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Browsing by Subject "ice nucleation"

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  • Juurikkala, Kasper (2023)
    Clouds and aerosols are among the key components of Earth's energy budget, and a major source of uncertainty in climate models, affecting the predictability of the future climate. This thesis focuses on the microphysical processes governing cirrus clouds, wispy clouds composed of ice crystals. Understanding these processes is crucial due to the extensive global coverage of cirrus clouds and their potential warming effect on the atmosphere. The study investigates ice nucleation, the process by which ice crystals form in the atmosphere. Ice nucleation occurs via two main pathways: homogeneous freezing and heterogeneous nucleation. Homogeneous freezing is a process where droplets spontaneously freeze without an aid of an ice nucleating particle (INP). It occurs in highly supersaturated conditions and at cold temperatures below -38°C. Heterogeneous nucleation occurs when INPs act as surfaces to trigger freezing at temperatures below 0°C. The study is conducted using UCLALES-SALSA Large Eddy Simulation (LES) model, which offers high spatial and temporal resolution for atmospheric simulation. The aim is to investigate ice nucleation with five well-established parameterizations. Simulations produced with these parameterizations are compared with cirrus cloud properties measured during the MACPEX campaign. Among heterogeneous nucleation mechanisms, deposition ice nucleation is considered as a primary contributor to the formation of cirrus clouds in the upper troposphere and used as a mechanism to generate ice in the model study. Heterogeneous nucleation requires the presence of INPs which are assumed to be mineral dust is used as it known to dominate ice nucleation. Results show good agreement between modeled and measured data for ice concentration (Ni) and ice water content (IWC). The comparison between parameterizations revealed a relatively similar performance, with variations in Ni and IWC falling within the same order of magnitude. However, conclusive determination of the best-performing parameterization within the temperature and humidity ranges of the study was challenging. The study sheds light on the fundamental difficulties when using parameterizations with ice nucleation processes in cirrus clouds without accurate initial conditions and knowledge about the history of ice nucleation of the measured cirrus clouds. Also, the importance of proper validation of each parameterization by using different scenarios was emphasized.