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

by A. Ehrlich, E. Bierwirth, M. Wendisch, A. Herber, J. F. Gayet
Atmospheric Chemistry and Physics ()
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Spectral radiance measurements by a digital single-lens reflex camera\nwere used to derive the directional reflectivity of clouds and different\nsurfaces in the Arctic. The camera has been calibrated radiometrically\nand spectrally to provide accurate radiance measurements with high\nangular resolution. A comparison with spectral radiance measurements\nwith the Spectral Modular Airborne Radiation measurement sysTem\n(SMART-Albedometer) showed an agreement within the uncertainties of both\ninstruments (6% for both). The directional reflectivity in terms of the\nhemispherical directional reflectance factor (HDRF) was obtained for sea\nice, ice-free ocean and clouds. The sea ice, with an albedo of = 0.96\n(at 530 nm wavelength), showed an almost isotropic HDRF, while sun glint\nwas observed for the ocean HDRF ( = 0.12). For the cloud observations\nwith = 0.62, the cloudbow - a backscatter feature typically for\nscattering by liquid water droplets - was covered by the camera. For\nmeasurements above heterogeneous stratocumulus clouds, the required\nnumber of images to obtain a mean HDRF that clearly exhibits the\ncloudbow has been estimated at about 50 images (10 min flight time). A\nrepresentation of the HDRF as a function of the scattering angle only\nreduces the image number to about 10 (2 min flight time).\nThe measured cloud and ocean HDRF have been compared to radiative\ntransfer simulations. The ocean HDRF simulated with the observed surface\nwind speed of 9 m s(-1) agreed best with the measurements. For the cloud\nHDRF, the best agreement was obtained by a broad and weak cloudbow\nsimulated with a cloud droplet effective radius of R-eff = 4 mu m. This\nvalue agrees with the particle sizes derived from in situ measurements\nand retrieved from the spectral radiance of the SMART-Albedometer.

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