The Southern Ocean and coastal Antarctica are key regions for global climate. Low level mixed-phase clouds strongly control the surface radiation budget of this region but remain challenging for climate models because of the complex interactions between cloud liquid water and ice crystals. Here we examine these interactions using the Unified Model (UM) regional climate model, with the Cloud AeroSol Interacting Microphysics (CASIM) and UK Chemistry and Aerosol (UKCA) models included for interactive aerosol and cloud microphysics. We simulate case studies from the second field campaign of Clouds Aerosols Precipitation Radiation and atmospheric Composition over the Southern Ocean Phase 2 (CAPRICORN-2). Compared to these observations, we find that the UM underestimates surface aerosol concentration by up to an order of magnitude and investigate the effect of this bias on the simulated cloud microphysical and radiative properties. We find that the cloud liquid water path and surface radiative fluxes are also biased in the model, with a 32% mean underestimation of liquid water path and 76% mean overestimation of downwelling surface shortwave flux in one case study. Sensitivity tests show that the cloud liquid water bias is largely caused by deficiencies in the representation of the meteorology, and less by aerosol or cloud microphysical properties. Our results provide key insights on the modeling of cloud processes in high southern latitudes.