Why icy plumes often appear before deadly ‘supercell’ storms

·3 min read
Extreme weather on the high plains of Nebraska with this stunning LP supercell mesocyclone with lightning, taken near Broken Bow, USA.
Icy plumes that form above supercell storms could help to explain their destructive power. (Getty)

Deadly tornadoes and hailstorms bigger than golf balls are often heralded by a cloudy plume of ice and water above a thunderstorm, researchers say. 

The icy plumes that form above the world’s most devastating "supercell" tornadoes could be linked to "hydraulic jumps" – first observed by Leonardo da Vinci 500 years ago. 

The plumes are often easy to spot in satellite imagery 30 minutes or more before severe weather reaches the ground.

Morgan O'Neill, lead author of the Stanford University study, said: "The question is, why is this plume associated with the worst conditions, and how does it exist in the first place? That's the gap that we are starting to fill.”

A week ago, supercell thunderstorms and tornadoes spun up among the remnants of Hurricane Ida as they barrelled into the US north-east.

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Understanding how and why plumes take shape above powerful thunderstorms could help forecasters issue more accurate warnings without relying on Doppler radar systems, which can be knocked out by wind and hail – and have blind spots even on good days.

O’Neill says: "If there's going to be a terrible hurricane, we can see it from space. We can't see tornadoes because they're hidden below thunderstorm tops. We need to understand the tops better.”

The thunderstorms that spawn most tornadoes are known as supercells, a rare breed of storm with a rotating updraft that can hurtle skyward at speeds faster than 150 miles an hour.

The storms have enough power to punch through the usual lid on Earth's troposphere, the lowest layer of our atmosphere.

In weaker thunderstorms, rising currents of moist air tend to flatten and spread out upon reaching this lid, called the tropopause, forming an anvil-shaped cloud. 

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A supercell thunderstorm's intense updraft presses the tropopause upward into the next layer of the atmosphere, creating what scientists call an overshooting top. 

"It's like a fountain pushing up against the next layer of our atmosphere," O'Neill said.

As winds in the upper atmosphere race over and around the protruding storm top, they sometimes kick up streams of water vapor and ice, which shoot into the stratosphere to form the tell-tale plume, technically called an Above-Anvil Cirrus Plume, or AACP.

Using computer simulations of idealised supercell thunderstorms, O'Neill and colleagues discovered that this excites a downslope windstorm at the tropopause, where wind speeds exceed 240 mph. 

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O’Neill said: "Dry air descending from the stratosphere and moist air rising from the troposphere join in this very narrow, crazy-fast jet. The jet becomes unstable and the whole thing mixes and explodes in turbulence.

"These speeds at the storm top have never been observed or hypothesised before."

The new modelling suggests the explosion of turbulence in the atmosphere that accompanies plumed storms unfolds through a phenomenon called a hydraulic jump. 

The same mechanism is at play when water speeding smoothly down a dam's spillway abruptly bursts into froth upon joining slower-moving water below.

Leonardo da Vinci observed the phenomenon in flowing water as early as the 1500s, and ancient Romans may have sought to limit hydraulic jumps in aqueduct designs. 

Watch: Monster supercell forms in Ontario

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