Many tornadoes are wider and have stronger winds than suggested by surveys of the destruction they leave behind, scientists reported this week, revealing a potential need for updating tornado hazard maps and building codes.
Researchers chased more than 100 tornadoes across the Great Plains in radar trucks and found that a whopping one-fifth are capable of causing the most intense levels of damage. The team published the findings March 22 in Proceedings of the National Academy of Sciences.
"We're really providing, for the first time, quantification of the distribution of sizes and intensities of tornadoes from maps of their wind, not just from looking at damage," said Joshua Wurman, the president of the Center for Severe Weather Research and first author of the paper.
Traditionally, ratings of tornado intensity and size are based on the paths of battered buildings and trees they create. The fiercest wind speeds can be inferred from the amount of damage well-engineered structures have sustained.
"Most of the time, they're going through fields, they're going through [rural] areas where there are weaker structures," Wurman said. "If a tornado is going through an open field or over a lake, you can't measure its damage swath very well."
And spring is a particularly active time for tornadoes. During the week of March 14 alone, more than two dozen touched down across multiple states, including Alabama and North Carolina.
Wurman and his colleagues created wind-speed maps of 120 supercell tornadoes — the most common and dangerous variety — observed in Texas, Oklahoma, Kansas and Nebraska from 1995 to 2006. They determined the size of each tornado from the diameter of the windiest area. The researchers then compared their estimates to the officially reported intensity and sizes available for 82 of the tornadoes, 7% of which were recorded as reaching the two most violent ratings on the Enhanced Fujita Scale, corresponding to wind gusts of 166 to over 200 mph.
However, Wurman and his colleagues calculated that 21% of the 120 tornadoes they examined were capable of causing the damage associated with these ratings, based on wind speed measurements collected from altitudes of up to 500 meters above the trucks.
"There are some tornadoes [that] are weak and small but they are fairly rare," Wurman said. "On average they are stronger and wider than those official intensities indicate."
For a smaller portion of the tornadoes, the team was able to collect measurements within 60 meters of the trucks.
"The numbers jump to 25% if you look at only the cases where we looked really close to the ground," Wurman said. "That's fewer cases so the statistics are noisier; on the other hand, it's perhaps more representative of what's really happening at building height."
The new findings have implications for efforts to assess and minimize tornado-related risks, he says.
"If one only uses the official ratings to develop risk models or inform building codes or community standards for shelters, one could come to the conclusion that there's lower risk of these tornadoes," Wurman said. "As urban areas increase, as suburban areas increase and there is increased sprawl, there are broader areas of the country that are potentially subject to these tornadoes, which are in fact larger and stronger than the official statistics indicate."
He and his colleagues will continue to refine their estimates as they observe more tornadoes. They're also investigating whether, in contrast with other kinds of windstorms, the strongest winds in tornadoes may be very near to the ground, as well as what happens in the 100 meters or more beyond the ring where the strongest winds in powerful tornadoes are concentrated.
"It still could damage or destroy your house; it's not the strongest wind but it's still quite high-impact and potentially very damaging or deadly winds," Wurman said. "So we're looking to quantify that better, too, again to really refine what the risks are associated with tornadoes."
The study, "Supercell tornadoes are much stronger and wider than damage-based ratings indicate," published March 22 in Proceedings of the National Academy of Sciences, was authored by Joshua Wurman, Karen Kosiba, Trevor White and Paul Robinson, Center for Severe Weather Research and University of Illinois.