NASA scientists have opened a new window for
understanding atmospheric water vapor, its implications for
climate change and ozone depletion.
Scientists have created the first detailed map of water,
containing heavy hydrogen and heavy oxygen atoms, in and out
of clouds, from the surface to some 25 miles above the Earth,
to better understand the dynamics of how water gets into the
stratosphere.
Only small amounts of water reach the arid stratosphere, 10 to
50 kilometers (6 to 25 miles) above Earth, so any increase in
the water content could potentially lead to destruction of
some ozone-shielding capability in this part of the
atmosphere. This could produce larger ozone depletions over
the North and South Poles as well as at mid-latitudes.
Water shapes Earth’s climate. The large amount of it in the
lower atmosphere, the troposphere, controls how much sunlight
gets through to the planet, how much is trapped in our skies,
and how much goes back out to space. Higher in the
stratosphere, where most of the Earth’s ozone shield protects
the surface from harmful ultraviolet rays, there is very
little water (less than .001 the surface concentration).
Scientists don’t fully understand how air is dried before it
gets to this region.
In the troposphere, water exists as vapor in air, as liquid
droplets in clouds, and as frozen ice particles in high
altitude cirrus clouds. Since there is so much water closer to
Earth and so little miles above, it is important to understand
how water enters and leaves the stratosphere. The “isotopic
content,” the natural fingerprint left by the heavy forms of
water, is key to understanding the process. An isotope is any
of two or more forms of an element having the same or very
closely related chemical properties and the same atomic
number, but different atomic weights. An example is oxygen 16
versus oxygen 18, both are oxygen, but one is heavier than the
other.
Heavy water is more readily condensed or frozen out from its
vapor, causing the nature of its distribution to differ
somewhat from the usual isotopic form of water. A measurement
of the isotopic make-up of water vapor enables scientists to
determine how water gets into the stratosphere.
“For the first time, we have water isotope content mapped in
incredible detail,” said Dr. Christopher R. Webster, a senior
research scientist at NASA’s Jet Propulsion Laboratory (JPL),
Pasadena, Calif. Webster is principal author of a scientific
paper announcing the new findings in Science Magazine today.
Dr. Andrew J. Heymsfield, of the National Center for
Atmospheric Research, Boulder, Colo., is co-author.
Measuring water isotopes is extremely challenging, because
they represent only a small fraction, less than one percent,
of the total water in the atmosphere. Detailed measurements
were made using an Aircraft laser Infrared absorption
spectrometer (Alias) flying aboard NASA’s WB-57F high-altitude
jet aircraft in July 2002. This new laser technique enables
mapping of water isotopes with sufficient resolution to help
researchers understand both water transport and the detailed
microphysics of clouds, key parameters for understanding
atmospheric composition, storm development and weather
prediction.
“The laser technique gives us the ability to measure the
different types of isotopes found in all water,” said Webster.
“With the isotopic fingerprint, we discovered the ice
particles found under the stratosphere were lofted from below,
and some were grown there in place.”
The data help explain how the water content of air entering
the stratosphere is reduced, and show that gradual ascent and
rapid upward motion associated with tall cloud systems
(convective lofting) both play roles in establishing the
dryness of the stratosphere.
The purpose of the aircraft mission was to understand the
formation, extent and processes associated with cirrus clouds.
The mission used six aircraft from NASA and other federal
agencies to make observations above, in and below the clouds.
By combining aircraft data with ground-based data and
satellites, scientists have a better picture of the
relationship between clouds, water vapor and atmospheric
dynamics than previously. They also can better interpret
satellite measurements routinely made by NASA.
The mission was funded by NASA’s Earth Science Enterprise. The
Enterprise is dedicated to understanding the Earth as an
integrated system and applying Earth System Science to improve
prediction of climate, weather and natural hazards using the
unique vantage point of space. For more information about
Alias, visit: http://laserweb.jpl.nasa.gov
For Information about NASA and JPL programs, visit:
