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The impact of ice clouds on solar disk and circumsolar radiances is\udinvestigated using a Monte Carlo radiative transfer model. The monochromatic\uddirect and diffuse radiances are simulated at angles of 0 to 8° from\udthe center of the sun. Input data for the model are derived from measurements\udconducted during the 2010 Small Particles in Cirrus (SPARTICUS) campaign\udtogether with state-of-the-art databases of optical properties of ice\udcrystals and aerosols. For selected cases, the simulated radiances are\udcompared with ground-based radiance measurements obtained by the Sun and\udAureole Measurements (SAM) instrument.\ud  \udFirst, the sensitivity of the radiances to the ice cloud properties and\udaerosol optical thickness is addressed. The angular dependence of the disk\udand circumsolar radiances is found to be most sensitive to assumptions about\udice crystal roughness (or, more generally, non-ideal features of ice\udcrystals) and size distribution, with ice crystal habit playing a somewhat\udsmaller role. Second, in comparisons with SAM data, the ice cloud optical\udthickness is adjusted for each case so that the simulated radiances agree\udclosely (i.e., within 3 %) with the measured disk radiances. Circumsolar\udradiances at angles larger than ≈ 3° are systematically\udunderestimated when assuming smooth ice crystals, whereas the agreement with\udthe measurements is better when rough ice crystals are assumed. Our results\udsuggest that it may well be possible to infer the particle roughness directly\udfrom ground-based SAM measurements. In addition, the results show the\udnecessity of correcting the ground-based measurements of direct radiation for\udthe presence of diffuse radiation in the instrument's field of view, in\udparticular in the presence of ice clouds

Disk and circumsolar radiances in the presence of ice clouds