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Maps
of Falling Water: Three Years of TRMM Data - Page Two
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INTRODUCTION:
Ever wonder
about the rain? Beyond the practicality of needing an umbrella,
climate researchers have wondered about the science of rainfall
for a long time. But it's only in the past few years that
they've begun to roll back some of its secrets. One of their
tools for doing so is a powerful satellite called the Tropical
Rainfall Measuring Mission, or TRMM. Now, after three years
of continual operation, project scientists have released dramatic
new maps of rainfall patterns gathered across a wide band
of the Earth. And with measurements from one of the satellite's
advanced sensors, meteorologists are now able to calibrate
ground-based rain monitoring systems with greater precision
than ever before.
For the
purposes of understanding color codes:
Color
bar for the monthly rainfall amounts (above).
Color
bar for the anomaly rainfall amounts (above).
Watching
the Atmospheric Engine at Work: Three
Year Rainfall Maps
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A
complete accounting of the world's total rainfall has long
been a major goal of climate researchers. Rain acts as the
atmosphere's fundamental engine for heat exchange; every time
a raindrop falls, the atmosphere gets churned up and latent
heat flows back into the total climate system. Considering
that rainfall is the primary driving force of heat in the
atmosphere, and that two thirds of all rain falls in the tropics,
these measurements are significant for our understanding of
overall climate. These images show three years of daily rainfall
measurements taken by the satellite's unique precipitation
radar. It's a revolutionary visualization in that it offers
experts the first comprehensive, long duration observations
of where it's raining and how much.
But why watch for rain from space? Why not use simple ground-based
monitoring systems and tally the results?
The
answer is simpler than you might think. The vast majority
of the Earth's rainfall happens over oceans, thus making it
difficult to obtain accurate, consistent rainfall measurements
in a sufficient number of regions for meaningful research.
TRMM is not only unfettered by geographic constraints, but
it can virtually blanket its observable region with collection
of data.
TRMM
cannot cover its total ground track in a single day. Generally
it takes about three days for complete coverage of the total
observable area. This presents certain mathematical challenges
for visualization, as absolute measurements for the entire
ground track are not possible every day. Here's the solution:
for each day shown in the visualizations, the development
team computed a moving average for each particular day shown.
As trails of rain move across the Earth's surface in this
visualization, each visible moment is composed of data gathered
fifteen days prior to and fifteen days following each individual
calendar day. As days progress, that thirty-day moving average
advances in synchrony with the calendar, changing the overall
rainfall picture as it slides through time. The resulting
images show rainfall trends for a given day, rather than precise
pictures of actual rainfall.
What's
Normal? Using TRMM to Generate Rainfall Averages
 
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Just
like the human body maintains a normal temperature that can
fluctuate under different conditions, there tends to be a
general rainfall average over much of the world. Whether those
averages for different regions change through time due human
factors is still an issue being debated. But TRMM is at least
enabling experts to generate data for tropical rainfall averages
taken over the past few years with a measure of accuracy never
before possible. The following images show rainfall averages
on a monthly time scale for territory beneath TRMM's ground
track.
Rainfall
Anomalies: When it Rain, it Pours
 
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About
two thirds of all the Earth's rain falls in the tropics. Moreover,
the energy released by that rain accounts for the vast majority
of energy released in the atmosphere. As we come to better
understand the rain on a planetary scale, we also begin to
understand the processes describing localized rainfall events
around the world.
The
following visualizations show rainfall anomalies-that is,
regions with rainfall amounts that significantly exceed or
fall short of averages. Blue regions show low quantities,
while yellows and reds show high quantities.
Notice
how there is generally very little distance separating high
and low regions. While we might expect to see a gradual transition
between regions of anomalous highs and lows, the two usually
appear together.
Here's
what's going on:
Regions
showing high rainfall must draw moisture and heat from a source.
Conversely, regions showing unusually little rain must lose
its moisture and heat to somewhere. In close proximity to
each other, each of the diametrically opposite conditions
appear precisely because of their opposite twin. To see an
example of this in action, watch this sequence with a close
eye on the central Pacific Ocean. When the strong signals
for anomalous high and low rainfall appear (corresponding,
incidentally with the El Nino and La Nina phenomena), notice
how they seem to run in parallel with each other. Areas of
heavy rain have drawn heat and moisture from adjacent areas.
Or, said the other way, areas of low rainfall have lost significant
heat and moisture to regions next door.
Rainfall
anomalies also allow experts to study local or regional changes
in climate. As we'll see in the next section, anomalous rainfall
measurements can be harbingers of change, as in the case of
desertification in parts of sub-Saharan Africa, or severe
short term events, such as the intense floods of Mozambique
in early 2000 or the pounding rains of Hurricane Mitch in
1998.
Capturing
Heavy Precipitation Events with TRMM
The following
visualizations focus on specific, localized rainfall events
in different parts of the world. Notice how the relative rainfall
intensity over each area suddenly increases around to the
dates of each event. By studying events like these, experts
hope to gain insight into better forecasting techniques, as
well as provide broader analysis into regional and global
climate change.
Mozambique
Devastated by Floods
  
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The worst
flooding in nearly fifty years devastated large regions of
Mozambique in late February and early March 2000. Intense
flooding following heavy rains displaced hundreds of thousands
of people and killed hundreds. As this visualization shows
regions of heavy precipitation over Mozambique, consider that
the southern part of that country received in the first three
weeks of February as much rain as usually falls there in a
whole year.
Hurricane
Floyd Pounds North Carolina
  
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In September of 1999, Hurricane Floyd precipitated massive
flooding across wide stretches of North Carolina and other
areas in the eastern United States. The hurricane moved slowly
across the region, prompting heavy rains not only to fall
over the area, but also to persist for days. Heavy rains from
the storm caused waters to rise quickly in a large stretch
of the region, while runoff and heavy sedimentation associated
with it caused serious problems for residents and officials
for weeks to come.
The
Grinding Wake of Hurricane Mitch
  
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It's
been more than two hundred years since a storm killed as many
people as Hurricane Mitch. In late October and early November
of 1998, this monster storm dumped as much as one to two feet
of rain per day for several days on parts of Central America.
More than 11,000 people died during Hurricane Mitch, while
it also caused billions of dollars of damage to fragile Central
American economies.
El
Nino and Tropical Rain: Causes and Effects
 
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Generally
tropical rains in the Pacific fall more heavily on the western
part of the ocean. But as seen in this visualization, the
El Nino event that recently came to an end caused an observable
shift in average rainfall patterns south of the equator. Heavier
than normal rains fell east of 150° West, while the western
Pacific showed greatly diminished rainfall rates.
Calibrating
Ground Based Radar from Orbit
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Measurements
are only as accurate as the instruments that take them. When
U.S. and Japanese scientists designed TRMM, they expected
to use ground based precipitation radar systems to calibrate
the precipitation radar onboard the spacecraft.But
after years of continual operation, the TRMM project has determined
that the satellite's level of accuracy and consistency is
great enough that ground-based systems can, in fact, be calibrated
to the system on the spacecraft.
Another
reason that TRMM has been shown to be effective for calibration
of ground-based rain radar is that it can provide an accurate
point of reference for engineers and technicians working to
keep terrestrial systems in top working order. Without TRMM
information, accurate calibration of ground based systems
usually took a lot of time and care. But because the satellite's
data has been proven so reliable, a shorter process in required
to keep ground systems working at peak efficiency by simply
using TRMM data as a reference point. Think of an ordinary
scale for measuring weight being calibrated by a series of
objects with previously validated measurements. Using TRMM
data, staff working with ground based systems can know if
they're taking accurate readings by making comparisons to
the satellite readings.
TRMM
does not communicate directly with ground based systems. Satellite
information actually helps experts develop algorithms and
techniques, enabling them to refine land-based systems. But
as a result of the orbiting rainfall research platform called
TRMM, those land-based systems can work at their peak efficiency,
helping experts both study and forecast changes in the weather.
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