Scientists solve mystery of icy plumes that could predict deadly supercell storms

When a cloudy plume of ice and water vapor rises above the top of a severe thunderstorm, there is a good chance that a severe tornado, strong winds, or hailstones larger than bullets. golf courses will soon hit the Earth below.

A new study led by Stanford University, published on September 10 in Science, reveals the physical mechanism of these plumes, which form above many of the world’s most damaging tornadoes.

Previous research has shown that they are easy to spot on satellite images, often 30 minutes or more before bad weather hits the ground. “The question is, why is this plume associated with the worst conditions, and how does it exist in the first place? This is the void that we are starting to fill,” said atmospheric scientist Morgan O’Neill, author principal of the new study. .

The research comes just over a week after thunderstorms and supercell tornadoes struck among the remnants of Hurricane Ida as they made their way to the northeastern United States, compounding the devastation wrought in the region with record rainfall and flash floods.

Understanding how and why plumes form over powerful thunderstorms could help forecasters recognize similar looming dangers and issue more accurate warnings without relying on Doppler radar systems, which can be destroyed by wind and hail. , and have blind spots even on good days. In many parts of the world, Doppler radar coverage is non-existent.

“If there is to be a terrible hurricane, we can see it from space. We cannot see the tornadoes because they are hidden under the tops of thunderstorms. We need to understand the peaks better,” O’Neill said. , who is an Assistant Professor of Earth System Science at the Stanford School of Earth, Energy and Environmental Sciences (Stanford Earth).

Supercell storms and explosive turbulence

The thunderstorms that spawn most tornadoes are known as supercells, a rare breed of storm with a rotating updraft that can rush skyward at speeds over 150 miles per hour, with enough power to pierce the usual cover of the Earth’s troposphere, the lowest layer of our atmosphere.

In weaker thunderstorms, updrafts of moist air tend to flatten and expand upon reaching this cover, called the tropopause, forming an anvil-shaped cloud. The intense updraft from a supercell storm pushes the tropopause upward into the next layer of the atmosphere, creating what scientists call a protruding peak. “It’s like a fountain pushing against the next layer of our atmosphere,” O’Neill said.

As the winds in the upper atmosphere rush over and around the protruding top of the storm, they sometimes initiate streams of water vapor and ice, which flow into the stratosphere to form the telltale plume, technically referred to as Cirrus feather above the anvil, or AACP.

The rising air from the overflowing spinning top returns quickly to the troposphere, like a ball accelerating downward after reaching the ridge. At the same time, air flows above the dome into the stratosphere, then descends along the sheltered side.

Using computer simulations of idealized supercell storms, O’Neill and his colleagues found that it excites a descending windstorm at the tropopause, where the wind speed exceeds 240 miles per hour. “The dry air coming down from the stratosphere and the humid air coming up from the troposphere come together in this very narrow and ultra-fast jet. The jet becomes unstable and everything mixes and explodes in the turbulence,” O ‘said. Neill. “These storm-top speeds have never been observed or hypothesized before.”

Hydraulic jump

Scientists have long recognized that passing peaks of humid air storms rising into the upper atmosphere can act as solid obstacles that block or redirect airflow. And it has been suggested that waves of moist air flowing over these peaks can break apart and throw water into the stratosphere. But no research to date has explained how all the pieces fit together.

The new modeling suggests that the explosion of turbulence in the atmosphere that accompanies plume storms takes place through a phenomenon called hydraulic jump. The same mechanism is in play when strong winds fall on the mountains and generate turbulence on the downslope side, or when the water gently rushing down the spillway of a dam suddenly bursts into foam, joining the slower water in below.

Leonardo DaVinci observed the phenomenon in flowing water as early as the 1500s, and the ancient Romans may have sought to limit hydraulic jumps in aqueduct designs. But so far atmospheric scientists have only seen the dynamics induced by solid topography. The new modeling suggests that a hydraulic jump can also be triggered by fluid obstacles in the atmosphere made almost entirely of air and changing shape every second, miles above the Earth’s surface.

Simulations suggest that the onset of the jump coincides with a surprisingly rapid injection of water vapor into the stratosphere, at over 7,000 kilograms per second. This is two to four times higher than previous estimates. Once it reaches the overworld, water can stay there for days or weeks, potentially influencing the amount and quality of sunlight that reaches Earth via the destruction of ozone in the stratosphere and warming. of the surface of the planet. “In our simulations that feature plumes, the water reaches deep into the stratosphere, where it could have a greater long-term climate impact,” said co-author Leigh Orf, an atmospheric scientist at the University of the Wisconsin-Madison.

According to O’Neill, NASA’s high-altitude research planes have only recently acquired the ability to observe three-dimensional winds atop thunderstorms and have yet to observe AACP production at close range. “We now have the technology to check our modeling results to see if they are realistic,” O’Neill said. “It really is a sweet spot in science.”