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

Date:
September 9, 2021
Source:
Stanford University
Summary:
The most devastating tornadoes are often preceded by a cloudy plume
of ice and water vapor billowing above a severe thunderstorm. New
research reveals the mechanism for these plumes could be tied to
'hydraulic jumps' -- a phenomenon Leonardo Da Vinci observed more
than 500 years ago.



FULL STORY ==========================================================================
When a cloudy plume of ice and water vapor billows up above the top of
a severe thunderstorm, there's a good chance a violent tornado, high
winds or hailstones bigger than golf balls will soon pelt the Earth below.


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A new Stanford University-led study, published Sept. 10 in Science,
reveals the physical mechanism for these plumes, which form above most
of the world's most damaging tornadoes.

Previous research has shown they're easy to spot in satellite imagery,
often 30 minutes or more before severe weather reaches the ground. "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," said atmospheric scientist Morgan O'Neill, lead author
of the new study.

The research comes just over a week after supercell thunderstorms and
tornadoes spun up among the remnants of Hurricane Ida as they barreled
into the U.S.

Northeast, compounding devastation wrought across the region by
record-breaking rainfall and flash floods.

Understanding how and why plumes take shape above powerful thunderstorms
could help forecasters recognize similar impending dangers and 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. In many parts of the world, Doppler radar coverage is nonexistent.

"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," said O'Neill, who is an assistant professor of Earth system science at Stanford's School of Earth, Energy & Environmental Sciences (Stanford Earth).



========================================================================== Supercell storms and exploding turbulence 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, with 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. 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.

The rising air of the overshooting top soon speeds back toward the
troposphere, like a ball that accelerates downward after cresting
aloft. At the same time, air is flowing over the dome in the stratosphere
and then racing down the sheltered side.



========================================================================== Using computer simulations of idealized supercell thunderstorms, O'Neill
and colleagues discovered that this excites a downslope windstorm at
the tropopause, where wind speeds exceed 240 miles per hour. "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," O'Neill said. "These
speeds at the storm top have never been observed or hypothesized before." Hydraulic jump Scientists have long recognized that overshooting storm
tops of moist air rising into the upper atmosphere can act like solid
obstacles that block or redirect airflow. And it's been proposed that
waves of moist air flowing over these tops can break and loft water
into the stratosphere. But no research to date has explained how all
the pieces fit together.

The new modeling 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 rushing winds tumble over mountains
and generate turbulence on the downslope side, or when water speeding
smoothly down a dam's spillway abruptly bursts into froth upon joining slower-moving water below.

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

The simulations suggest the onset of the jump coincides with a
surprisingly rapid injection of water vapor into the stratosphere,
upwards of 7000 kilograms per second. That's two to four times higher
than previous estimates. Once it reaches the overworld, water may stay
there for days or weeks, potentially influencing the amount and quality of sunlight that reaches Earth via destruction of ozone in the stratosphere
and warming the planet's surface. "In our simulations that exhibit
plumes, water reaches deep into the stratosphere, where it possibly could
have more of a long-term climate impact," said co- author Leigh Orf,
an atmospheric scientist at the University of Wisconsin- Madison.

According to O'Neill, high-altitude NASA research aircraft have only
recently gained the ability to observe the three-dimensional winds at
the tops of thunderstorms, and have not yet observed AACP production
at close range. "We have the technology now to go verify our modeling
results to see if they're realistic," O'Neill said. "That's really a
sweet spot in science." This research was supported by the National
Science Foundation and the NASA Precipitation Measurement Mission and
Ground Validation program.

========================================================================== Story Source: Materials provided by Stanford_University. Original written
by Josie Garthwaite. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Morgan E O'Neill, Leigh Orf, Gerald M. Heymsfield, Kelton Halbert.

Hydraulic jump dynamics above supercell thunderstorms. Science,
2021; 373 (6560): 1248 DOI: 10.1126/science.abh3857 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210909141231.htm

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