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Journal article

Monte Carlo-based subgrid parameterization of vertical velocity and stratiform cloud microphysics in ECHAM5.5-HAM2

Tonttila J, Räisänen P, Järvinen H...(+3 more)

Atmospheric Chemistry and Physics, vol. 13, issue 15 (2013) pp. 7551-7565 Published by Copernicus GmbH

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A new method for parameterizing the subgrid variations of vertical
velocity and cloud droplet number concentration (CDNC) is presented for
general circulation models (GCMs). These parameterizations build on top
of existing parameterizations that create stochastic subgrid cloud
columns inside the GCM grid cells, which can be employed by the Monte
Carlo independent column approximation approach for radiative transfer.
The new model version adds a description for vertical velocity in
individual subgrid columns, which can be used to compute cloud
activation and the subgrid distribution of the number of cloud droplets
explicitly. Autoconversion is also treated explicitly in the subcolumn
space. This provides a consistent way of simulating the cloud radiative
effects with two-moment cloud microphysical properties defined at
subgrid scale. The primary impact of the new parameterizations is to
decrease the CDNC over polluted continents, while over the oceans the
impact is smaller. Moreover, the lower CDNC induces a stronger
autoconversion of cloud water to rain. The strongest reduction in CDNC
and cloud water content over the continental areas promotes weaker
shortwave cloud radiative effects (SW CREs) even after retuning the
model. However, compared to the reference simulation, a slightly
stronger SW CRE is seen e. g. over mid-latitude oceans, where CDNC
remains similar to the reference simulation, and the in-cloud liquid
water content is slightly increased after retuning the model.

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