![Figure 1: AIRS VIS Daylight Snapshot July 26](https://webarchive.library.unt.edu/eot2008/20090831090722im_/http://photojournal.jpl.nasa.gov/figures/PIA00439_fig1_thumb.jpg) |
![Figure 2: AIRS Infrared Image July 28](https://webarchive.library.unt.edu/eot2008/20090831090722im_/http://photojournal.jpl.nasa.gov/figures/PIA00439_fig2_thumb.jpg) |
Figure 1: AIRS VIS Daylight Snapshot July 26 | Figure 2: AIRS Infrared Image July 28 |
Hurricane Darby as observed by NASA's spaceborne Atmospheric Infrared
Sounder (AIRS). This daylight image of Hurricane Darby on July 28 was
made with the visible sensor in the AIRS instrument suite. After reaching
sustained winds on July 27 of 100 knots (115 mph) with gusts to 120 knots
(138 mph), the intensity of the storm is now lowered to 75 knots (86 mph).
Located in the eastern north Pacific Ocean located about 1,165 miles
west-southwest of the southern tip of Baja California, the storm
continues its west/northwest path at 14 knots (16mph). Figure 1 is a
daylight snapshot from AIRS visible/near-infrared sensor before Darby
became a tropical storm. Darby is in the upper right-hand corner.
Circulation is not apparent because the storm was not organized
sufficiently to allow the nascent eye to appear. At this time, winds
were approximately 35 mph. Figure 2 is an AIRS infrared image. Darby
falls on the edge of two AIRS data granules, which have been "stitched"
together in this image. Storm intensity is lowered to 75 knots (86 mph),
down from 100 knots (115 mph).
Frame from July 27 movie, slicing down the atmosphere with the AIRS infrared sensor
The major contribution to radiation (infrared light) that AIRS infrared
channels sense comes from different levels in the atmosphere, depending
upon the channel wavelength. To create the movies, a set of AIRS infrared
channels were selected which probe the atmosphere at progressively deeper
levels. If there were no clouds, the color in each frame would be nearly
uniform until the Earth's surface is encountered. The tropospheric air
temperature warms at a rate of 6 K (about 11 F) for each kilometer of
descent toward the surface. Thus the colors would gradually change from
cold to warm as the movie progresses.
Clouds block the infrared radiation. Thus wherever there are clouds we can
penetrate no deeper in infrared. The color remains fixed as the movie
progresses, for that area of the image is "stuck" to the cloud top
temperature. The coldest temperatures around 220 K (about -65 F) come
from altitudes of about 10 miles.
We therefore see in a 'surface channel' at the end of the movie, signals
from clouds as cold as 220 K and from Earth's surface at 310 K (about 100
F). The very coldest clouds are seen in deep convection thunderstorms over
land.
Movies
July 26 Spiral rain bands are visible
July 27 Spiral rain bands are visible, as well as water vapor stream
coming in from the SW (band of convection cells leading to storm from
lower right).
July 28 Darby is seen in the lower left, right on the edge of the
AIRS data granule.
The Atmospheric Infrared Sounder Experiment, with its visible, infrared,
and microwave detectors, provides a three-dimensional look at Earth's
weather. Working in tandem, the three instruments can make simultaneous
observations all the way down to the Earth's surface, even in the presence
of heavy clouds. With more than 2,000 channels sensing different regions
of the atmosphere, the system creates a global, 3-D map of atmospheric
temperature and humidity and provides information on clouds, greenhouse
gases, and many other atmospheric phenomena. The AIRS Infrared Sounder
Experiment flies onboard NASA's Aqua spacecraft and is managed by NASA's
Jet Propulsion Laboratory, Pasadena, Calif., under contract to NASA. JPL
is a division of the California Institute of Technology in Pasadena.