On March 2, 2003, near-surface winds carried a large amount of Saharan
dust aloft and transported the material westward over the Atlantic
Ocean. These observations from the Multi-angle Imaging
SpectroRadiometer (MISR) aboard NASA's Terra satellite depict an area
near the Cape Verde Islands (situated about 700 kilometers off of
Africa's western coast) and provide images of the dust plume along with
measurements of its height and motion. Tracking the three-dimensional
extent and motion of airmasses containing dust or other types of
aerosols provides data that can be used to verify and improve computer
simulations of particulate transport over large distances, with
application to enhancing our understanding of the effects of such
particles on meteorology, ocean biological productivity, and human
health.
MISR images the Earth by measuring the spatial patterns of reflected
sunlight. In the upper panel of the still image pair, the observations
are displayed as a natural-color snapshot from MISR's vertical-viewing
(nadir) camera. High-altitude cirrus clouds cast shadows on the
underlying ocean and dust layer, which are visible in shades of blue
and tan, respectively. In the lower panel, heights derived from
automated stereoscopic processing of MISR's multi-angle imagery show
the cirrus clouds (yellow areas) to be situated about 12 kilometers
above sea level. The distinctive spatial patterns of these clouds
provide the necessary contrast to enable automated feature matching
between images acquired at different view angles. For most of the dust
layer, which is spatially much more homogeneous, the stereoscopic
approach was unable to retrieve elevation data. However, the edges of
shadows cast by the cirrus clouds onto the dust (indicated by blue and
cyan pixels) provide sufficient spatial contrast for a retrieval of the
dust layer's height, and indicate that the top of layer is only about
2.5 kilometers above sea level.
Motion of the dust and clouds is directly observable with the
assistance of the multi-angle "fly-over" animation. The frames of the
animation consist of data acquired by the 70-degree, 60-degree,
46-degree and 26-degree forward-viewing cameras in sequence, followed
by the images from the nadir camera and each of the four
backward-viewing cameras, ending with 70-degree backward image. Much of
the south-to-north shift in the position of the clouds is due to
geometric parallax between the nine view angles (rather than true
motion), whereas the west-to-east motion is due to actual motion of the
clouds over the seven minutes during which all nine cameras observed
the scene. MISR's automated data processing retrieved a primarily
westerly (eastward) motion of these clouds with speeds of 30-40 meters
per second. Note that there is much less geometric parallax for the
cloud shadows owing to the relatively low altitude of the dust layer
upon which the shadows are cast (the amount of parallax is proportional
to elevation and a feature at the surface would have no geometric
parallax at all); however, the westerly motion of the shadows matches
the actual motion of the clouds. The automated processing was not able
to resolve a velocity for the dust plume, but by manually tracking dust
features within the plume images that comprise the animation sequence
we can derive an easterly (westward) speed of about 16 meters per
second. These analyses and visualizations of the MISR data demonstrate
that not only are the cirrus clouds and dust separated significantly in
elevation, but they exist in completely different wind regimes, with
the clouds moving toward the east and the dust moving toward the west.
The Multi-angle Imaging SpectroRadiometer observes the daylit Earth
continuously and every 9 days views the entire globe between 82 degrees
north and 82 degrees south latitude. These data products were generated
from a portion of the imagery acquired during Terra orbit 17040. The
panels cover an area of about 312 kilometers x 242 kilometers, and use
data from blocks 74 to 77 within World Reference System-2 path 207.
Image credit: NASA/GSFC/LaRC/JPL, MISR Team.
Text acknowledgment: Clare Averill (Acro Service Corporation/Jet Propulsion Laboratory).
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