Collocated Airborne Measurements of Radiative Properties of Cirrus Layers

A new innovative approach to study inhomogeneous cirrus by collocated airborne radiation and microphysical measurements is presented. The close collocation of the measurements, above, beneath and inside the cirrus cloud, is obtained by two platforms connected to each other: a research aircraft (Learjet) and the towed platform AIRTOSS (AIRcraft TOwed Sensor Shuttle). The AIRTOSS can be released from and retracted to the aircraft by means of a towing cable up to a distance of 4 km. In the framework of the AIRTOSS – Project (a cooperation with the University and the Max Planck Institute for Chemistry in Mainz, and the Forschungszentrum Jülich GmbH) two field campaigns were performed in spring and late summer 2013 above the North Sea. First preliminary results illustrate the quality of the measurements which emphasizes the benefit of such collocated measurements. The effects of cloud optical layer properties on the radiative energy budget of cirrus are studied by measurements of spectral up- and downward solar irradiances. From the collocated measurements spectral transmissivity, absorptivity and reflectivity of the observed cirrus layer are derived directly and set into relation to the in situ measurements of cirrus microphysical properties. To constrain the cloud radiative effect found in the measurements and to improve our understanding of the impact of cloud inhomogeneities we applied additional radiative transfer simulations. Using the measured cirrus microphysical properties as input of the simulations in a first step a good agreement between measured and simulated cloud radiative properties was obtained. This illustrates, that the new approach of collocated measurements is well suited for such closure experiments and will help to investigate cloud inhomogeneities in future stages of the presented study.