Airborne hyperspectral surface and cloud bi-directional reflectivity observations in the Arctic using a commercial, digital camera
1Leipzig Institute for Meteorology (LIM), University of Leipzig, Leipzig, Germany
3Alfred Wegener Institute for Polar and Marine Research (AWI), Potsdam, Germany
4Laboratoire de Météorologie Physique (LAMP), Université Blaise Pascal, Aubière Cedex, France
Abstract. Spectral radiance measurements by a digital single-lens reflex camera were used to derive the bi-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 SMART-Albedometer showed an agreement within the uncertainties of both instruments. The bi-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, 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 fog bow – a backscatter feature typically for scattering by liquid water droplets – was covered by the camera. For measurements above a heterogeneous stratocumulus clouds, the required number of images to obtain a mean HDRF which clearly exhibits the fog bow has been estimated with about 50 images (10 min flight time). A representation of the HDRF as 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 fog bow simulated with a cloud droplet effective radius of Reff = 4 μm. This value agrees with the particle sizes from in situ measurements and retrieved from the spectral radiance of the SMART-Albedometer.