Cloud climatologies from the infrared sounders AIRS and IASI: strengths and applications
Global cloud climatologies have been built from 13 years of Atmospheric Infrared Sounder (AIRS) and 8 years of Infrared Atmospheric Sounding Interferometer (IASI) observations, using an updated Clouds from Infrared Sounders (CIRS) retrieval. The CIRS software can handle any infrared (IR) sounder...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-11-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/13625/2017/acp-17-13625-2017.pdf |
| Summary: | Global cloud climatologies have been built from 13 years of Atmospheric
Infrared Sounder (AIRS) and 8 years of Infrared Atmospheric Sounding
Interferometer (IASI) observations, using an updated Clouds from Infrared
Sounders (CIRS) retrieval. The CIRS software can handle any infrared (IR)
sounder data. Compared to the original retrieval, it uses improved radiative
transfer modelling, accounts for atmospheric spectral transmissivity changes
associated with CO<sub>2</sub> concentration and incorporates the latest ancillary
data (atmospheric profiles, surface temperature and emissivities). The global
cloud amount is estimated to be 0.67–0.70, for clouds with IR optical depth
larger than about 0.1. The spread of 0.03 is associated with ancillary data.
Cloud amount is partitioned into about 40 % high-level clouds, 40 %
low-level clouds and 20 % mid-level clouds. The latter two categories are
only detected in the absence of upper clouds. The A-Train active instruments,
lidar and radar of the CALIPSO and CloudSat missions, provide a unique
opportunity to evaluate the retrieved AIRS cloud properties. CIRS cloud
height can be approximated either by the mean layer height (for optically
thin clouds) or by the mean between cloud top and the height at which the
cloud reaches opacity. This is valid for high-level as well as for low-level
clouds identified by CIRS. IR sounders are particularly advantageous to
retrieve upper-tropospheric cloud properties, with a reliable cirrus
identification, day and night. These clouds are most abundant in the tropics,
where high opaque clouds make up 7.5 %, thick cirrus 27.5 % and thin
cirrus about 21.5 % of all clouds. The 5 % annual mean excess in
high-level cloud amount in the Northern compared to the Southern Hemisphere
has a pronounced seasonal cycle with a maximum of 25 % in boreal summer,
in accordance with the moving of the ITCZ peak latitude, with annual mean of
4° N, to a maximum of 12° N. This suggests that this excess
is mainly determined by the position of the ITCZ. Considering interannual
variability, tropical cirrus are more frequent relative to all clouds when
the global (or tropical) mean surface gets warmer. Changes in relative amount
of tropical high opaque and thin cirrus with respect to mean surface
temperature show different geographical patterns, suggesting that their
response to climate change might differ. |
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| ISSN: | 1680-7316 1680-7324 |