Retrieval of cirrus optical thickness and assessment of ice crystal shape from ground-based imaging spectrometry

A ground-based hyperspectral imaging spectrometer (AisaEAGLE, manufactured by Specim Ltd., Finland) 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. A new method is presented to retrieve the cirrus optical thickness (τ_ci) using the spectral radiance data collected by AisaEAGLE. The data were collected during 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 τ_ci as well as the width of its frequency distribution. By comparing measured and simulated downward solar spectral radiance as a function of scattering angle, some evidence of the prevailing cirrus ice crystal shape can be obtained and subsequently used to substantiate the retrieval of τ_ci. The sensitivity of the retrieval method with respect to surface albedo, effective radius (r_eff), cloud height and ice crystal shape is quantified. An enhanced sensitivity of the retrieved τ_ci is found with respect to the surface albedo (up to 30%) and ice crystal shape (up to 90%). The sensitivity with regard to the effective ice crystal radius (5%) and the cloud height (0.5%) is rather small and can be neglected.