An investigation and discussion on the performance and use of cloud droplet activation parameterisations
Results from the evaluation show that the parameterisations overestimate the fraction of activated drops when the aerosol particle ''median diameter'' is large (between 250 and 2000 nm). This is a result of an infinite ''effective simulation time'' in the parameterisations compared to a prescribed simulation time in the parcel model. This problem arises in the parameterisations because it is assumed that a parcel of air rises to the altitude where maximum supersaturation occurs, regardless of whether this altitude is above cloud top. Such behaviour is problematic because, in some cases, large aerosol can completely suppress the activation of drops. Secondly, it is found that in the dual-mode cases there is a systematic tendency towards underestimation of the fraction of activated drops, which results from the methods used by the parameterisations to approximate the sink of water vapour.
The results from the evaluation will be used to direct a discussion of the use of cloud droplet activation parameterisations in large-scale global and regional models.