Satellite instruments captured numerous
images of
Hurricane Sandy—using both daylight and moonlight—as
the enormous storm approached the heavily-populated East Coast of the United
States in late October 2012. In most cases, these instruments observed just the
uppermost layer of clouds at the top of the storm.
However, one satellite—Cloudsat—peered
inside the storm and observed its vertical structure. It did so with a cloud-profiling radar that sent pulses of energy toward
Earth and recorded the strength of the signal that bounced off ice and water
particles.
The bottom map shows the storm as observed by Cloudsat around 2
p.m. local time (18:00 Universal Time) on October 29, 2012. The image shows a
cross-section—what the storm would look like if it had been sliced near the
middle and viewed from the side. (Watch this animation to see how Cloudsat collects data).
The top image, acquired the same day by the Moderate Resolution Imaging
Spectroradiometer on
the Aqua satellite,
is shown for reference. The yellow line is the north-to-south track that
CloudSat took over the storm.
In the Cloudsat data, the darkest blues represent areas where
clouds and raindrops reflected the strongest signal back to the satellite
radar. These areas had the heaviest precipitation and the largest water
droplets. The blue line in the center of the image is the freezing line; ice
particles formed above it, raindrops below it. Though they look similar from
above, the thin clouds on the far left of the image at an altitude between
about 6 and 10 kilometers (4 and 6 miles) are cirrus clouds producing little or
no precipitation.
Another clue to the intensity and concentration of precipitation
is visible at the bottom of the image. In areas with fewer clouds and less
vigorous precipitation, enough radar signals made it all the way to the ground
to create the blue line along the bottom. In areas with heavy precipitation,
the line is absent because so much of the radar’s signal was scattered before
reaching the ground.
Sandy was an unusual hybrid
storm that
included features of both a tropical
cyclone and
an extratropical winter
storm. The offshore tropical storm merged with a storm system approaching from
the west, giving Sandy a cloud structure that was different than most
hurricanes and typhoons.
Sandy’s clouds, for instance, reached about 10 kilometers (6
miles) at their highest point—not as high as the clouds of many tropical
cyclones. For comparison, Hurricane
Earl and Super
Typhoon Choi-Wan’s both
reached about 15 kilometers (9 miles).
Sandy’s cloud field was also unusually wide—about 1,200
kilometers (750 miles) from one end to the other—because the storm was in the
process of becoming an extratropical cyclone. Tropical cyclones tend to have tight,
circular cloud bands that are about 400 kilometers (250 miles) from edge to
edge (or end to end). Extratropical cyclone bands often stretch more than 800
kilometers (500 miles).
Scientists are creating a database of Cloudsat overpasses in order to
better understand how storms work. Natalie
Tourville, a
research meteorologist at Colorado State University who studies tropical
cyclones, explained some of the ways that Cloudsat has proven useful since it
launched in 2006.
“CloudSat is providing exceptional views of the internal
structure of
tropical cyclones, with at least 30 eye
overpassesglobally,” she said. “We’ve been able to sample eye
wall slope
and view the vertical details of the convective
core regions. This
includes where and how large these cores are and how high the cloud tops are.
We've also sampledovershooting
cloud tops (some
extend over 17 kilometers into the atmosphere) and provide detailed analysis of
exactly what is underneath the cirrus clouds of these storm systems.”
Scientists have also developed an innovative technique that uses
Cloudsat measurements, in conjunction with data from other satellites, to
estimate storm intensity—something that has proven challenging to do from
space. A team of researchers first described the technique in an article published
in IEEE
Geoscience and Remote Sensing Letters in 2008. Tourville is currently
working on a follow-up project that aims to improve the technique by using more
recent Cloudsat data.
1.
Further Reading
2.
Durden, S. (2009) Cloudsat
and A-Train Observations of Tropical Cyclones. The Open Atmospheric Journal.
5.
Posselt, D. (2008,
May) Cloudsat Adding a New Dimension to a Classical View of
Extratropical Cyclones. Bulletin of the American
Meteorological Society.
6.
Stephens, G.L.
(2008, January) On the Use of CloudSat and
MODIS Data for Estimating Hurricane Intensity.Geosciences
and Remote Sensing Letters.
NASA Earth Observatory image by Jesse Allen, using CloudSat
FirstLook data provided courtesy of the CloudSat
team at Colorado State University. Caption by Adam Voiland.
Instrument:
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