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JOURNAL OF THE ATMOSPHERIC SCIENCES, 64, 2670-2682, 2007.
Entrainment-Mixing and Radiative Transfer Simulation in Boundary Layer Clouds.
F. Chosson, J.-L. Brenguier, and L. Schuller
Abstract
In general circulation models, clouds are parameterized and radiative transfer calculations are performed
using the plane-parallel approximation over the cloudy fraction of each model grid. The albedo bias
resulting from the plane-parallel representation of spatially heterogeneous clouds has been extensively
studied, but the impact of entrainment-mixing processes on cloud microphysics has been neglected up to
now. In this paper, this issue is examined by using large-eddy simulations of stratocumulus clouds and
tridimensional calculations of radiative transfer in the visible and near-infrared ranges. Two extreme
scenarios of mixing are tested: the homogeneous mixing scheme with constant concentration and reduced
droplet sizes, against the inhomogeneous mixing scheme, with reduced concentration and constant droplet
sizes. The tests reveal that entrainment-mixing effects at cloud top may substantially bias the simulated
albedo. In the worse case, which corresponds to a fragmented and thin stratocumulus cloud, the albedo bias
changes from 3% to 31% when using both mixing schemes alternatively.
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