11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Tuesday, 4 June 2002: 3:45 PM
Parameterizations of solar single-scattering radiative properties for tropical ice clouds
Greg M. McFarquhar, Univ. of Illinois, Urbana, IL; and P. Yang, A. Macke, A. Baran, S. Iacobellis, and R. Somerville
Poster PDF (639.3 kB)
Parameterizations of single-scattering properties currently used in cloud resolving and general circulation models are somewhat limited in that they typically assume the presence of single particle habits, do not adequately account for the numbers of ice crystals with diameters smaller than 100 micrometers, and contain no information about the variance of parameterization coefficients. In this presentation, new parameterizations of mean single-scattering properties (single-scatter albedo, asymmetry parameter, and extinction efficiency) for distributions of ice crystals in tropical ice clouds are developed. Using information about the size and shape of ice crystals acquired by a two-dimensional cloud probe during the Central Equatorial Pacific Experiment (CEPEX), a self-organized neural network defines shape based on simulations of how the particle maximum dimension and area ratio (ratio of projected area to that of circumscribed circle with maximum dimension) vary for random orientations of different idealized shapes (i.e., columns, bullet rosettes, rough aggregates, and particles represented by Chebyshev polynomials). The size distributions for ice crystals smaller than 100 micrometers are based on parameterizations developed using representative samples of crystals imaged by a Video Ice Particle Sampler (VIPS). The mean scattering properties for distributions of ice crystals are then determined by weighting the single-scattering properties of individual ice crystals, determined using an improved geometric ray-tracing method, according to number concentration and scattering cross-section.

The new parameterizations of the single-scattering properties are determined by functional fits in terms of cloud particle effective radius; there was no statistically significant dependence on either ice water content or temperature. Uncertainty estimates incorporated into the parameterization coefficients are based upon a Monte Carlo approach. Comparisons with previously used parameterizations and with parameterizations developed using single crystal habits are made to show that the determination of representative crystal habits is still a major unknown in the development of parameterization schemes.

The new parameterization is incorporated into the Scripps SCM to show the sensitivity of modeled radiative fluxes to the single-scattering radiative properties of clouds.

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