Application of ground-based hyperspectral imaging to retrieve ice crystal shape and fields of cirrus optical thickness

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 evaluation procedures are introduced. A method is presented to retrieve the cirrus optical thickness τci using ground-based spectral radiance data collected 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 the standard deviation of the retrieved τci, as well as the width of the frequency distribution of the retrieved τci. 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 τci. The sensitivity of the retrieval method with respect to surface albedo, effective radius reff, cloud height, and ice crystal 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.