolution of the secondary surge in more detail,” Wurman says. “Seeing this process where the rear-flank gust front and the tornado strengthened together gives us confidence that the gust front is involved in strengthening.”
He says researchers don’t know why the rear-flank downdraft and its gust front strengthens and weakens. “In science you often solve one problem and it raises another problem.”
Wurman says if further observations continue to find similar links between secondary rear-flank gust fronts and the birth, growth and weakening of tornadoes, “it means we may be on our way to answering: ‘When will a supercell make a tornado?’ and ‘Why did that supercell make a tornado and not that other one?’ The answers to these questions could lead to more precise forecasts with fewer false alarms.”
One stumbling block is that for today’s weather radars to detect rear-flank and secondary rear-flank gust fronts a supercell has to be relatively near the radar. The answer could lie in computer models that create more realistic supercells using available data that would, in effect, supply storm data. Wurman thinks this could happen in the next 10 years.
Still, he says: “It’s going to be difficult years of slogging through the trenches; it’s a very difficult problem.”