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FINDING
TINY PARTICLES IN HURRICANES MAY HELP WITH PREDICTIONS
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1 - NCAR GPS Dropsonde
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NASA-funded
scientists are looking at microscopic ice particles inside
hurricanes to determine if they contribute to the stormís
strengthening or weakening. Researchers have discovered that
greater numbers of ice particles higher up in a hurricane
reflect more energy from the Sun out to space, creating a
temperature difference that helps power the hurricane. The
particles could also indicate a loss of energy into the surrounding
atmosphere.
During
a flight into Hurricane Humberto in 2001, researchers identified
a large number of ice particles up to 7 millimeters in diameter
in the upper reaches of the storm, around 39,000 feet high.
The research was part of CAMEX-4 (The Convection And Moisture
EXperiment), a joint effort between NASA, the National Oceanic
and Atmospheric Administration (NOAA) and a number of universities.
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2 - Hurricane Humberto |
"We've
found that a high concentration of small particles at the top
portion of a hurricane are very reflective, and they send more
energy back to space than previously thought," said Andrew
J. Heymsfield, researcher from the National Center for Atmospheric
Research (NCAR), Boulder, Colo., and a lead author on a paper
on the experiment.
Large
concentrations of small ice crystals can reflect more sunlight
and can lead to strong cooling near the tops of hurricanes,
creating more of a temperature contrast between the bottom
and top of the storm. That contrast of air temperatures could
possibly contribute to its strength, just as a cold front
moving into warm, moist air triggers thunderstorms.
"Updrafts
of wind in hurricanes are generally pretty small, so some
researchers did not expect large (up to 7 millimeters) particles,
which generally form at warm temperatures, to exist at higher
altitudes," Heymsfield said. Because large particles
weigh more, they fall faster than small particles, and small
particles can evaporate.
Aaron
R. Bansemer, co-author on the paper and on the experiment
noted that the data from Humberto showed some of the larger
particles were transported to at least 35,000 feet, or may
have grown to large sizes through aggregation of smaller particles,
a process that is usually thought to occur at warmer temperatures,
usually around 18,000 feet.
Scientists
are curious about how ice particles and condensed water is
distributed in a hurricane because it tells them where most
of the mass or ìweightî is in a hurricane. Knowing
this distribution of mass will help researchers determine
if a hurricane is strengthening. If most of the hurricaneís
mass (condensed water and ice particles) is high in the storm,
some may escape into other parts of the upper troposphere
where it evaporates, weakening it by reducing the overall
precipitation efficiency of the hurricane.
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Ice crystal particles found in Hurricane Humberto |
This
discovery and observations of particles higher up in hurricanes
are being entered into computer models that simulate hurricanes,
and will give researchers a better idea of the workings of
hurricanes. "The bottom line here is that by identifying
where larger ice particles are in a hurricane, we are adding
to the data we put in computer models simulating the storms,
and that will improve predictions on whether a hurricane is
strengthening or weakening," Heymsfield said.
This
paper, "In Situ Measurements of Particle Size Distributions
in Hurricane Humberto," will be presented on Monday,
April 29, 2002, at the American Meteorological Society Hurricane
and Tropical Meteorology Conference, San Diego, Calif.
This
research was entirely funded by NASA through the CAMEX-4 Mission.
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