Influence of Giant CCN on warm rain processes in the ECHAM5 GCM R. Posselt and U. Lohmann Institute for Atmospheric and Climate Science, ETH Zurich, Universitaetsstrasse 16, 8092 Zurich, Switzerland
Abstract. Increased Cloud Condensation Nuclei (CCN) load due to anthropogenic
activity might lead to non-precipitating clouds because the
cloud drops become smaller (for a constant liquid water content)
and, therefore, less efficient in rain formation (aerosol indirect
effect). Adding giant CCN (GCCN) into such a cloud can initiate
precipitation (namely, drizzle) and, therefore, might counteract the
aerosol indirect effect.
The effect of GCCN on global climate on warm clouds and
precipitation within the ECHAM5 General Circulation Model (GCM) is
investigated. Therefore, the newly introduced prognostic rain scheme
(Posselt and Lohmann, 2007) is applied so that GCCN are directly activated
into rain drops. The ECHAM5 simulations with incorporated GCCN show
that precipitation is affected only locally. On the global scale,
the precipitation amount does not change. Cloud properties like
total water (liquid + rain water) and cloud drop number show a
larger sensitivity to GCCN. Depending on the amount of added GCCN,
the reduction of total water and cloud drops account for up to
20% compared to the control run without GCCN. Thus, the
incorporation of the GCCN accelerate the hydrological cycle so that
clouds precipitate faster (but not more) and less condensed water is
accumulated in the atmosphere.
An estimate of the anthropogenic aerosol indirect effect on the
climate is obtained by comparing simulations for present-day and
pre-industrial climate. The introduction of the prognostic rain
scheme lowered the anthropogenic aerosol indirect effect
significantly compared to the standard ECHAM5 with the diagnostic
rain scheme. The incorporation of the GCCN changes the model state,
especially the cloud properties like TWP and Nl. The
precipitation changes only locally but globally the precipitation is
unaffected because it has to equal the global mean evaporation rate.
Changing the cloud properties leads to a local reduction of the
aerosol indirect effect and, hence, partly compensating for the
increased anthropogenic CCN concentrations in that regions.
Globally, the aerosol indirect effect is nearly the same for all
Citation: Posselt, R. and Lohmann, U.: Influence of Giant CCN on warm rain processes in the ECHAM5 GCM, Atmos. Chem. Phys., 8, 3769-3788, doi:10.5194/acp-8-3769-2008, 2008.