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

摘要

Spectral radiance measurements by a digital single-lens reflexcamera were used to derive the directional reflectivity of cloudsand different surfaces in the Arctic. The camera has beencalibrated radiometrically and spectrally to provide accurateradiance measurements with high angular resolution. A comparisonwith spectral radiance measurements with the Spectral ModularAirborne Radiation measurement sysTem (SMART-Albedometer) showedan agreement within the uncertainties of both instruments (6%for both). The directional reflectivity in terms of thehemispherical directional reflectance factor (HDRF) was obtainedfor sea ice, ice-free ocean and clouds. The sea ice, with analbedo of ρ = 0.96 (at 530 nm wavelength), showed an almostisotropic HDRF, while sun glint was observed for the oceanHDRF (ρ = 0.12). For the cloud observations with ρ = 0.62,the cloudbow – a backscatter feature typically for scattering byliquid water droplets – was covered by the camera. For measurementsabove heterogeneous stratocumulus clouds, the required number ofimages to obtain a mean HDRF that clearly exhibits the cloudbowhas been estimated at about 50 images (10 min flight time). Arepresentation of the HDRF as a function of the scattering angleonly reduces the image number to about 10 (2 min flight time).The measured cloud and ocean HDRF have been compared toradiative transfer simulations. The ocean HDRF simulated withthe observed surface wind speed of 9 m s?1 agreed best withthe measurements. For the cloud HDRF, the best agreement wasobtained by a broad and weak cloudbow simulated with a clouddroplet effective radius of R_eff = 4 μm. This valueagrees with the particle sizes derived from in situ measurementsand retrieved from the spectral radiance of the SMART-Albedometer.