An investigation and discussion on the performance and use of cloud droplet activation parameterisations

Cloud droplet number concentration is central to large-scale weather and climate modelling. The benchmark cloud parcel model calculation of aerosol particle growth and activation, by diffusion of vapour to aerosol particles in a rising parcel of air experiencing adiabatic expansion, is too computationally expensive for use in large scale global models. Therefore the process of activation of aerosol particles into cloud droplets is parameterised with an aim to strike the optimum balance between numerical expense and accuracy. Here a summary of a detailed systematic evaluation of four cloud droplet activation parameterisations will be given as well as a discussion on the use of cloud droplet activation parameterisations in large-scale models. Issues that will be considered include: 1. Implementation of parameterisations in models in ways to overcome the issues raised in the evaluation, 2. The current interpretation of parameterisations results and 3. The treatment within models of large unactivated drops that contribute to cloud radiative properties.

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.