Effect of dynamics on the formation of mixed phase regions in stratiform clouds

Supercooled cloud droplets may exist in a metastable liquid phase resulting in colloidal three-phase component system consisting of water vapor, ice particles and liquid droplets. Such a system is usually referred to as "mixed phase". Since the saturation vapor pressure over liquid water is higher than that over ice at cold temperatures, the mixed phase system is condensational unstable. It is usually assumed that all mixed phase clouds exist only for a limited time: in the final stage, all the liquid droplets will evaporate and the cloud will consist only of ice particles. The glaciation process due to ice growth by deposition at the expense of co-existing liquid droplets is known as the Wegener-Bergeron-Findeisen mechanism. It is shown that vertical oscillations of a cloud parcel under certain conditions may result in a periodic evaporation and activation of liquid droplets in the presence of ice particles. Such a periodic process can continue indefinitely (subject to external forcing) and it does not result in an irreversible glaciation of the cloud parcel. Analytical expressions for necessary and sufficient conditions for such periodic processes are presented here. The results of analytical calculations were found to be in a good agreement with numerical modeling of mixed phase formation. The phenomenon of periodic activation and evaporation liquid water in ice clouds may explain the existence of long-lived mixed phase stratiform layers observed from in-situ measurements.