On the importance of the condensation sink of pre-existing droplets for aerosol-cloud interactions

The activation of aerosols to form cloud droplets is fundamentally dependent on the ambient supersaturation, which is controlled by a balance between adiabatic cooling due to the vertical velocity of the rising air and the condensational sink of the activated droplet spectrum. Current droplet activation parameterisations used in many global circulation models (GCMs) are usually based upon some form of adiabatic parcel model theory. However, not all air parcels are initiated in clear sky conditions as this theory assumes, and therefore, as current activation parameterisations neglect the condensational sink on the existing cloud droplet spectrum they are likely to overestimate the maximum supersaturation, i.e. the number of activated aerosols.

We demonstrate the importance of this effect through offline simulations with an adiabatic parcel model, accounting for the condensation sink of a pre-existing droplet population during the calculation of the maximum supersaturation reached in the air parcel. For a range of environmental conditions the inclusion of this additional sink term significantly reduces the number of additional activated droplets after the first nucleation.

Based on the large impact observed from the offline simulations we have added this additional condensational sink term to the droplet activation parameterisation scheme used within a GCM, ECHAM6.1-HAM2.2, in order to assess changes in the global cloud droplet number concentrations (CDNC) and radiative effects.

Significant reductions in the annual mean CDNC are observed, especially over regions experiencing high aerosol concentrations, indicating a possible large reduction in the indirect radiative aerosol forcing. Further simulations will be performed to obtain estimates of the change in radiative flux perturbation (RFP) due to total anthropogenic aerosol effects that can be attributed to this process.