The group investigates microphysical cloud processes like the nucleation of liquid water and ice, droplet collisions and fragmentation and secondary ice processes in laboratory experiments. The main tools are single droplet electrodynamic balances, cold stages, particle traps, mass spectrometry, nonlinear optics and environmental scanning electron microscopy.
Reliable modeling of cloud processes for weather predictions and climate change projections requires a sound understanding of the ice formation in mixed-phase clouds. However, ice crystal concentrations measured in the clouds often exceed the concentration of ice nucleating aerosol particles by many orders of magnitude. To elucidate this discrepancy, we simulate the atmospheric processes of ice multiplication upon freezing of water droplets levitated in an electrodynamic trap (EDB) coupled with sophisticated optical and infrared video recording system. In the past few years, we have achieved significant progress in understanding the physics of freezing of freely suspended cloud droplets and currently aim at full quantification of the secondary ice production mechanism.
Ice Nucleation and Growth via Deposition from Vapor Phase
The ice crystals in atmospheric clouds governs their life time, optical properties, and are part of the precipitation pathway. In the troposphere, ice crystals form mostly via heterogeneous nucleation, assisted by the presence of aerosol. However, only few aerosol particles would serve as efficient ice nucleators. To understand the role of aerosol particles and to identify the ice nucleating active sites, we conduct laboratory experiments with an Environmental Scanning Electron Microscope (ESEM), allowing us to study morphology and chemical composition of individual aerosol particles and conduct the ice nucleation experiments directly in the specimen chamber of the ESEM.
Biological Ice Nucleators in the Atmosphere
Living organisms have developed ways of controlling ice nucleation and growth to prevent cell damage caused by freezing of intracellular water. As a result, many microorganisms, such as bacteria, fungi, or plant pollen, have become able to trigger freezing of water at just a few degrees of supercooling, as compared to -40°C degrees required to freeze microscopic droplet of supercooled water homogeneously. Some bacterial and animal proteins are also responsible for inhibiting the ice crystal growth by attaching themselves to the crystalline faces of growing ice. In cooperation with microbiologists from KIT and Aarhus university (Denmark), we study how certain bacteria affect freezing of atmospheric water and how this affects their chance of survival and dissemination pathways.