Mixed-phase Clouds

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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.

Direct measurements are important to improve our understanding of MPCs. We perform field campaigns to measure:

  • ambient background IN concentrations with PINC and HINC at the high altitude research station Jungfraujoch (JFJ).
  • the cloud phase of MPCs with a high spatial resolution with the holographic imager HOLIMO at the Jungfraujoch and on Gondola.

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

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