Scientists solve mystery of icy plumes that may foretell deadly supercell storms

3D rendering of the simulation experiment producing the AACP on the sheltered side, or lee, of the overshooting summit. Image credit: Leigh Off and David Semeraro. Credit: Leigh Off and David Semeraro
A cloudy plume of water vapor and ice rising above the top of severe thunderstorms indicates that a strong tornado, high winds, or hailstones larger than golf balls could soon hit the Earth below.

A Stanford University study has revealed the physical mechanism of these plumes that form over most of the most destructive tornadoes in the world. It was published in Science on Sept. 10.

They are easy to spot on satellite imagery and can be seen for up to 30 minutes before severe weather hits the ground. Previous research has confirmed this. "The question is: Why is this plume associated to the worst conditions and how did it get there? Morgan O'Neill (atmospheric scientist), is the lead author of the study.

This research is just over a week following supercell thunderstorms, tornadoes, and remnants from Hurricane Ida that barrelled into the Northeast United States. They added to the devastation caused by flash floods and record-breaking rainfall.

Forecasters could learn more about the formation of plumes above strong thunderstorms to help them spot impending dangers before they happen. They can also use Doppler radar systems to issue warnings that are more accurate than Doppler radar systems. These systems can be affected by hail and wind, and have blind spots even when it is good. Doppler radar coverage in many areas of the globe is limited.

"If there is going to be a horrible hurricane, we can view it from space. Because tornadoes are hidden beneath thunderstorm tops, we can't see them. We need to better understand the tops," O'Neill, an assistant professor of Earth system sciences at Stanford's School of Earth, Energy & Environmental Sciences, (Stanford Earth), said.

Supercell storms, exploding Turbulence

Supercells are the rarest kind of thunderstorm that produce tornadoes. They have a rotating updraft and can travel at speeds of more than 150 miles per hour. With enough power to smash through the troposphere (the lowest layer of the atmosphere), the lid on Earth's atmosphere, it is possible to create supercells.

Rising currents of moist atmosphere tend to become anvil-shaped clouds in weaker thunderstorms. The tropopause is pushed upwards by a supercell thunderstorm's strong updraft, creating an "overshooting top" according to scientists. O'Neill described it as "a fountain pushing against the next layer in our atmosphere."

The winds from the upper atmosphere racing over and around the storm top can sometimes produce streams of water vapor or ice that shoot into the stratosphere, forming the distinctive plume. This is technically known as an Above-Anvil Cirrus Plume (or AACP).

Soon, the rising air from the top will speed back towards the troposphere like a ball which accelerates down after it has crested aloft. The air is also flowing in the stratosphere, passing over the dome and racing down the sheltered side.

O'Neill and his colleagues created computer simulations of supercell thunderstorms. They discovered that this triggers a downslope storm at the tropopause. Wind speeds exceeding 240 miles an hour. This jet is made up of moist and dry air from the troposphere, which descends from the stratosphere. O'Neill stated that the jet becomes unstable, and the entire thing explodes in turbulence. These speeds at the storm top are not known or hypothesized.

Hydraulic jump

Scientists have known for a long time that moist air from the storm tops can act as solid barriers that restrict or redirect airflow. It has been suggested that the tops of moist air can cause waves to break apart and lift water into the stratosphere. However, no research has been done to explain how the pieces fit together.

New modeling indicates that the hydraulic jump is a phenomenon that causes turbulence to explode in the atmosphere when plumed storms occur. This same mechanism is also at work when rushing winds blow over mountains and create turbulence on their downslope side.

Leonardo DaVinci saw the phenomenon in flowing waters as early as 1500s. Ancient Romans may have tried to limit hydraulic jumps by designing aqueducts. Atmospheric scientists have seen only the dynamic caused by solid topography. New modeling shows that hydraulic jumps can also be caused by fluid obstacles in the atmosphere, which is almost entirely made of air. These fluid obstacles are constantly changing shape miles above the Earth's surface.

Simulations show that the jump occurs with an amazingly rapid injection of water into the stratosphere at speeds of up to 7000 kilograms per second. This is two to four times more than the previous estimates. Water can stay in the overworld for days or even weeks after it reaches Earth's surface. This could have an impact on the amount and quality sunlight reaching Earth through the destruction of the stratosphere and warming its surface. "In our simulations that show plumes, water reaches deeper into the stratosphere where it could possibly have a greater long-term climate effect," stated Leigh Orf (an atmospheric scientist at University of Wisconsin-Madison).

O'Neill claims that NASA's high-altitude research aircraft has only recently been able to see the three-dimensional winds at thunderstorm tops. They have also not yet seen AACP production close up. O'Neill stated that "we have the technology now for going to verify our modeling results and see if these are realistic." "That's a really sweet spot in science."

Learn more Size of thunderstorm cloud domes may help predict tornado intensity

Further information: Hydraulic leap dynamics above supercell storms, Science (2021). Journal information: Science Hydraulic jump dynamics above supercell thunderstorms,(2021). DOI: 10.1126/science.abh3857


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