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April
27, 2007: Lightning is more than light and noise:
It's an intense chemical factory that affects both local air
quality and global climate. But how big is the effect? Researchers
aren't sure. To answer the question they're developing a new
technique to estimate the factory's output.
If
successful, the method will be applied to the Geostationary
Lightning Mapper (GLM) that will monitor the Western hemisphere
from a next-generation weather satellite slated for launch
in 2014.
Right:
Lightning over the Swiss Alps. Credit and copyright: Olivier
Staiger.
"Atmospheric
chemists are very interested in trace gases produced by lightning,
particularly nitrogen oxides ('NOx' for short)," explains
William Koshak, a lightning researcher at NASA's Marshall
Space Flight Center. NOx includes nitric oxide (NO), a toxic
air pollutant produced by automobile engines and power plants,
and nitrogen dioxide (NO2), a poisonous reddish-brown
gas with a sharp odor.
"We
know that lightning is the most important source of NOx in
the upper troposphere, where our weather takes place,"
Koshak continues. "NOx indirectly influences our climate
because it partly controls the concentration of ozone (O3)
and hydroxyl radicals (OH) in the atmosphere. Ozone is an
important greenhouse gas, and OH is a highly reactive molecule
that controls the oxidation of several greenhouse gases."
While
the output from cars and industry can be measured, lightning
is a wildcard in models of regional air quality and global
climate because it is difficult to realistically model several
important lightning characteristics--e.g., lightning energy
and the thermochemical yield of NOx produced by a lightning
stroke. As such, the global production rate of lightning NOx
is still uncertain, and ranges anywhere from 2 to 20 teragrams
per year (1 teragram = 1 trillion grams).
Below:
The worldwide distribution of lightning strikes. Each flash
produces a tiny puff of NOx, individually negligible, but
adding up to as much as 20 trillion grams per year when summed
over the entire globe.
"Fortunately,
space-based atmospheric chemistry measurements made by NASA's
Aura satellite provide a 'top-down' constraint on global chemistry
and climate models," Koshak says. "With these new
constraints, the best estimate to date is closer to 6 teragrams
per year. However, more work must be done to better model
lightning and other chemical processes before full confidence
in this estimate is achieved."
To
better understand lightning flash energy -- a critical parameter
in lightning NOx production -- Koshak and his colleagues are
using data from the Lightning Imaging Sensor (LIS) aboard
the Tropical Rainfall Measuring Mission (TRMM) satellite and
two arrays of ground instruments at NASA's Kennedy Space Center
in Florida. LIS is a special camera that uses a very narrow
spectral filter and other techniques to detect the lightning
optical emissions during both day and night. The filter is
centered near 777.4 nm, which is just below the deep red limit
of human vision.
Their
results will be reported in Lightning charge retrieval:
dimensional reduction, LDAR constraints, and a first comparison
with LIS satellite data, a paper recently accepted for
publication in the Journal of Atmospheric & Oceanic
Technology of the American Meteorological Society. His
co-authors are E. Philip Krider, Natalie Murray, and Dennis
Boccippio.
Right:
Koshak and colleagues are using data from the Lightning Imaging
Sensor (LIS) aboard the Tropical Rainfall Measuring Mission
(TRMM) satellite to study lightning NOx production. [TRMM]
[LIS]
"The
idea is to investigate what correlation may exist between
the optical characteristics of flashes seen by LIS vs. ground-based
lightning measurements at Kennedy. The ground-based sensors
allow us to probe deep within the thundercloud to determine
the geometry of the lightning channel, the charges deposited
by the flash, and the energy of the flash. The key is to see
if the space-based optical measurements can be related to
the ground-based flash energy estimates. If this can be done,
it would be possible to use sensors in space to remotely retrieve
flash energetics over a much larger region of the globe,"
says Koshak.
"It's
a formidable task, and this is just a preliminary look,"
he says of the forthcoming paper. The cloud medium is variable
and therefore scatters the light emitted from lightning in
complex ways. Energetic flashes embedded deep within an "optically
thick" thundercloud could appear relatively dim to a
space sensor, while weak-energy flashes occurring near cloud-top
could appear relatively bright. All these complexities must
be unraveled, and it is "tricky business."
Ultimately
Koshak hopes to provide a technique that will use GLM data
to estimate lightning flash energy. "In practice we'll
do it in a statistical fashion. We would like to give the
atmospheric chemistry modelers a realistic probability distribution
function for lightning flash energies that they can use in
their models to better simulate lightning (be it ground or
cloud flashes)."
From
that, scientists will start to better understand the global
production of one of the key pollutants in the atmosphere
pertinent to global climate and air quality.
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Author: Dave Dooling | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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