When a strong tornado roars through a city, it often leaves behind demolished buildings, broken tree limbs and trails of debris. But a similarly powerful storm touching down over barren, unvegetated land is much harder to spot in the rearview mirror.
Now, satellite imagery has revealed a 60-kilometer-long track of moist earth in Arkansas that was invisible to human eyes. The feature was presumably excavated by a tornado when it stripped away the uppermost layer of the soil, researchers report in the March 28 Geophysical Research Letters. This method of looking for “hidden” tornado tracks is particularly valuable for better understanding storms that strike in the winter, when there’s less vegetation, the researchers suggest. And recent research has shown that wintertime storms are likely to increase in intensity as the climate warms (SN: 12/16/21).
Over 1,000 tornadoes strike the United States each year, according to the National Weather Service. But not all are equally likely to be studied, says Darrel Kingfield, a meteorologist at the National Oceanic and Atmospheric Administration in Boulder, Colo., who was not involved in the research.
For starters, storms that pass over populated areas are more apt to be analyzed. “There’s historically been a pretty big population bias,” Kingfield says. Storms that occur over vegetated regions also tend to be well studied, simply because they leave obvious scars on the landscape. Ripped-up grasses or downed trees function like beacons to indicate the path of a storm, says Kingfield, who has studied forests damaged by tornadoes.
Spring and summer are peak storm seasons in the United States — more than 70 percent of tornadoes strike from March through September, according to NOAA. But on December 10, 2021, a cluster of storms started racing across the central and southern United States. Those tornadoes, which claimed more than 80 lives, swept across cities and also farmland, much of which had already been harvested for the season.
Jingyu Wang, a physical geographer at Nanyang Technological University in Singapore, and his colleagues set out to detect the signatures of those deadly storms in unpopulated, barren landscapes.
Swirling winds, even relatively weak ones, can suction up several centimeters of soil. And since deeper layers of the ground tend to be wetter, a tornado ought to leave behind a telltale signature: a long swath of moister-than-usual soil. Two properties linked with soil moisture level — its texture and temperature — in turn impact how much near-infrared light the soil reflects.
Wang and his collaborators analyzed near-infrared data collected by NASA’s Terra and Aqua satellites and looked for changes in soil moisture consistent with a passing tornado.
When the team looked at data obtained shortly after the 2021 storm outbreak, they noticed a signal in northeastern Arkansas. The feature was consistent with a roughly 60-kilometer-long track of wet soil. Tornadoes had been previously reported in that area — outside the city of Osceola — so it’s likely that this feature was created by a powerful storm, the team concluded.
That makes sense, Kingfield says, and observations like these can reveal tornado signatures that might otherwise be missed. However, it’s important to acknowledge that this new technique works best in places where soils are capable of retaining water, he says. “You need to have clay-rich soils.”
Even so, these results hold promise for analyzing other tornadoes, Kingfield says. It’s always useful to have a new tool for estimating the strength, path and structure of a storm, but many storms go relatively unexamined simply because of where and when they occur, he says. “Now we have this new ground truth.”