The influence of CCN spectra and updraft velocities on the validity of the b parameterization of the condensation process

A detailed parcel model of condensational droplet growth, including surface tension and dissolved salts effects, is used for studying the validity of a parameterization based on the simplified droplet growth equation, the b² scheme. In the b² scheme, spectra are derived from the droplet distribution at the activation level, and the liquid water mixing ratio using a translation of the initial spectrum in the r² scale. Spectra derived from the detailed model are thus compared to translated spectra. Four simplified CCN distributions are tested for updraft velocities in the range of 0.5 m/s to 5.0 m/s. It is shown that the b² scheme is suitable for use in cloud models, except when the proportion of large nuclei in the CCN distribution is extremely high.