Doctoral defence: Jorma Rahu „Novel insights into aerosol-cloud interactions by analysing the temporal evolution of strong anthropogenic cloud perturbations“

On 12 June 2025, Jorma Rahu will defend his doctoral thesis "Novel insights into aerosol-cloud interactions by analysing the temporal evolution of strong anthropogenic cloud perturbations".

Supervisors:
Velle Toll, Institute of Physics, University of Tartu
Piia Post, Institute of Physics, University of Tartu

Opponent:
Prof. Harri Kokkola, Finnish Meteorological Institute, Finland

Summary

The Earth’s climate depends on a delicate balance between absorbed solar radiation and the thermal radiation leaving our atmosphere. Clouds play a crucial role in this equation, scattering solar radiation back to space before it even reaches the ground, absorbing solar radiation, and absorbing and emitting thermal radiation. Clouds themselves are affected by tiny air pollution particles in the air called aerosols. Aerosols change cloud properties like the size of cloud droplets, cloud thickness, cloud amount, and cloud lifetime. This research examines how industrial pollution modifies cloud properties and leads to so-called aerosol-polluted cloud tracks - line-shaped cloud disturbances visually identifiable in satellite images in cloud decks downwind of strong industrial air pollution sources.

Aerosol particles act as condensation sites for water vapour. When anthropogenic pollution adds more aerosols into the atmosphere, the clouds form with more numerous but smaller water droplets. Smaller cloud droplets mean the polluted clouds are brighter than the surrounding unpolluted clouds. The polluted cloud tracks can be observed in satellite images, and this research focuses on the temporal evolution of aerosol-induced changes in cloud properties, using high temporal resolution geostationary satellite data. In some cases, we found an increase in cloud water in the afternoon, suggesting suppression of precipitation formation in the aerosol-polluted clouds. In addition, we saw that cloud tracks polluted by industrial aerosols can persist for multiple consecutive days, allowing for the clouds to adjust to aerosol perturbations. This indicates that such aerosol-polluted cloud tracks can be used to study the causal impacts of aerosols on clouds.

The most novel part of this thesis was the discovery of direct observational evidence that the impact of man-made aerosol pollution is not limited only to changing the properties of liquid-water clouds, but industrial aerosols can also act as ice-seeding particles. These particles transform supercooled liquid water cloud droplets into ice crystals, which are prone to growing larger through the Wegener-Bergeron-Findeisen process. Such a glaciation process initiates a sequence of processes where the ice crystal formation leads to snowfall, which results in reduced cloud cover downwind of large industrial sites. It is important to note that the emission of heat and water vapour may also play a role in the observed glaciation events.

In conclusion, the dissertation advances the understanding of how anthropogenic aerosols alter cloud properties. This was achieved by analysing the temporal evolution of cloud perturbations caused by industrial aerosols, as multiple insights into the relevant physical processes at play were established. The improved understanding of aerosol impacts on clouds could ultimately lead to more reliable climate projections.