Four images of Tropical Cyclone Ramasun were obtained July 3, 2002 by the
Atmospheric Infrared Sounder experiment system onboard NASA's Aqua
spacecraft. The AIRS experiment, with its wide spectral coverage in four
diverse bands, provides the ability to obtain complete 3-D observations
of severe weather, from the surface, through clouds to the top of the
atmosphere with unprecedented accuracy. This accuracy is the key to
understanding weather patterns and improving weather predictions.
Viewed separately, none of these images can provide accurate 3-D
descriptions of the state of the atmosphere because of interference from
clouds. However, the ability to make simultaneous observations at a wide
range of wavelengths allows the AIRS experiment to "see" through clouds.
This visible light picture from the AIRS instrument provides important
information about the location of the cyclone, cloud structure and
distribution.
The AIRS instrument image at 900 cm-1 (Figure 1) is from a 10 micron
transparent "window channel" that is little affected by water vapor but
still cannot see through clouds. In clear areas (like the eye of the
cyclone and over northwest Australia) it measures a surface temperature
of about 300K (color encoded red). In cloudy areas it measures the cloud
top temperature, about 200K for the cyclone, which translates to a cloud
top height of about 50,000 feet.
On the other hand, most clouds are relatively transparent in microwave,
and the Advanced Microwave Sounding Instrument channel image (Figure 2)
can see through all but the densest clouds. For example, Taiwan, which is
covered by clouds, is clearly visible.
The Humidity Sounder for Brazil instrument channel (Figure 3), also in
the microwave, is more sensitive to both clouds and humidity. Only in
clear, dry regions, such as the eye of the cyclone or the area north of
Australia, does it see the surface. It is also severely affected by
suspended ice particles formed by strong convection, which causes
scattering and appears to be extremely cold. These blue areas indicate
intense precipitation.
The Atmospheric Infrared Sounder is an instrument onboard NASA's Aqua
satellite under the space agency's Earth Observing System. The sounding
system is making highly accurate measurements of air temperature,
humidity, clouds and surface temperature. Data will be used to better
understand weather and climate. It will also be used by the National
Weather Service and the National Oceanic and Atmospheric Administration
to improve the accuracy of their weather and climate models.
The instrument was designed and built by Lockheed Infrared Imaging
Systems (recently acquired by British Aerospace) under contract with JPL.
The Aqua satellite mission is managed by NASA's Goddard Space Flight
Center.
