Mixed-phase Clouds

Mixed-phase clouds (MPCs) are clouds that consists of supercooled cloud droplets and ice crystals.

MPCs are thermodynamically unstable because saturation with respect to water is reached at lower humidity than with respect to water. In MPCs without any noticeable vertical velocity and turbulence, the ice crystals will grow at the expense of cloud droplets to precipitation sizes on relative short time scales. This process, known as the Wegener-Bergeron-Findeisen process (WBF-process), thoroughly glaciates the cloud. However, observations have shown that MPCs are a common phenomenon and have been observed in all seasons, under a variety of conditions and at all latitudes worldwide.

The level of understanding of MPCs is low because of their complicated structure, dynamics, and aerosol-cloud interaction:

• The rate of the glaciation process depends on the spatial scale at which the phase composition of the cloud changes, which can be less than the resolution of cloud instrumentation and models.
• The importance of turbulence and orographic lifting inside MPCs is unclear. Large updraft velocities can raise the humidity above water saturation and consequently stabilize the MPC.
• Ice crystals inside MPCs form via heterogeneous nucleation. Ice nuclei measurement capabilities are still insufficient. This may also lead to insufficient parameterization in models.

Modelling will help us to understand the behavior of MPCs better. We work on:

• a better representation of freezing processes to improve MPCs representation in climate modeling and therefore climate
  prediction.
• regional modeling to study the interaction of MPCs with dynamical processes.