Accurate representations of clouds are required in large-scale weather and climate models to make detailed and precise predictions of the Earth's weather and climate. Representations of clouds within these models are limited by the present understanding of the role of aerosols in the microphysical processes responsible for cloud formation and development.As part of a NERC funded CASE studentship with the Met Office, this thesis aims to test new aerosol-dependent mixed-phase cloud parametrizations by obtaining extensive cloud microphysical measurements in-situ and comparing and contrasting them with model simulations. Cloud particle concentrations were measured during the Ice NUcleation Process Investigation And Quantification (INUPIAQ) field campaign at Jungfraujoch in Switzerland. A new probe was used to separate droplet and small ice concentrations by using depolarisation ratio and size thresholds. Whilst the new small ice crystal and droplet number concentrations compared favourably with other instruments, the size and depolarisation ratio thresholds were found to be subjective, and suggested to vary from cloud to cloud.An upwind site was chosen to measure out-of-cloud aerosol particle concentrations during INUPIAQ. During periods where the site was out-of-cloud and upwind of Jungfraujoch, several large-scale model simulations were run using the aerosol concentrations in an aerosol-dependent ice nucleation parametrization. The inclusion of the parametrization failed to increase the simulated ice crystal number concentrations, which were several orders of magnitude below those observed in-situ at Jungfraujoch.Several possible explanations for the high observed ice crystal number concentrations at Jungfraujoch are tested using further model simulations. Further primary ice nucleation was ruled out, as the inclusion of additional ice nucleating particles in the model simulations suppressed the liquid water content, preventing the simulation of the mixed-phase clouds observed during INUPIAQ. The addition of ice crystals produced via the Hallett-Mossop process upwind of Jungfraujoch into the model only infrequently provided enough ice crystals to match the observed concentrations. The inclusion of a simple surface flux of hoar crystals into the model simulations was found to produce ice crystal number concentrations of a similar magnitude to those observed at Jungfraujoch, without depleting the simulated liquid water content. By confronting models with in-situ observations of cloud microphysical process, this thesis highlights interactions between surface ice crystals and mixed-phase clouds, and their potential impact on large-scale models.