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.
From:
Stanford University
September 9, 2021 -- 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.
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."
https://www.sciencedaily.com/releases/2021/09/210909141231.htm
No comments:
Post a Comment