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| Titel |
On cloud ice induced absorption and polarisation effects in microwave limb sounding |
| VerfasserIn |
P. Eriksson, B. Rydberg, S. A. Buehler |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 4, no. 6 ; Nr. 4, no. 6 (2011-06-30), S.1305-1318 |
| Datensatznummer |
250002024
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| Publikation (Nr.) |
 copernicus.org/amt-4-1305-2011.pdf |
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| Zusammenfassung |
| Microwave limb sounding in the presence of ice clouds was studied by detailed
simulations, where clouds and other atmospheric variables varied in three
dimensions and the full polarisation state was considered. Scattering
particles were assumed to be horizontally aligned oblate spheroids with a size
distribution parameterized in terms of temperature and ice water content.
A general finding was that particle absorption is significant for limb
sounding, which is in contrast to the down-looking case, where it is usually
insignificant. Another general finding was that single scattering can be
assumed for cloud optical paths below about 0.1, which is thus an important
threshold with respect to the complexity and accuracy of retrieval algorithms.
The representation of particle sizes during the retrieval is also discussed.
Concerning polarisation, specific findings were as follows: Firstly, no
significant degree of circular polarisation was found for the considered
particle type. Secondly, for the ±45° polarisation components,
differences of up to 4 K in brightness temperature were found, but
differences were much smaller when single scattering conditions
applied. Thirdly, the vertically polarised component has the smallest cloud
extinction.
An important goal of the study was to derive recommendations for future limb
sounding instruments, particularly concerning their polarisation setup. If
ice water content is among the retrieval targets (and not just trace gas
mixing ratios), then the simulations show that it should be best to observe
any of the ±45° and circularly polarised components. These pairs of
orthogonal components also make it easier to combine information measured from
different positions and with different polarisations. |
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