Lightning
Strikes an Odd Pattern Over the Plains
Satellite sensor points to most
violent storm activities
Jan
4, 2000: Where there's lightning, many people worry that
tornadoes may follow. Lightning is associated with energetic
storms since it takes large upward movements of air - plus
water in various forms including raindrops and ice crystals -
to produce a large electric potential.
Right: This map depicts the ratio of
cloud-to-cloud lightning to cloud-to-ground lightning over the
continental United States. The red areas show where the ratio
is as high as 10 cloud-to-cloud strikes for every ground
strike. Blue areas indicate ratios as low as 1:1. Links to 911x650-pixel,
300KB JPG. Or, click here for 212KB
Acrobat PDF version that can be edited in Illustrator.
Credit: Dennis Boccippio, NASA/Marshall.
When the potential becomes great enough,
electricity punches its way through air that normal insulates
and builds a narrow bridge of electrified gas or plasma. The
current burrows its way in search of an oppositely charged
region where the imbalance can be relieved. When the two are
joined, current flows freely and ionizes even more air on its
path, thus creating the glowing hydra that we see as a
lightning bolt. The heated air expands and, when the discharge
is suddenly stopped, it slams back together to produce the
thunderclap.
The feeling that many people have about
lightning and tornadoes is gradually getting scientific
support. The latest find is that storms with far more
cloud-to-cloud lightning than cloud-to-ground are more
energetic and are more likely to produce violent storms.
"The really interesting area is the
Great Plains region where the ratio goes as high as
10-to-1," said Dr. Dennis Boccippio of the Global
Hydrology and Climate Center, affiliated with NASA's Marshall
Space Flight Center in Huntsville, Ala.
Boccippio, working with Drs. Hugh Christian and
Steve Goodman of the GHCC and Dr. Cummins of Global
Atmospherics in Tucson, Ariz., combined four years of
satellite and ground-based data to estimate the ratio of
intercloud (IC) and cloud-to-ground (CG) lightning. He
presented their findings to the American Geophysical Union's
fall meeting in San Francisco on Thursday.
The space segment of Boccippio's data came
from two similar instruments developed by the GHCC and
NASA/Marshall. The Optical Transient Detector, launched in
1995, records the locations of lightning flashes in clouds
beneath the satellite.
"After four years of observations, the
OTD has enough data to merge with the National Lightning
Detection Network," or NLDN, Boccippio explained. The
NLDN is ground-based and detects where lightning strikes the
ground. This information is vital to utilities and
communications companies so they can tell which facilities are
at increased risk as a weather system moves through an area.
But the NLDN can only measure strikes that
reach the ground, and those are only part of the story.
Right:
Storm clouds from nighttime thunderstorm Illuminated by
cloud-to-cloud lightning. Credit: NOAA Photo Library, NOAA
Central Library
Scientists have known for some years that
storms have a significant amount of energy tied up in
lightning that never reaches the ground. While the lightning
itself poses no direct threat to people on the ground, it is a
strong indicator of vertical motion in a storm.
"That ratio has never been known very
well because it's very hard to measure," Boccippio said.
Much of what was known was based on small data sets collected
by ground observers and instruments carried by balloons.
The two satellite sensors let improve our
understanding with a great deal of confidence.
"This is the first time we've been able
to map the ratio across the entire United States," he
continued.
Two areas on the map stand out right away.
First is Tornado Alley, an area covering much of Kansas and
Nebraska. Second is most of Oregon and part of northwest
California.
While one might react with, "Well, I
could have told them that," it's one thing to
"know" something, and quite another to assign
numerical values that tell a scientific story that eventually
might help predict the weather.
"Strong updrafts, which cause
lightning, correlate to stronger storms," Boccippio said.
"The absolute flash rate value from satellites is useful.
Combined with the ground rate, we can break the two types of
lightning out, and be more confident in saying that a given
storm is going to be a supercell and have hail or
tornadoes."
Oregon, though, is not known for tornadoes.
One possibility is that the area is not as densely populated
with sensors as the rest of the United States, so the ratio is
artificially high. Also, more lightning in that region occurs
in winter storms, which behave differently than summertime
thunderstorms.
Also striking is the number of blue areas,
indicating very low ratios, almost 1:1, of intercloud and
cloud-ground lightning. These correspond with the Appalachian
Mountains in the East, and the Rockies and Sierra Nevada
mountains in the West.
Two factors may literally short-circuit
lightning there, he suggested.
"The conventional wisdom is that the
ground is closer to the main negative charge layer," he
said. With less insulating air between opposite charges,
lightning will discharge more easily.
"Also, the topography may keep storms
from organizing supercells," he added.
With further study, Boccippio expects that
understanding the lightning ratio can benefit people on the
ground and in the air. The two sensors can only be used for
limited research because they are in low orbits and see only a
tiny fraction of the Earth at any given time.
A Lightning Mapping Sensor in geostationary
orbit, though, would see an entire hemisphere, and would have
optics to monitor danger areas as they brewed.
This would help severe storm forecasters
provide quicker, more precise warnings of storms that are
likely to unleash damaging hail, high winds, and tornadoes.
They could also reduce false alarm rates for severe weather
hazards.
It would also help airlines route aircraft
around storm centers where intercloud lightning poses a
significant hazard to aircraft electronics.
Intercloud and
cloud-to-ground lightning:
Comparisons between OTD, NLDN and the GAI Long Range Network.
The long range component of the North
American Lightning Detection Network has been providing
experimental data products since July 1996, offering
cloud-to-ground lightning coverage throughout the Atlantic and
Western Pacific oceans, as well as south to the Intertropical
Convergence Zone. The network experiences a strong decrease in
detection efficiency with range, which is also significantly
modulated by differential propagation under day, night and
terminator-crossing conditions. A climatological comparison of
total lightning data observed by NASA's Optical Transient
Detector (OTD) and CG lightning observed by the long range
network is conducted, with strict quality control and
allowance for differential network performance before and
after the activation of the Canadian Lightning Detection
Network. This yields a first-order geographic estimate of long
range network detection efficiency and its spatial
variability. Intercomparisons are also performed over the
continental US, allowing large scale estimates of the
midlatitude climatological IC:CG ratio. We find significant
spatial variability in this parameter, little latitude
dependence, some indication of elevation dependence, and a
strong anomaly possibly associated with enhanced MCS activity
in the upper midwest.
Other lightning stories
Details
on the Optical Transient Detector.
Global
Atmospherics, operator of the National Lightning
Detection Network.
Human
Voltage (June 18,1999) What happens when lightning meets
people
News
shorts from Atmospheric Electricity Conference (June
16,1999) Poster papers on hurricanes and tornadoes
summarized.
Soaking
in atmospheric electricity (June 15, 1999) 'Fair
weather' measurements important to understanding
thunderstorms.
Lightning
position in storm may circle strongest updrafts (June
11, 1999) New finding could help in predicting hail,
tornadoes
Lightning
follows the Sun (June 10, 1999) Space imaging team
discovers unexpected preferences
Spirits
of another sort (June 10, 1999) Thunderstorms generate
elusive and mysterious sprites.
Getting
a solid view of lightning (June 9, 1999): New Mexico
team develops system to depict lightning in three
dimensions.
Learning
how to diagnose bad flying weather (June 8, 1999):
Scientists discuss what they know about lightning's effects
on spacecraft and aircraft.
Three
bolts from the blue (June 8, 1999): Fundamental
questions about atmospheric electricity posed at conference
this week.
Lightning
Leaders Converge in Alabama (May 24, 1999): Preview of
the 11th International Conference on Atmospheric
Electricity.
What
Comes Out of the Top of a Thunderstorm? (May 26, 1999):
Gamma-rays (sometimes).
More links
National
Severe Storms Laboratory, Norman, OK
National
Severe Storms Laboratory Photo Library, where we got a
lot of the neat pictures for these lightning stories.
Lightning
research at NASA/Marshall and the Global Hydrology and
Climate Center.
More
Space Science Headlines - NASA research on the web
NASA's
Earth Science Enterprise Information on Earth Science
missions, etc.
TOP
