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| Titel |
Airborne hyperspectral observations of surface and cloud directional reflectivity using a commercial digital camera |
| VerfasserIn |
A. Ehrlich, E. Bierwirth, M. Wendisch, A. Herber, J.-F. Gayet |
| 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 ; 12, no. 7 ; Nr. 12, no. 7 (2012-04-11), S.3493-3510 |
| Datensatznummer |
250011024
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| Publikation (Nr.) |
 copernicus.org/acp-12-3493-2012.pdf |
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| Zusammenfassung |
Spectral radiance measurements by a digital single-lens reflex
camera were used to derive the directional reflectivity of clouds
and different surfaces in the Arctic. The camera has been
calibrated radiometrically and spectrally to provide accurate
radiance measurements with high angular resolution. A comparison
with spectral radiance measurements with the Spectral Modular
Airborne Radiation measurement sysTem (SMART-Albedometer) showed
an agreement within the uncertainties of both instruments (6%
for both). The directional reflectivity in terms of the
hemispherical directional reflectance factor (HDRF) was obtained
for sea ice, ice-free ocean and clouds. The sea ice, with an
albedo of ρ = 0.96 (at 530 nm wavelength), showed an almost
isotropic HDRF, while sun glint was observed for the ocean
HDRF (ρ = 0.12). For the cloud observations with ρ = 0.62,
the cloudbow – a backscatter feature typically for scattering by
liquid water droplets – was covered by the camera. For measurements
above heterogeneous stratocumulus clouds, the required number of
images to obtain a mean HDRF that clearly exhibits the cloudbow
has been estimated at about 50 images (10 min flight time). A
representation of the HDRF as a function of the scattering angle
only reduces the image number to about 10 (2 min flight time).
The measured cloud and ocean HDRF have been compared to
radiative transfer simulations. The ocean HDRF simulated with
the observed surface wind speed of 9 m s−1 agreed best with
the measurements. For the cloud HDRF, the best agreement was
obtained by a broad and weak cloudbow simulated with a cloud
droplet effective radius of Reff = 4 μm. This value
agrees with the particle sizes derived from in situ measurements
and retrieved from the spectral radiance of the SMART-Albedometer. |
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