Measurements
The InfraRed Atmospheric Sounding Interferometers (IASI) aboard the EUMETSAT platforms Metop-A, Metop-B and Metop-C provide very high spectral resolution measurements (8461 samples) of Earth emitted radiation between 3.6 and 15.5 mm, since October 2007. 2 x 2 IASI measurements correspond to one footprint of the Advanced Microwave Sounding Unit (AMSU-A).
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
9:30 AM and 9:30 PM for Metop-A (2008-2018) and Metop-B (2013-2018)
Spatial Resolution
12 km at nadir
Cloud Detection
a posteriori: spectral coherence of retrieved cloud emissivity (6 channels between 9 and 12 μm). Only clouds with CEM > 0.1 were kept in CIRS version 1. CIRS version 2 (provided in the GEWEX database) keeps high-level clouds with CEM > 0.05 and lower clouds with CEM > 0.1. The difference in high-level cloud amount between both versions is negligible on global average, but CAH in the deep tropics increased from 0.39 to 0.41.
Retrieval Methodology
The CIRS (Clouds from IR Sounders) retrieval, developed at LMD, is being operated on AIRS and IASI L1 data and is easily adaptable to any other IR sounder. The synergy between AIRS and IASI allows to study the diurnal cycle of high-level clouds.
CP and CEM are determined from 8 CO2 absorbing channels (similar to AIRS) by a weighted χ2 method based on spectral emissivity coherence. 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 weighting 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 ancillary atmospheric T and virtual T profiles. COD = – cphase ln (1 – CEM), with cliquid = 2.56 and cice = 2.66, as in ISCCP. Ice clouds are defined by CT < 250K and liquid clouds by CT > 260K.
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
- atmospheric T and H2O profiles, surface T from ECMWF meteorological reanalysis data (ERA Interim, available until mid-2019)
- spectral atmospheric transmissivities from TIGR database
- 4A/OP radiative transfer (https://4aop.aeris-data.fr/)
- spectral surface emissivities (monthly climatology) from IASI
- single scattering properties of ice crystals (hex. columns, aggregates, Baran), bimodal size distribution, 4A-DISORT radiative transfer
References
Stubenrauch, C. J., A. G. Feofilov, E.-S. Protopapadaki, and R. Armante, 2017: Cloud climatologies from the InfraRed Sounders AIRS and IASI: Strengths and Applications, Atmosph. Chem. Phys., 17, 13625-13644, doi :10.5194/acp-17-13625-2017.
Feofilov, A. G., and C. J. Stubenrauch, 2019: Diurnal variation of high-level clouds from the synergy of AIRS and IASI space-borne infrared sounders, Atmos. Chem. Phys., 19, 13957–13972, doi:10.5194/acp-19-13957-2019.
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, doi:10.5194/acp-12-503-2012.
L2 Data Distribution
CIRS Version 1 can be downloaded from the AERIS Data and Services Center at https://cirs.aeris-data.fr/