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NASA
SCIENTISTS TAKE FIRST "FULL-BODY SCAN"
OF EVOLVING THUNDERSTORM
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A
doctor gets a better view inside a patient by probing the
body with CAT and MRI scanning equipment. Now, NASA meteorologists
have done a kind of "full-body scan" of an evolving
thunderstorm in the tropics, using advanced radar equipment
to provide a remarkable picture of the storm's anatomy. The
observations are expected to help double-check satellite rainfall
measurements, improve computer models of storms, and make
the skies safer for airplanes to navigate.
David
Atlas of NASA's Goddard Space Flight Center, Greenbelt, Md.,
has gathered the data collected from an unusual storm over
the Amazon rainforest in February 1999 and arranged it into
an intriguing image of the storm clouds' inner workings.
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The
research, co-authored by University of Colorado's Christopher
Williams, appears in the January 2003 American Meteorological
Society's Journal of the Atmospheric Sciences.
Storms
often form precipitation in one of two ways, either by forming
rain at lower altitudes or by forming frozen particles higher
in the atmosphere. But this storm was unusual in that both
processes operated as the storm evolved.
In
the tropics, the air is warm even at considerably high altitudes,
so rain can occur in high clouds by forming liquid droplets
without freezing first. During the warm rain stage only the
larger drops fall quickly enough to overcome the strength
of the updraft, but the smaller ones are carried very high
into the clouds, where they freeze into snow and hail. This
storm possessed a very strong updraft and also formed frozen
precipitation in its upper levels even as rain fell closer
to the ground.
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"While
such a two-phase process should occur in many vigorous storms,
it has rarely been observed in a single storm," Atlas
said.
"The
'full-body scan' also provides new insight into the intensity
and hazards within storms, which should be avoided by aircraft.
Even the aircraft used in this study did not go into the core
of the storm because of the hazards," Atlas said.
At
the higher altitudes, cold temperatures created larger frozen
particles that were able to fall through the updraft. The
smaller ones continued to rise, colliding with the larger
falling ones. These collisions caused friction and electrical
charges that generated lightning.
Radar
was the primary means by which the "body scan" was
taken. A team of scientists from NASA, the National Oceanic
and Atmospheric Administration (NOAA), the National Center
for Atmospheric Research (NCAR), and other universities used
radar equipment sensitive enough to detect the different kinds
of particles from the storm's base up to its top, some 14
km (8.7 miles) above the jungle floor.
Different
types of radar examined different aspects of the storm. These
included a scanning Doppler radar, often seen on television
weather broadcasts, but specially designed with the capability
to measure particle types and sizes and rain rates, provided
by NCAR; and a vertically oriented Doppler radar, which measures
particle motion and size, and vertical air motions, supplied
by NOAA. The analysis was also greatly aided by measurements
within the storm made by a jet aircraft operated by the University
of North Dakota. NASA provided funds for the use of the two
radars and the operation of the jet aircraft during this field
experiment."
One
purpose of the study was to validate the Tropical Rainfall
Measuring Mission (TRMM) satellite measurements. Satellites
like TRMM provide data about how these storms operate and
help atmospheric scientists better understand how wind circulates
above the planet. A continuous array of these evolving tropical
thunderstorms around the world acts as a heat engine that
warms the upper atmosphere. That warming maintains a gradient
of temperature and pressure from the tropics to the poles,
and driving the global wind circulation.
"Our
particular study attempted to validate what the satellites
were showing us with an up-close view," Atlas said. The
validation of TRMM data will help to fine-tune and set the
stage for the Global Precipitation Mission (GPM) satellite,
which is planned to succeed TRMM.
This
research was funded in part by NASA's Tropical Rainfall Measuring-
Mission.
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