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Journal article

Airborne hyperspectral observations of surface and cloud directional reflectivity using a commercial digital camera

Ehrlich A, Bierwirth E, Wendisch M, Herber A, Gayet J ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 7 (2012) pp. 3493-3510

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Abstract

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 = 0.96
(at 530 nm wavelength), showed an almost isotropic HDRF, while sun glint
was observed for the ocean HDRF ( = 0.12). For the cloud observations
with = 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 R-eff = 4 mu m. 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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Authors

  • Andreas HerberAlfred Wegener Institute for Polar and Marine Research Research Unit Potsdam

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  • A. Ehrlich

  • E. Bierwirth

  • M. Wendisch

  • J. F. Gayet

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