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How do raindrops
form? It's a
simple question, but the answer is far from elementary. Tiny water
droplets
somehow merge to become full-sized raindrops, but the details remain a
mystery.
Now, scientists at
the Scientists think
they have a good
theoretical understanding of how raindrops form in cold clouds, where
ice
crystals play a key role, Chuang said. So he and his graduate student,
Jennifer
Small, are tackling the more puzzling problem of how raindrops form in
warm
clouds, like those that produce summer showers. They will present their
findings on December 11 at the annual meeting of the American
Geophysical Union
(AGU) in Knowing how clouds
make rain is
critical for improving the accuracy of climate models and predicting
the
effects of global warming, Chuang said. Water vapor is a potent
greenhouse gas,
and rain is the primary regulator of how much is in the air, he said.
Rain is
also one of the ways a cloud reaches the end of its life. This is
important
because clouds reflect sunlight, affecting global temperatures by
regulating
how much solar energy reaches the Earth's surface. Condensation of
water vapor in
clouds creates tiny water droplets about 10 to 20 microns in
diameter--less
than the width of a human hair. These droplets are far too small to
fall as
raindrops, which are about a millimeter (a thousand microns) across and
a
million times heavier. To make raindrops, the droplets need to collide
and
stick together to create larger and larger droplets, Chuang said. When scientists
simulate this
process with computers, raindrops form in one or two hours. In the real
world,
however, rain can start to fall within 15 minutes of cloud formation.
Something
must be speeding up the mergers of tiny droplets, which are so light
they float
around in clouds and avoid the collisions necessary to make raindrops. Droplets with a
diameter of about
55 microns or larger are heavy enough to fall through the cloud,
merging with
other droplets at a rapid pace. The real mystery, then, is how the
tiny, 10- to
20-micron droplets become 55-micron droplets, Chuang said. One way to
speed up
the coalescence of droplets is to stir them up. "If you stir up the
droplets, they will more readily collide with one another," he said.
"There are many ideas as to what the mixing mechanism is, but no one
knows
for sure how it comes about." For decades,
scientists have been
debating over two main mechanisms. One proposal involves chaotic swirls
of
turbulence that churn on millimeter- to centimeter-sized scales.
Another idea
is a process called entrainment that happens when dry air mixes with
moist air
at the edges of clouds. It has been
difficult to find
unequivocal evidence for either hypothesis because there have been no
instruments that could measure water droplets in the key size range of
30 to
100 microns. So Chuang led the development of a new instrument to do
just that,
working in collaboration with Artium Technologies, Inc. Data gathered
with the
new instrument suggest raindrops form through a combination of both
entrainment
and turbulence. "Until now, no one's
been
able to look at that process of how drops form--it's a missing link
that has
been in contention for 50 years," said Small, who will present their
work
at an AGU poster session. Chuang's
device--called a phase
Doppler interferometer--attaches to the wing of an airplane and uses
lasers to
measure droplet sizes while the plane flies through clouds. Small and
Chuang
first used it over the course of six weeks in December 2004 and January
2005
above the Caribbean Small and Chuang
measured the
sizes of droplets in clouds, and found droplets larger than 55 microns
in
sinking pockets of drier air at the tops of clouds. This implies that
the
process of entrainment, which tends to occur at the tops of clouds,
helped
create the large droplets. The fact that the researchers found large
droplets at
cloud tops contradicts suggestions that large droplets form from
particles like
dust in the cloud, because such big particles would have sunk to the
bottom.
The fact that large droplets were grouped together and not randomly
distributed
throughout the cloud also challenges the notion that large droplets
come from
an outside source like sea spray. Entrainment could
explain most of
the droplets they saw, but the findings suggest turbulence also plays a
role,
Small said. Turbulence, however, is inherently complicated and
difficult to
study. As a next step, the researchers will try to incorporate both
entrainment
and turbulence into a simple computer model and see if it re-creates
their
observations.
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