A supercell thunderstorm, which can generate violent tornadoes, rumbles over Texas.Credit: Getty

Atmospheric scientists will soon get an unprecedented view of the conditions that trigger some of the United States’ most devastating tornadoes.

Starting on 15 May, a fleet of up to four drones will help researchers to monitor supercell thunderstorms. These rotating storms pummel the central United States with lightning, tennis ball-sized hail and damaging winds every spring and summer, and some, but not all, develop tornadoes.

Previous projects have used single drones flying at an altitude of around 300 metres to take measurements from supercells. But the US$2.4-million project, called Targeted Observation by Radars and Unmanned aircraft systems of Supercells (TORUS), will send as many as four drones into these storms, flying up to nearly 800 metres in the air, to gather data simultaneously on the atmosphere.

This means researchers will be able to see multiple parts of the storm at the same time, says TORUS lead investigator Adam Houston, an atmospheric scientist at the University of Nebraska–Lincoln.

“We are attacking the storm from every angle to see what is going on,” adds Brian Argrow, an aerospace engineer at the University of Colorado Boulder, who leads TORUS’ drone team.

The machines will fly into regions of the storms that are too dangerous to send people into, due to strong winds. Scientists hope that the resulting information will help them to more accurately forecast which supercells will generate tornadoes ― information that could save lives. Current tornado warning systems have a high false-positive rate, says Christopher Weiss, an atmospheric scientist at Texas Tech University in Lubbock, who leads the TORUS mobile-radar team. “If you get over-warned, it’s a natural human reaction to heed those warnings less in the future.”

Stalking supercells

TORUS will deploy fifty researchers and students split into about a dozen teams, who will chase after supercell thunderstorms that form in a 950,000-square-kilometre region stretching from North Dakota to Texas, and from Iowa to Wyoming and Colorado. This year's field season will end on 16 June and project scientists will collect data during a second season in May and June 2020. The National Science Foundation provided the bulk of the funding for the project.

In addition to the drones, researchers will use mobile radar systems, weather balloons and an aeroplane outfitted with meteorological sensors to monitor supercells (see ‘Instrument arsenal’).

Atmospheric scientists have a handle on the large-scale conditions that whip up a tornado: they include temperature differences between layers of the atmosphere and big changes in wind speed with increasing altitude that generate a rotating column of air. But researchers know that this isn’t the whole picture.

Houston and his colleagues will use their array of instruments to gather information on supercell conditions, including humidity, air pressure, temperature, and wind speed and direction. These measurements will help them to pinpoint smaller-scale structures in these thunderstorms that they think trigger tornadoes.﻿

“There are structures within the storm that we either know exist — and we just don’t have enough information about them — or we think exist based on preliminary evidence,” says Houston.

One hypothetical structure that Houston hopes to spot is a stream of cool air located near the ground that computer simulations indicate helps to drive tornado formation. This cool current speeds up the rate at which warm air is sucked into a supercell, which increases the chances that a tornado will form. The cool stream of air ends up flowing around the outside of a tornado.

Completing the picture

“I’m really psyched about this,” says Leigh Orf, an atmospheric scientist at the University of Wisconsin–Madison who developed the simulation that predicts this cool air current. TORUS is a “big first step toward looking the right way in the right part of the storm”, for these smaller structures, he says.

The project won’t revolutionize what atmospheric scientists know in a single year, says Joshua Wurman, a meteorologist at the Center for Severe Weather Research in Boulder, Colorado. But with drones, TORUS has the potential to obtain better information on supercell thunderstorms than previous studies, and change what researchers know about these storms, he says.

If these drones succeed in helping researchers to better monitor supercells, the US National Weather Service might one day integrate the aircraft into their data-gathering and tornado-forecasting process, Houston says. “The technology that we advance as a consequence of this project could be used in that new surveillance network,” he says.