Measurements
The Atmospheric InfraRed Sounder (AIRS) aboard the NASA EOS platform AQUA provides very high spectral resolution measurements
(2378 channels) of Earth emitted radiation
in 3 spectral bands in the IR since May 2002. 3 x 3 AIRS measurements correspond to one footprint of the Advanced Microwaive
Sounding Unit (AMSU).
By assuming that CO2 is mixed uniformly in the atmosphere, absorption and emission due to CO2 can be
used to sense the temperature
and the uppermost cloud level in the atmosphere: measured radiances from near the center of a CO2 absorption
band are sensitive only to the upper atmosphere while radiances from the wings of the band see successively lower levels
of the atmosphere.
Local Observation Time and Length of Data Record
1:30 AM and 1:30 PM ; 2003-2009, microphysics available for 2004-2009
Spatial Resolution
14 km
Cloud Detection
a posteriori: CEM > 0.05, spectral emissivity coherence (between 9 and 12 μm),
and over land or snow: CT-Tsurf(air) < -3 K.
Retrieval Methodology
CP and CEM are determined from 8 CO2 absorbing channels by a weighted χ2 method based on spectral
emissivity coherence (as TOVS PathB).
Therefore spectral emissivities are calculated from measured radiance, clear sky radiance and radiance of an opaque cloud
for 40 pressure levels.
The method takes into account the vertical weigthing of the different channels and the growing uncertainty in the
computation of CEM with increasing pressure.
CT and CZ are derived from CP using the retrieved atmospheric T and virtual
T profiles. COD = -2 ln ( 1 - CEM ). Ice clouds are defined by CT < 230 K.
CREIH and CIWPH of semi-transparent cirrus ( 0.2 < CEM < 0.85 )
are retrieved from 6 spectral cirrus emissivities between 8 and 12 μm, applying a Look-Up Table approach.
Ancillary Input
References
L2 Data Distribution
The files can be downloaded from the
ICARE Data and Services Center (University of Lille).