Disk and circumsolar radiances in the presence of ice clouds
- 1Department of Physics, University of Helsinki, P.O. Box 64, 00014 University of Helsinki, Finland
- 2Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
- 3Department of Atmospheric Science, University of Illinois at Urbana-Champaign, Urbana, 105 South Gregory St., IL 61801-3070, USA
- 4Leibniz Institute for Tropospheric Research, Permoserstraße 15, 04318 Leipzig, Germany
- 5Research Department, Swedish Meteorological and Hydrological Institute, Folkborgsvägen 17, 601 76 Norrköping, Sweden
- 6Department of Earth and Space Science, Chalmers University of Technology, 412 96 Gothenburg, Sweden
- 7Visidyne, Inc., 429 Stanley Drive, Santa Barbara, CA 93105, USA
Abstract. The impact of ice clouds on solar disk and circumsolar radiances is investigated using a Monte Carlo radiative transfer model. The monochromatic direct and diffuse radiances are simulated at angles of 0 to 8° from the center of the sun. Input data for the model are derived from measurements conducted during the 2010 Small Particles in Cirrus (SPARTICUS) campaign together with state-of-the-art databases of optical properties of ice crystals and aerosols. For selected cases, the simulated radiances are compared with ground-based radiance measurements obtained by the Sun and Aureole Measurements (SAM) instrument.
First, the sensitivity of the radiances to the ice cloud properties and aerosol optical thickness is addressed. The angular dependence of the disk and circumsolar radiances is found to be most sensitive to assumptions about ice crystal roughness (or, more generally, non-ideal features of ice crystals) and size distribution, with ice crystal habit playing a somewhat smaller role. Second, in comparisons with SAM data, the ice cloud optical thickness is adjusted for each case so that the simulated radiances agree closely (i.e., within 3 %) with the measured disk radiances. Circumsolar radiances at angles larger than ≈ 3° are systematically underestimated when assuming smooth ice crystals, whereas the agreement with the measurements is better when rough ice crystals are assumed. Our results suggest that it may well be possible to infer the particle roughness directly from ground-based SAM measurements. In addition, the results show the necessity of correcting the ground-based measurements of direct radiation for the presence of diffuse radiation in the instrument's field of view, in particular in the presence of ice clouds.