TINIA - Influence of turbulence on ice crystal formation and diffusional growth by the example of stratiform mixed-phase clouds

The DFG-funded project TINIA aims to improve our fundamental understanding of the physical processes that occur in stratiform mixed-phase clouds (thin layer clouds). This type of cloud is characterized by the simultaneous presence of supercooled cloud droplets and ice crystals and has a major impact on weather and climate. For example, stratiform mixed-phase clouds in the Arctic are considered to play a crucial role in Arctic warming. Although knowledge about this type of cloud has increased significantly in recent decades, the relevant microphysical processes and interactions are still poorly understood and quantified. For example, key questions remain on how, in general, turbulent fluctuations in temperature and saturation influence the formation and diffusional growth of ice crystals and how, in particular, cloud (top) turbulence in stratiform mixed-phase clouds influences these microphysical processes.

The project uses a combination of laboratory studies, atmospheric observations, and direct numerical simulation to answer these questions.

• The laboratory studies are conducted in the turbulent moist-air wind tunnel LACIS-T and in the PI cloud chamber (Michigan Tech, Houghton (MI), USA, in cooperation with Prof. Raymond A. Shaw) to investigate the effects of defined turbulence on ice crystal formation and their diffusive growth under controlled conditions.
• The atmospheric observations including data obtained during an aircraft measurement campaign (BACSAM II) serve as the starting point for the laboratory studies and direct numerical simulations, providing them with crucial initial and boundary conditions.
• The direct numerical simulations are being carried out to investigate the influence of turbulence on ice crystal formation and diffusional growth in stratiform mixed-phase clouds. The simulations combine an Eulerian turbulence solver developed in the group of Prof. Juan Pedro Mellado (University of Hamburg) with a Lagrangian microphysics model implemented and further developed by our team.

First results indicate that turbulent temperature fluctuations shift ice crystal formation to higher mean temperatures. This could potentially have a significant impact on the properties and behavior of stratiform mixed-phase clouds, which is now being further investigated in the simulation studies. Overall, the combination of laboratory studies, atmospheric observations and numerical simulation holds great potential for expanding our fundamental understanding of the behavior of stratiform mixed-phase clouds and their role in weather and climate.