Investigating ice crystal formation and growth in wintertime stratus clouds over the Swiss Plateau (CLOUDLAB project)

Wintertime stratus clouds over the Swiss Plateau can last for days.  They dissipate either due to airmass changes, absorption of solar radiation during the day, or after glaciation when a sufficiently large number of cloud droplets freezes. After formation, the ice crystals grow in an ice-supersaturated environment via vapor deposition until they are large enough to sediment from the cloud as drizzle or freezing drizzle.

To better understand how quickly ice crystals of various habits grow in real clouds with turbulence, we conduct glaciogenic seeding experiments in wintertime stratus clouds over the Swiss Plateau in our project CLOUDLAB[1]. During these experiments, a drone releases silver iodide (AgI) particles into the cloud, upwind of our measurement site, and we use various ground-based remote sensing and in-situ cloud and aerosol instruments to detect the microphysical changes induced by seeding. Preliminary results from the first CLOUDLAB field campaign proved that our method successfully allows us to detect the seeding signal in the cloud radar. In addition to our field measurements, we conduct numerical model simulations with ICON at different horizontal resolutions and different seeding particle concentrations to understand which seeding AgI concentration is theoretically needed for partial or full glaciation of the cloud, i.e. how fast the ice crystals grow at the expense of the evaporating cloud droplets due to the Wegener-Bergeron-Findeisen process.

First results will be presented in this talk.

[1] https://cloudlab.ethz.ch/