Assessing the Potential of Cirrus Cloud Thinning through Cloud Chamber Experiments and Parcel Model Simulations — Thesis

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https://publikationen.bibliothek.kit.edu/1000168905

Author 

Schorr, Tobias

Aerosolforschung (IMKAAF), Karlsruher Institut für Technologie (KIT)

28 February 2024

Abstract

Cirrus cloud thinning (CCT) is a climate engineering approach to achieve regional cooling by reducing the coverage of effectively warming cirrus clouds. Seeding with ice-nucleating particles (INPs) would affect the natural cirrus cloud formation process and is expected to change the cloud properties to a thinner cloud with a shorter lifetime. With cirrus clouds having a warming effect (on average), diminishing these clouds could lead to a surface cooling. The Arctic could particularly benefit from such a intervention, due to regional feedback effects.

In this work cloud chamber experiments and parcel model simulations on CCT are presented. These results contribute to a better understanding of the competition between heterogeneous and homogeneous freezing and will therefore support a more rigorous evaluation of CCT effectiveness.

In our cloud chamber studies, CCT effectiveness is probed by investigating the competition between homogeneous freezing of sulfuric acid solution droplets and heterogeneous ice nucleation by three different seeding agents, i.e. fumed silica, quartz and calcium carbonate. These cloud chamber experiments show that CCT effectiveness (i.e. minimizing

the total ice crystal number concentration) is dependent on the ambient temperature and the concentration of the seeding aerosol. The Lagrangian parcel model MAID (Model for Aerosol and Ice Dynamics) is validated against our experimental results and used to further analyze CCT effectiveness beyond the experimentally accessible parameter space. As part of this work, the model as improved and expanded by a new heterogeneous freezing scheme, internally calculated trajectories and the representation of gravity wave driven fluctuations. After validation we conduct atmospheric CCT simulations with smaller seeding concentrations and slower updraft velocities along adiabatic updraft trajectories. The results show regimes of optimal seeding conditions, as well as regimes with the opposite effect (overseeding). If the updraft trajectories are superimposed with gravity wave driven fluctuations, the characteristics of those regimes become less distinct and the effect of CCT is significantly reduced.

Our results underline the complexity of CCT effectiveness and highlight the sensitivity with regard to variations of the seeding concentration, updraft velocity and gravity wave fluctuations. Due to the strong impact and statistical nature of gravity wave fluctuations a controlled application of CCT is challenging. Yet, a statistical analysis of stochastic updraft fluctuations shows thinned cirrus in 20 % to 30 % of the scenarios with low to moderate seeding. Our model simulations emphasize the importance of the competition

between heterogeneous and homogeneous freezing, as well as gravity wave driven updraft

fluctuations.

Source: KIT