High-latitude ice-nucleating particles and the effects on mixed-phase clouds

High-latitude mixed-phase cloud systems are important to the climate as their reflectivity will increase as the climate warms, since ice in the clouds will be replaced by liquid. This mechanism is called the cloud-phase feedback. Ice-nucleating particles (INPs) play a key role in these clouds, underpinning the amount of ice formation and modulating the strength of this feedback. However, there is limited understanding of the concentrations and sources of INPs in high-latitude mixed-phase clouds, or how INPs are processed by these cloud systems. To address these challenges, this thesis presents measurements and computer modelling of INPs in cold-air outbreaks (CAOs). These high-latitude mixed-phase cloud systems form a useful natural laboratory since they have predictable formation locations and well-defined structures. This work found that springtime CAOs over the Norwegian and Barents seas had some of the highest INP concentrations observed in the Arctic. These high concentrations were attributed to aged fertile dust aerosol in Arctic haze transported from lower-latitude regions. These observations also suggested that the INP concentration reduced through a CAO as INPs were removed through precipitation. To investigate the magnitude of this change, CASIM, a double-moment microphysics module with interactive aerosol was used to simulate a CAO. This modelling confirmed that INPs are removed through a CAO but the simulated rate of removal was less than measured. Though INPs were removed, there was a negligible impact on INP concentrations in the cloud, which were predominantly controlled by temperature. However, the ice nucleation scheme in CASIM did not track INP sites, instead assuming there was a constant relationship between the number of INPs and the aerosol concentration. Using a new ice nucleation scheme that tracked active sites, we were able to replicate the observed reduction of INP concentrations, but not while accurately simulating the magnitude of the INP concentrations.