Subvisible and thin cirrus clouds formed by homogeneous freezing

Subvisible and thin cirrus clouds occur frequently in the tropopause region. The ice crystal number concentrations of such clouds are usually quite small. The dominant pathway for forming such optically thin clouds is not known well; however, it is often assumed that heterogeneous nucleation should be the dominant formation mechanism.

We developed a simplified analytical cloud model, as derived from complex microphysics parameterisations. This model basically consists of a set of ordinary differential equations, which can be analyzed using standard methods from theory of dynamical systems. The model agrees quite well with more sophisticated detailed cloud models for scenarios with external forcing of slow synoptic updrafts, but also for perturbations with higher updrafts.

We used the analytical cloud model to investigate the formation of subvisible and thin cirrus clouds by homogeneous freezing of aqueous solution droplets. Depending on environmental conditions (temperature, updraft velocity etc.) two qualitatively different solutions can be found, i.e. a damped oscillation of ice crystal mass/number concentration (positive attractor) or a saw tooth like behavior of these variables (limit cycle). In both cases, the microphysical variables agree reasonably well with in situ observations. Thus, homogeneous nucleation of ice crystals might still be a possible pathway for forming subvisible and thin cirrus clouds.