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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.
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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.
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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.
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Capturing
Heavy Precipitation Events with TRMM
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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.
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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.
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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.
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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.
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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.
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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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To
continue to the second page of images and animations, click here.
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