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| Titel |
Cloud phase identification of Arctic boundary-layer clouds from airborne spectral reflection measurements: test of three approaches |
| VerfasserIn |
A. Ehrlich, E. Bierwirth, M. Wendisch, J.-F. Gayet, G. Mioche, A. Lampert, J. Heintzenberg |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 8, no. 24 ; Nr. 8, no. 24 (2008-12-16), S.7493-7505 |
| Datensatznummer |
250006513
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| Publikation (Nr.) |
 copernicus.org/acp-8-7493-2008.pdf |
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| Zusammenfassung |
Arctic boundary-layer clouds were investigated with remote sensing and in
situ instruments during the Arctic Study of Tropospheric Aerosol, Clouds and
Radiation (ASTAR) campaign in March and April 2007. The clouds formed
in a cold air outbreak over the open Greenland Sea. Beside the predominant
mixed-phase clouds pure liquid water and ice clouds were observed. Utilizing
measurements of solar radiation reflected by the clouds three methods to
retrieve the thermodynamic phase of the cloud are introduced and compared.
Two ice indices IS and IP were obtained by
analyzing the spectral pattern of the cloud top reflectance in the near
infrared (1500–1800 nm wavelength) spectral range which is characterized by
ice and water absorption. While IS analyzes the spectral slope
of the reflectance in this wavelength range, IS utilizes a
principle component analysis (PCA) of the spectral reflectance. A third ice
index IA is based on the different side scattering of spherical
liquid water particles and nonspherical ice crystals which was recorded in
simultaneous measurements of spectral cloud albedo and reflectance.
Radiative transfer simulations show that IS, IP
and IA range between 5 to 80, 0 to 8 and 1 to 1.25 respectively
with lowest values indicating pure liquid water clouds and highest values
pure ice clouds. The spectral slope ice index IS and the PCA
ice index IP are found to be strongly sensitive to the
effective diameter of the ice crystals present in the cloud. Therefore, the
identification of mixed-phase clouds requires a priori knowledge of the ice
crystal dimension. The reflectance-albedo ice index IA is
mainly dominated by the uppermost cloud layer (τ<1.5). Therefore,
typical boundary-layer mixed-phase clouds with a liquid cloud top layer will
be identified as pure liquid water clouds. All three methods were applied to
measurements above a cloud field observed during ASTAR 2007. The
comparison with independent in situ microphysical measurements shows the
ability of the three approaches to identify the ice phase in Arctic
boundary-layer clouds. |
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