The impact of ice crystal shapes, size distributions and spatial structures of cirrus clouds on solar radiative fluxes.

Schlimme, Ingo, Macke, Andreas and Reichardt, J. (2005) The impact of ice crystal shapes, size distributions and spatial structures of cirrus clouds on solar radiative fluxes. Open Access Journal of the Atmospheric Sciences, 62 (7). pp. 2274-2283. DOI 10.1175/JAS3459.1.

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The solar radiative properties of cirrus clouds depend on ice particle shape, size, and orientation, as well as on the spatial cloud structure. Radiation schemes in atmospheric circulation models rely on estimates of cloud optical thickness only. In the present work, a Monte Carlo radiative transfer code is applied to various cirrus cloud scenarios to obtain the radiative response of uncertainties in the above-mentioned microphysical and spatial cloud properties (except orientation). First, plane-parallel homogeneous (0D) clouds with different crystal shapes (hexagonal columns, irregular polycrystals) and 114 different size distributions have been considered. The resulting variabilities in the solar radiative fluxes are in the order of a few percent for the reflected and about 1% for the diffusely transmitted fluxes. Largest variabilities in the order of 10% to 30% are found for the solar broadband absorptance. However, these variabilities are smaller than the flux differences caused by the choice of ice particle geometries.

The influence of cloud inhomogeneities on the radiative fluxes has been examined with the help of time series of Raman lidar extinction coefficient profiles as input for the radiative transfer calculations. Significant differences between results for inhomogeneous and plane-parallel clouds were found. These differences are in the same order of magnitude as those arising from using extremely different crystal shapes for the radiative transfer calculations. From this sensitivity study, the ranking of cirrus cloud properties according to their importance in solar broadband radiative transfer is optical thickness, ice crystal shape, ice particle size, and spatial structure.

Document Type: Article
Keywords: ice crystal, cirrus clouds, solar radiative fluxes
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-ME Maritime Meteorology
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1175/JAS3459.1
ISSN: 0022-4928
Date Deposited: 03 Dec 2008 16:50
Last Modified: 16 Apr 2018 11:14

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