Impact of heterogeneous ice nuclei on homogeneous freezing events in cirrus clouds

The influence of initial heterogeneous nucleation on subsequent homogeneous nucleation events in cirrus clouds is investigated using a box model which includes the explicit impact of aerosols on the nucleation of ice crystals and a new sedimentation scheme. Different effects are discussed, namely the impact of external versus internal mixtures of heterogeneous ice nuclei and the influence of size-dependent freezing thresholds. Several idealized experiments are carried out, which show that the treatment of external mixtures of ice nuclei can strongly change subsequent homogeneous nucleation events (i.e., the ice crystal number densities) over a large variety of environmental conditions relevant for the upper troposphere. In most cases a strong reduction in ice crystal number concentrations formed in subsequent homogeneous freezing events is observed. The use of size-dependent freezing thresholds can change cloud properties when compared to more simple parameterizations and is most important at largest ice nuclei concentrations.