Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2016-967
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
25 Nov 2016
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
Disk and circumsolar radiances in the presence of ice clouds
Päivi Haapanala1, Petri Räisänen2, Greg M. McFarquhar3, Jussi Tiira1, Andreas Macke4, Michael Kahnert5,6, John DeVore7, and Timo Nousiainen2 1Department of Physics, University of Helsinki, P.O. Box 48, 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 S. 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 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 with the Sun and Aureole Measurement (SAM) instrument.

First, the sensitivity of the radiances to the ice cloud properties and aerosol optical thickness was addressed. The angular dependence of the disk and circumsolar radiances was 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 the comparisons with SAM data, the ice-cloud optical thickness was adjusted for each case so that the simulated radiances agreed closely (i.e., within 3 %) with the measured disk radiances. Circumsolar radiances at angles larger than ≈ 3° were systematically underestimated when assuming smooth ice crystals, but the agreement with the measurements was better when rough ice crystals were assumed. Our results suggest that it may well be possible to infer the particle roughness (or more generally, non-ideality) 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.


Citation: Haapanala, P., Räisänen, P., McFarquhar, G. M., Tiira, J., Macke, A., Kahnert, M., DeVore, J., and Nousiainen, T.: Disk and circumsolar radiances in the presence of ice clouds, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-967, in review, 2016.
Päivi Haapanala et al.
Päivi Haapanala et al.
Päivi Haapanala et al.

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Short summary
The dependence of solar-disk and circumsolar radiances on ice cloud properties is studied with a Monte Carlo radiative transfer model. Ice crystal roughness (or more generally, non-ideality) is found to be the most important parameter influencing the circumsolar radiance, and ice crystal sizes and shapes also play significant roles. When comparing with radiances measured with the SAM instrument, rough ice crystals reproduce the measurements better than idealized smooth ice crystals do.
The dependence of solar-disk and circumsolar radiances on ice cloud properties is studied with...
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