Observations and Model Simulations of Phase Heterogeneity in Arctic Clouds

Mixed-phase clouds (MPCs) play a key role in Earth's radiation budget particularly in the Arctic where they are ubiquitous year-round. An important characteristic of MPCs is how the cloud phases are mixed, which affects interactions between ice and liquid. Observations show that phase tends to be nonuniform with ice and liquid forming spatially separated single-phased “pockets” not accounted for in climate models. These pockets may vary in size from the micron-scale to several hundred kilometers making them notoriously difficult to study, and the factors influencing cloud-phase heterogeneity remain uncertain. We quantify size distributions of phase pockets in an observed and modeled Arctic MPC occurring 12 November 2019 in Ny-Ålesund, Svalbard. The case is simulated with the Weather Research and Forecasting model constrained with representative aerosol concentrations following measurements from the Ny-Ålesund Aerosol Cloud Experiment. We find that phase pockets exhibit broad size distributions with the smallest pockets occurring most frequently. Observations reveal mean pocket lengths of 2 km, whereas the simulated pockets are about 5 times longer on average. Simulated pocket size distributions are highly sensitive to prescribed aerosols. Moreover, we observe a pronounced increase of 6.5 km in the mean length of mixed-phase pockets when secondary ice production is enhanced in simulations. These results shed light on the link between cloud microphysics and the in-cloud distribution of phase and provide a potential framework for representation of sub-grid scale phase variability in climate models.