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| Titel |
Satellite retrieval of the liquid water fraction in tropical clouds between −20 and −38 °C |
| VerfasserIn |
D. L. Mitchell, R. P. D'Entremont |
| 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 ; 5, no. 7 ; Nr. 5, no. 7 (2012-07-18), S.1683-1698 |
| Datensatznummer |
250003008
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| Publikation (Nr.) |
 copernicus.org/amt-5-1683-2012.pdf |
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| Zusammenfassung |
| This study describes a satellite remote sensing method
for directly retrieving the liquid water fraction in mixed phase clouds, and
appears unique in this respect. The method uses MODIS split-window channels
for retrieving the liquid fraction from cold clouds where the liquid water
fraction is less than 50% of the total condensate. This makes use of the
observation that clouds only containing ice exhibit effective
12-to-11 μm absorption optical thickness ratios (βeff) that are
quasi-constant with retrieved cloud temperature T. This observation was made
possible by using two CO2 channels to retrieve T and then using the 12
and 11 μm channels to retrieve emissivities and βeff. Thus
for T < −40 °C, βeff is constant, but for T > −40 °C,
βeff slowly increases due to the
presence of liquid water, revealing mean liquid fractions of ~ 10% around
−22 °C from tropical clouds identified as cirrus by the cloud mask.
However, the uncertainties for these retrievals are large, and extensive in
situ measurements are needed to refine and validate these retrievals. Such
liquid levels are shown to reduce the cloud effective diameter De such
that cloud optical thickness will increase by more than 50% for a given
water path, relative to De corresponding to pure ice clouds. Such
retrieval information is needed for validation of the cloud microphysics in
climate models. Since low levels of liquid water can dominate cloud optical
properties, tropical clouds between −25 and −20 °C may be
susceptible to the first aerosol indirect effect. |
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