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Airborne hyperspectral surface and cloud bi-directional reflectivity observations in the Arctic using a commercial, digital camera

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Ehrlich, A. , Bierwirth, E. , Wendisch, M. , Herber, A. and Gayet, J. F. (2012): Airborne hyperspectral surface and cloud bi-directional reflectivity observations in the Arctic using a commercial, digital camera , Atmospheric Chemistry and Physics., 12 , pp. 3493-3510 . doi: 10.5194/acp-12-3493-2012
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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 measure- ments with the Spectral Modular Airborne Radiation mea- surement sysTem (SMART-Albedometer) showed an agree- ment within the uncertainties of both instruments (6 % for both). The directional reflectivity in terms of the hemispher- ical 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 al- most 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 cam- era. For measurements above heterogeneous stratocumu- lus 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 com- pared 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 cloud- bow 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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