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Abstract

We recently developed a novel retrieval of cloud top heights and particle size distributions from polarized 2D observations by our airborne specMACS instrument (Kölling et al. 2019; Pörtge et al., 2022). The cloud height is derived by a stereographic approach and effective droplet radius and width of the size distribution are derived from the shape of the cloudbow. Validation of such methods with independent observations is difficult because in-situ observations with sufficient co-location in space and time are rarely available. An alternative is the use of radiative transfer simulations based on the output of high-resolution cloud models where not only the observations but also the "truth" is known. Realistic 3D radiative transfer simulations of specMACS measurements were performed with the 3D radiative transport modelMYSTIC (Mayer, 2009). Overflights over a field of shallow cumuli based on high-resolutionLES simulations carried out with the PALM model were simulated. The clouds develop realistically as the aircraft passed overhead. It is shown that the cloud top heights differ by less than 40m from the model truth. The uncertainty of the retrieved effective radius was shown to be a few micron for complex droplet size distributions within the field-of-view of the instrument. The consideration of the mean wind by the retrieval was shown to be crucial for the accuracy of the results. The temporal evolution of the clouds adds some uncertainty which, in contrast to the effect of the wind cannot be corrected.