Laboratory studies on the influence of turbulence on heterogeneous ice formation

Mixed-phase clouds are ubiquitous in the troposphere during all seasons, from polar to tropical regions, and have a significant impact on weather and climate (e.g., Korolev et al., 2017). Although the knowledge about mixed-phase clouds has increased significantly in recent decades, the relevant microphysical processes and interactions are still poorly understood and insufficiently quantified. For example, how turbulent fluctuations in temperature affect the immersion freezing of supercooled cloud droplets in these clouds remains a key question. To investigate the immersion freezing behavior of supercooled droplets in a turbulent environment, laboratory studies are carried out in the turbulent moist-air wind tunnel LACIS-T (Turbulent Leipzig Aerosol Cloud Interaction Simulator, Niedermeier et al. (2020)). The experiments use size-selected, monodisperse Snomax particles as ice-nucleating particles. These particles are injected into the measurement section of LACIS-T where the formation of supercooled droplets, their growth, and the potential freezing occur. The study includes several experiments varying the mean temperature and the magnitude of temperature fluctuations. Droplet freezing is quantified for the different conditions by determining the fraction of frozen droplets as a function of mean temperature and temperature fluctuations. One main result is the observation of immersion freezing at higher mean temperatures compared to conditions without temperature fluctuations. In other words, turbulence affects the number of frozen droplets. The obtained results will be presented in detail and its atmospheric implications will be discussed.

References:
Korolev et al. (2017), Meteorol. Monogr., 58, 5.1-5.50, https://doi.org/10.1175/AMSMONOGRAPHS-D-17-0001.1.
Niedermeier et al. (2020), Atmos. Meas. Tech., 13, 2015-2033, https://doi.org/10.5194/amt-13-2015-2020.

Acknowledgement:
We gratefully acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Project Number NI 2231/1-1 (Project name: TINIA).