Modelling studies on the impact of heterogeneous ice nucleation on mixed-phase clouds

Clouds are a critical component of Earth’s climate and hydrological cycle. The formation of ice in the atmosphere, especially at lower altitudes, can substantially impact the evolution of clouds and their radiative properties, and represents the initiation of the cold rain precipitation process. In mixed-phase clouds quantitatively understanding the interactions between ice and liquid, and the subsequent impact on the cloud development, is fundamentally dependent on the process of ice formation and its representation within cloud models. Experiments show that ice nucleating particles (INPs) exhibit variability in both freezing efficiency and time-dependent behaviour. The variability in freezing efficiency is currently well characterised and represented, but variability in time-dependence is poorly characterised and rarely represented in models and parameterisations. The primary aim of this thesis is to understand the role that time-dependence plays in the freezing behaviour of droplets, and secondly to examine the sensitivity of mixed-phase clouds to time-dependence in immersion mode freezing. It is initially found that CNT-based models are unable to reproduce the observed time-dependent behaviour. A new model is therefore presented that uniquely incorporates the variability in both freezing efficiency and time-dependent behaviour; this is applied to experimental data to understand the manifestation of time-dependence in experiments. The model is then used to derive a new theoretical framework for use in experimental analysis and cloud modelling studies. The framework is underpinned by the finding that the temperature dependence (named λ) of the nucleation rate coefficient solely determines the time-dependent behaviour observed in droplet freezing experiments. New and existing experimental data is used to demonstrate the ability for the framework to reconcile data obtained on different timescales with different experimental methods. Finally, an efficient and representative parameterisation is used to explore the sensitivity of mixed-phase clouds to time-dependence. Using a series of increasingly complex models (0D to 2D) it is shown that the inclusion of time-dependence impacts cloud properties in regimes where the updraught speed is relatively low.