How well can a nudged global cloud resolving model represent mesoscale cloud features and precipitation within Southern Ocean boundary layer clouds?

The Southern Ocean is frequently blanketed in low to mid-level boundary layer clouds, which feature mesoscale open and closed cells 10s of kilometers across that modulate cloud radiative properties and precipitation. Here, we evaluate Southern Ocean clouds within a global simulation with 2-3 km horizontal grid spacing, which resolves mesoscale features evolving within a realistic large-scale environment but still must parameterize most turbulence. Temperature and horizontal winds are nudged towards ERA5 reanalysis data with a 24 hour timescale so that the model output can faithfully be compared with observations. The simulation is run for a period of five days overlapping three flights from the 2018 Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) aircraft campaign. Of the three overlapping flights, one sampled open-cell cumulus and two sampled extensive stratocumulus decks featuring mesoscale closed cells. Four microphysical schemes are used, including three two-moment schemes (Morrison, Thompson and P3) and one single moment scheme (SAM). We evaluate simulated cloud macrophysics and cloud radiative effects using satellite data and we compare simulated particle size distributions and synthetic radar from the two-moment microphysical schemes with in-situ data from particle probes and measurements from an airborne W-band radar. All microphysical schemes skillfully reproduce observed boundary layer moisture and maintain supercooled liquid water-dominated clouds in all three cases, but struggle to reproduce the scale of the closed cells and the frequency of precipitation observed during the two stratocumulus flights. In all cases, the microphysical schemes disagree on the order of magnitude of the frozen ice mass, an important control on cloud feedbacks, and the rain number concentration, which has implications for the marine aerosol cycle. Morrison microphysics best simulates open cellular convection and the two moment schemes outperform the single moment scheme in representing mesoscale closed cells.