A ground-based hyperspectral imaging spectrometer (AisaEAGLE) is applied to measure downward spectral radiance fields with high spatial (1024 spatial pixels within 36.7 • field of view), spectral (488 spectral pixels, 400-970 nm, 1.25 nm full width at half maximum) and temporal (4-30 Hz) resolution. The calibration, measurement, and data 5 evaluation procedures are introduced. A method is presented to retrieve the cirrus optical thickness τ ci using ground-based spectral radiance data collected

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... by AisaEAGLE. On the basis of four measurement cases during the second campaign of the Cloud Aerosol Radiation and tuRbulence of trade wInd cumuli over BArbados (CARRIBA) project in 2011 the spatial inhomogeneity of the investigated cirrus is characterized by 10 the standard deviation of the retrieved τ ci , as well as the width of the frequency distribution of the retrieved τ ci . By comparing measured and simulated downward solar radiance as a function of scattering angle, a first estimation of the detected cirrus ice crystal shape is given and used in the retrieval of the τ ci . The sensitivity of the retrieval method with respect to surface albedo, effective radius r eff , cloud height, and ice crys-15 tal shape was characterized. Significant sensitivities of the retrieval method were found for the assumed surface albedo (up to 30 %) and ice crystal shape (up to 90 %). The sensitivity with regard to the effective radius (≤ 5 %) and the cloud height (≤ 0.5 %) is rather small and can be neglected.