As PI on the NASA Scatterometer (NSCAT) Science Working Team and
as the former Project Engineer for the NSCAT Project, Dr. Long has
been involved in the calibration of the NSCAT instrument and in
science data analysis.
NSCAT picture (59K)
NSCAT OCEAN WIND MEASUREMENT
NSCAT was designed to measure near-surface winds over the ocean.
This image [Low-Res
(74kb) Hi-Res (810kb)]
shows ocean surface wind speeds and directions over the Pacific
Ocean on 21 September 1996 as they were measured by the NASA Scatterometer
(NSCAT) onboard Japan's Advanced Earth Observing Satellite (ADEOS).
The background color indicates wind speed and the white arrows show
the direction of the wind. The basin-wide wind field is representative
of near-Equinox atmospheric circulation. The strong Trade Winds
(red) blow steadily from the cooler subtropical ocean to the warm
water of the Intertropical Convergence Zone (ITCZ) located just
north the Equator. Instead of blowing in the north-south direction,
the winds are deflected westward by the Corriolis Force due to the
Earth's rotation. The air rises over the warm water of ITCZ and
sinks in the subtropics at the Horse Latitudes, forming the Hadley
Circulation. Both the convergence area at the ITCZ and the divergence
area at the Horse Latitudes are indicated by low wind speed of blue
color. In the mid-latitudes, the high vorticity due to the Corriolis
Force generates cyclones (yellow spirals) moving in the eastward
direction. Two typhoons are observed in the western Pacific. Typhoon
Violet is just south of Japan. After these data were taken, Typhoon
Violet struck the East Coast of Japan causing damage and deaths.
Typhoon Tom is located further east and did not land.
The image is based on preliminary processing of the first set
of NSCAT observations, using prelaunch model function and calibration.
Improvement is expected after the standard calibration and beam
balancing procedures. The image is produced by objective interpolation
as described by Tang and Liu [JPL Publication 96-19, 1996] based
entirely on NSCAT data. This preliminary analysis clearly demonstrates
that the high spatial resolution of NSCAT data improves the monitoring
of sever storms, such as typhoons, which are usually grossed over
by conventional methods. It also shows that the repeated global
coverage provides a better description of atmospheric circulation
over ocean that is not adequately sampled in the past.
NSCAT LAND, ICE, AND VEGETATION APPLICATIONS
In addition to their primary role of measuring oceanic winds, spaceborne
scatterometers can significantly contribute to non-ocean studies
in vegetation and polar ice. The recently launched NASA Scatterometer
(NSCAT) is an important asset in such studies. Several applications
are highlighted below.
The polar regions play a central role in regulating global climate,
and it is important to accurately record and monitor the extent
and surface conditions of the earth's major ice masses. Such monitoring
can only be done using spaceborne sensors. Spaceborne radar remote
sensors are uniquely well-suited for mapping the polar regions since
the radar can image the surface through clouds and both day and
night. Active radar instruments are useful for studying snow and
ice. Similarly, radars are also useful for vegetation studies because
different vegetation types and densities have different radar responses.
A radar scatterometer measures the radar backscattering cross-section
(termed "sigma-0" by scientists) of the Earth's surface.
Measurements of sigma-0 over the ocean are used to infer the near-surface
wind speed and velocity. Measurements of sigma-0 in the polar regions
and over land can be used to study ice and vegetation.
Dr. David Long of Brigham Young University has developed a technique
for making sigma-0 images of the Earth's surface. Areas which reflect
more microwaves are typically rougher and appear brighter in the
images than smoother areas which reflect less. The electrical properties
of the surface also affect the image brightness.
Figure 1
(gif 356K) illustrates an image formed from NSCAT sigma-0 measurements
and demonstrate the wealth of information contained in the scatterometer
data. This image shows Antarctica and the surrounding sea ice constructed
from 6 days of scatterometer data in Sept. 1996. The black circle
in the center of the image is where no data was collected owing
to NSCAT's orbital and sampling geometry. The dark band around the
continent is sea-ice pack surrounding Antarctica. The variations
in sea ice show the circulation patterns and are due to the snow
cover, thickness, and history of the ice since formation. This information
is essential to understand the effects of the ice pack on the ocean
and climate systems. The white, rectangular object in the ice pack
on the lower left of the image is a 50 km x 100 km "super-berg"
which broke off the Thwaites ice tongue in 1995 and is now circulating
in the sea-ice pack. Other large icebergs are also visible in the
image. A time-sequence (movie) of these images is helping scientists
to understand more about how the ice is formed and circulates.
Antarctica is overed with a thick ice sheet which appears very
bright in the image due to snow crust and refrozen ice in the snow
cover. Details visible in the glacial ice cover show the locations
of ice "hills" and "valleys" which reveal information
about the flow of the ice over the subsurface topography. The relative
brightness can be useful for determining the annual snowfall. For
reference Figure
2 (gif 358K) shows a view of the Arctic ice pack.
Antarctic Images and Movies
Ice masked NSCAT images of the polar regions are animated in the
following movies. The images have been down sampled from original
sizes. These movies are available in mov, avi, Animated gif formats.
A full global image as produced from NSCAT data is shown in Figure
3 (b/w gif 402K)
(color gif 383K) .
The brightest regions are glacial ice sheets in Greenland and Antarctica
as described above. For other regions, the brightness of the image
is related to the vegetation cover and soil moisture. Tropical rainforests
along the equator in South America, Africa, and Southeast Asia are
relatively bright while desert regions are dark. Very dry, sandy
deserts show up as black in this image. Some examples are the Empty
Quarter in Saudia Arbia, the Gobi desert in Western China, and the
great erg (sand dune) seas in Sahara desert in North Africa. The
light area just below the wide, dark band in Africa is known as
the Sahel. This area lightens and darkens with the changing season
and drought conditions in Africa. The seasonal radar response of
the Sahel is thought to be a sensitive indicator of desertification
due to global warming and climate change.
Tropical rainforests are critical to the climate health of the
world and are thought to contain 1/2 of all the world's species.
Figure 4 [hires b/w
gif 523K] [lores
b/w gif 99K]
[color gif 492K) shows the Amazon rainforest over South America
as observed by NSCAT. Because the radar response is sensitive to
the type and density of vegetation, the scatterometer data can provide
information useful for discriminating and mapping vegetation. A
false color image helps discriminate general areas of tropical rainforest
(blues and purples) from woodlands and savannah (greens and yellows).
Mountains and degraded farm lands show up as black. [Note: data
is not calibrated so this IS NOT a classified image.] The NSCAT
data is able to delineate the extent of the tropical rainforest.
Comparison of this image taken in 1996 with images made from Seasat
scatterometer data collected in 1978 may help reveal the extent
of tropical deforestation in this sensitive area.
Statistical Modeling
We have been developing improved statistical models for the scatterometer
measurement process. Some of the areas understudy are:
NSCAT NASA Press Releases
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NSCAT picture (59K)
![[hires b/w gif 523K]](images/NSCAT/NSCAT_415-631_cal4.gif){kind=link}
![[lores b/w gif 99K]](images/NSCAT/NSCAT_amazon_lowres.gif){kind=link}
