An aerosol-cloud module for inclusion in the KNMI regional climate model RACMO2

This report describes the development and testing of a cloud condensation nuclei (CCN) parameterization in the KNMI regional climate model RACMO2. This requires the parameterization of the cloud droplet number concentration (CDNC) as a function of aerosol loading to allow the computation of the so called indirect aerosol radiative forcing effect. Four CDNC parameterizations were selected and implemented into the Single Column version (SCM) of RACMO2.

The evaluation of the CDNC schemes and their effect on the radiative transfer of clouds has been performed with a set of comprehensive observations made at Cabauw on 30 January 2007. Sensitivity runs with the radiation module of the SCM forced with observations of the atmospheric column at Cabauw show that the internal degrees of freedom of the radiation module overshadow the differences in radiation output arising from the use of different CDNC parameterizations. Moreover, Integrated Profiling Technique retrievals of liquid water path (LWP) available for the testing day were found unrealistically low and required an enhancement factor before being used as input values to the SCM.

At this stage of the investigation we tend to recommend, based on the limited amount of evidence, but also on its potential, the parameterization by Menon et al., (2002) as the most appropriate aerosol/cloud module for application in future studies with RACMO2 of the cloud albedo effect over Europe, in order to assess differences in radiative forcing between pre-industrial and today or future conditions