AIRS-LMD Description

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

  • retrieved T and H2O profiles (NASA AIRS): instantaneous or average
  • spectral atmospheric transmissivities from TIGR database
  • 4A radiative transfer
  • spectral surface emissivities from AIRS and MODIS
  • single scattering properties of ice crystals (hex. columns, aggregates, Baran), bimodal size distribution, 4A-DISORT radiative transfer

  • References

  • Stubenrauch. C. J., S. Cros. A. Guignard, and N. Lamquin, 2010: A six-year global cloud climatology from the Atmospheric InfraRed Sounder aboard the AQUA Satellite: statistical analysis in synergy with CALIPSO and CloudSat. Atmos. Chem. Phys., 10, 7197-7214.
  • Guignard, A., C. J. Stubenrauch, A. J. Baran, and R. Armante, 2012: Bulk microphysical properties of semi-transparent cirrus from AIRS: a six year global climatology and statistical analysis in synergy with geometrical profiling data from CloudSat-CALIPSO. Atmos, Chem. Phys., 12, 503-525.
  • L2 Data Distribution

    The files can be downloaded from the ICARE Data and Services Center (University of Lille).