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ATMOSPHERIC RESEARCH, 72, 291-315, 2004
Thermal radiative fluxes through inhomogeneous cloud fields: a sensitivity study using a new stochastic cloud generator
A. Benassi, F. Szczap, A. Davis, M. Masbou, C. Cornet, P. Bleuyard
Abstract
We analyze the effects of flat and bumpy top, fractional and internally inhomogeneous cloud layers on large area-averaged
thermal radiative fluxes. Inhomogeneous clouds are generated by a new stochastic model: the tree-driven mass accumulation
process (tdMAP). This model is able to provide stratocumulus and cumulus cloud fields with properties close to those observed
in real clouds. A sensitivity study of cloud parameters is done by analyzing differences between 3D fluxes simulated by the
spherical harmonic discrete ordinate method and three "standard" models likely to be used in general circulation models:
plane-parallel homogeneous cloud model (PPH), PPH with fractional cloud coverage model (FCPPH) and independent pixel
approximation model (IPA). We show that thermal fluxes are strong functions of fractional cloud coverage, mean optical depth,
mean geometrical thickness and cloud base altitude. Fluctuations of "in-cloud" horizontal variability in optical depth and
cloud-top bumps have negligible effects in the whole. We also showed that PPH, FCPPH and IPA models are not suitable to
compute thermal fluxes of flat top fractional inhomogeneous cloud layer, except for completely overcast cloud. This implies
that horizontal transport of photon at thermal wavelengths is important when cloudy cells are separated by optically
thin regions.
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