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| Titel |
Airborne observations and simulations of three-dimensional radiative interactions between Arctic boundary layer clouds and ice floes |
| VerfasserIn |
M. Schäfer, E. Bierwirth, A. Ehrlich, E. Jäkel, M. Wendisch |
| 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 ; 15, no. 14 ; Nr. 15, no. 14 (2015-07-23), S.8147-8163 |
| Datensatznummer |
250119923
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| Publikation (Nr.) |
 copernicus.org/acp-15-8147-2015.pdf |
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| Zusammenfassung |
Based on airborne spectral imaging observations, three-dimensional
(3-D) radiative effects between Arctic boundary layer clouds and highly variable Arctic
surfaces were identified and quantified. A method is presented to
discriminate between sea ice and open water under cloudy conditions based on airborne nadir reflectivity γλ
measurements in the visible spectral range.
In cloudy cases the transition of γλ from open water to
sea ice is not instantaneous but horizontally smoothed. In
general, clouds reduce γλ above bright surfaces in
the vicinity of open water, while
γλ above open sea is enhanced. With the help of
observations and 3-D radiative transfer simulations, this effect
was quantified to range between 0 and 2200 m distance to the sea ice edge
(for a dark-ocean albedo of αwater = 0.042
and a sea-ice albedo of αice = 0.91 at 645 nm wavelength). The affected distance Δ L was found to depend
on both cloud and sea ice properties. For a low-level cloud
at 0–200 m altitude, as observed during the Arctic field
campaign VERtical Distribution of Ice in Arctic clouds (VERDI) in 2012,
an increase in the cloud optical thickness τ
from 1 to 10 leads to a decrease in Δ L from 600 to
250 m. An increase in the cloud base altitude or cloud
geometrical thickness results in an increase in Δ L;
for τ = 1/10 Δ L = 2200 m/1250 m in case of a cloud at
500–1000 m altitude. To quantify the effect for
different shapes and sizes of ice floes, radiative transfer simulations were performed with various albedo fields
(infinitely long straight ice edge, circular ice floes, squares, realistic ice floe
field). The simulations show that Δ L increases with
increasing radius of the ice floe and reaches maximum values for ice floes with radii larger than 6 km
(500–1000 m cloud altitude), which matches the results found for an infinitely long, straight ice edge.
Furthermore, the influence of these 3-D radiative effects on the retrieved
cloud optical properties was investigated. The enhanced
brightness of a dark pixel next to an ice edge results in
uncertainties of up to 90 and 30 % in retrievals of τ
and effective radius reff, respectively.
With the help of Δ L, an estimate of the distance to the ice
edge is given, where the retrieval uncertainties due to 3-D radiative effects are negligible. |
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