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| Titel |
Determination of optical and microphysical properties of thin warm clouds using ground based hyper-spectral analysis |
| VerfasserIn |
E. Hirsch, E. Agassi, I. Koren |
| 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. 4 ; Nr. 5, no. 4 (2012-04-27), S.851-871 |
| Datensatznummer |
250002783
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| Publikation (Nr.) |
 copernicus.org/amt-5-851-2012.pdf |
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| Zusammenfassung |
| Clouds play a critical role in the Earth's radiative budget as they modulate
the atmosphere by reflecting shortwave solar radiation and absorbing long
wave IR radiation emitted by the Earth's surface. Although extensively
studied for decades, cloud modelling in global circulation models is far
from adequate, mostly due to insufficient spatial resolution of the
circulation models. In addition, measurements of cloud properties still need
improvement, since the vast majority of remote sensing techniques are
focused in relatively large, thick clouds. In this study, we utilize ground
based hyperspectral measurements and analysis to explore very thin water
clouds. These clouds are characterized by liquid water path (LWP) that spans
from as high as ~50g m−2 and down to 65 mg m−2 with a minimum
of about 0.01 visible optical depth. The retrieval methodology relies on
three elements: a detailed radiative transfer calculations in the longwave
IR regime, signal enhancement by subtraction of a clear sky reference, and
spectral matching method which exploits fine spectral differences between
water droplets of different radii. A detailed description of the theoretical
basis for the retrieval technique is provided along with a comprehensive
discussion regarding its limitations. The proposed methodology was validated
in a controlled experiment where artificial clouds were sprayed and their
effective radii were both measured and retrieved simultaneously. This
methodology can be used in several ways: (1) the frequency and optical
properties of very thin water clouds can be studied more precisely in order
to evaluate their total radiative forcing on the Earth's radiation budget.
(2) The unique optical properties of the inter-region between clouds (clouds'
"twilight zone") can be studied in order to more rigorously understanding of
the governing physical processes which dominate this region. (3) Since the
optical thickness of a developed cloud gradually decreases towards its
edges, the proposed methodology can be used to study the spatial
microphysical behaviour of these edges. (4) A spatial-temporal analysis can
be used to study mixing processes in clouds' entrainment zone. |
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