Seminar 06 (August 28, 2024) of the CleanCloud series. 

Speaker is Prof. Ulrike Lohmann (ETH Zürich in Switzerland).

Abstract:
Clouds are not only fascinating to observe for their myriad of shapes, but are also scientifically challenging because their formation requires both knowledge about the large-scale meteorological environment as well as knowledge about the details of cloud droplet and ice crystal formation on the micro-scale. Mixed-phase clouds consist of liquid droplets and ice particles. They are thermodynamically unstable because of the lower vapor pressure over ice than over liquid water. In a series of field experiments we perturbed supercooled stratus clouds over the Swiss plateau by injecting AgI particles from an uncrewed aerial vehicle upwind of our measurement site. These seeding particles caused ice crystal formation and triggered the Wegener-Bergeron-Findeisen (WBF) process, by which ice crystals grow at the expense of the evaporating cloud droplets. Accompanying large-eddy simulations showed that model can reproduce the observed ice crystal number concentrations most of the time, but not the observed fast reductions in cloud droplet number concentrations. Our detailed analysis shows that the WBF process appears to be less efficient in the model than in the field. In the talk, I will discuss both our field experiments as well as simulations of mixed-phase clouds and discuss an extrapolation to potential climate implications of an enhanced WBF process.