Fortunately these events are rare, although we often forget that the
potential violence of a thunderstorm is huge however localized as
anyone who has watched a field of grain reduced to a mat by a few
inches of hailstones. That event rarely hits more than a square mile
but is no fun.
The Derocho is something special and is simply unpredictable except
to recognize pending conditions.
The result is a moving thunderstorm front that simply refuses to fade
out.
This recent event was huge and will act as a standard to measure
other such events against. These conditions have prevailed before
but simply not to this extent. Thus we have a new name that is also
a warning to batten down.
"Super
Derecho" Ambushed Mid-Atlantic before We Knew What One Was
These
rare storms feature bands of turbulence that feed on warm, moist air
to create a cycle of warming and cooling that can drive itself for
hundreds of kilometers
By Nathanael Massey
and ClimateWire July 9, 2012
Few denizens of the
East Coast were familiar with the term "super derecho"
before its hurricane-like winds carved a devastating path through the
mid-Atlantic on June 29. It snapped off trees and triggered sprawling
power outages as it went.
Since then, the term
has become ubiquitous, tossed around easily as news anchors rattle
off the storm's mounting toll: 13 dead, more than 2 million without
power at the storm's peak and yet-untold billions of dollars in
damages.
By now, we know what a
super derecho does. But few know what a rare and complex breed of
storm it is.
A time-lapse video
made from radar images that tracked the destructive path of the June
29 super derecho storm system as it developed over Iowa and gathered
strength moving across the Midwest and onto the East Coast. Colors
indicate wind speed, with red marking the highest intensity. Video
courtesy of Daryl Herzmann and the Iowa Environmental Mesonet. Click
to watch the video.
The term was coined in
1888 by a meteorologist named Gustavus Hinrichs. Derived from a
Spanish word meaning "direct" or straight ahead," it
describes a forward-moving band of turbulent weather that
feeds on the warm, moist air in front of it, creating a cycle of
warming and cooling that can whip up violent winds and drive itself
for hundreds of miles in a single direction.
Only rarely do such
phenomena occur, said Daryl Herzmann, an assistant scientist at the
Iowa Environmental Mesonet.
"It's a complex
interaction of forces, and the environment has to be conducive,"
he said. "You need a very special setup to get a derecho --
obviously, that was the case [on June 29]."
Recipe for an ugly
surprise
First of all, Herzmann said, you need a large reservoir of energy. For weather systems, this ordinarily takes the form of heat energy trapped in water molecules in the atmosphere.
Then, you need wind.
But not the kind of spastic east, west and south winds that
ordinarily crisscross the atmosphere. Derecho wind is unidirectional
and has to be strongly sustained, at least at first.
Like many extreme
weather events, a derecho starts with a storm, Herzmann said. That
strong, unidirectional wind pushes the storm along, its precipitation
and evaporation cooling the air below and around it. As the storm
moves, a "blob" of cold air builds, particularly in the
upper reaches of the towering cumulonimbus clouds that march along at
the storm's front.
In time, the blob of
cold air becomes denser then the air around it and pours down out of
the sky like water from a bucket. Already moving in the direction of
the winds, the fall gives the cold air significant additional speed,
and it flows forward as wind, the new vanguard of the storm.
Meteorologists call this wave of cold air a "downburst."
At this point, most
storms would dissipate, their accumulated energy spent. But when the
system is moving into a region of high heat and, more importantly,
high humidity, the storm can actually feed off that heat energy,
rebuilding and repeating the cycle. And as anyone east of the Rocky
Mountains can tell you, there's been plenty of excess heat energy in
the country for the past several weeks.
Easy to identify,
difficult to predict
"All that humid sticky air that's been stuck with us for a week or so, that's the rich fuel the storm feeds off of," Herzmann said.
As cold air pours down
from above, driving out in front of the storm, warm air is pushed up
and over it like dirt rising over a bulldozer's shovel. The upward
trajectory of the air gives shape to the towering cumulonimbus
clouds, and if the air is heavily laden with water vapor, it adds
mass to the clouds, as well.
With the hot air
swelling and spreading the storm, rain continues to fall behind the
derecho. This forms a cold pool behind the storm front -- "like
a bubble or a dome of cooler air," according to Herzmann --
that, in turn, allows more downbursts to form and the cycle to
repeat itself.
Looking at radar
images of the derecho, the system appears as a kind of
boomerang-shaped band of high-intensity winds with a circular,
cooler mass of air behind it. That boomerang -- called a "bow
echo" by meteorologists -- is in fact a long line of clustered
downbursts, each 2 to 4 miles wide.
That makes the
derecho easy to identify, although given the speed with which one
can form, that doesn't necessarily give communities much time to
prepare, said Herzmann.
"The problem is
that, on any given day, you're going to see small weather events
that have the potential to turn into a derecho," he said. "It's
still really hard to know which ones are going to roll into the
full-fledged derecho and which ones will just dissipate."
No one could say
climate change was the direct cause of this storm, but rising
temperatures and more powerful storms have been predicted by climate
models for more than two decades.
Reprinted from
Climatewire with permission from Environment & Energy
Publishing, LLC. www.eenews.net, 202-628-6500
1 comment:
Well, is it a Derocho or a Derecho?
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