The infrared region of the spectrum that is used for atmospheric
remote sensing ranges from around 600 to 2800 cm-1
(16.6 to 3.3 microns in wavelength). In this part of the spectrum
we can use CO2 spectral bands at 15 and 4.3 microns
to give us information on the temperature structure of the atmosphere.
Information on water vapour content can be gained from a large
number of H2O lines between 5 and 8 microns. The CO2
and H2O bands have been used for many years in satellite
sounding applications. In addition, parts of the infrared spectrum
are sensitive to ozone (8.9-10.1 micron band), and other trace
gases such as CH4 and N2O.
The Infrared region offers a complementary data set to the microwave
frequencies. Clouds are opaque in the infrared, meaning that
to date it has not been possible to use these data in cloudy regions.
However the new generation of infrared sounding instruments contain
information at a greater vertical resolution than the microwave
The High-resolution InfraRed Sounder (HIRS), a 20 channel radiometer,
has been used operationally at the Met Office for many years to
provide information on the temperature and water vapour structure
of the atmosphere. Together with microwave instruments AMSU-A
and AMSU-B, HIRS forms part of the ATOVS sounding system (on NOAA
and Metop satellites), which is the backbone of satellite data
assimilation for numerical weather prediction.
Sounding page contains more information about the use of ATOVS
data at the Met Office.
|High Spectral Resolution
The next generation of infrared sounding instruments are now
beginning to be launched. These instruments, such as the Atmospheric
Infrared Sounder (AIRS) on EOS-Aqua and the Infrared Atmospheric
Sounding Interferometer (IASI) on Metop, boast very high spectral
resolution (0.4-2.1 cm-1 for AIRS; 0.25-0.5 cm-1
for IASI) and thousands of individual channels. EOS-Aqua was launched
in 2002, providing us with AIRS data which has been assimilated
to good effect since May 2004.