Simulating the Impact of Cloud Microphysics and Ice Nucleation on Southern Ocean Clouds using Models Constrained by Observations

Supercooled liquid clouds are common at higher latitudes (especially over the Southern Ocean) and are critical for constraining climate projections. We take advantage of recent Southern Ocean field campaigns, including the Macquarie Island Cloud and Radiation Experiment (MICRE) and the Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES) to perform a detailed analysis of observed and simulated cloud processes over the Southern Ocean in a region and season dominated by supercooled liquid clouds. We compare two different cloud microphysical schemes in a single framework to detailed ground based observations of cloud microphysics and radiation over a 3 month period (January - March 2017), and confirm some of the results with comparisons to SOCRATES flights. Results indicate that both schemes are able to reproduce cloud and radiation observations during MICRE to within the uncertainty of the data, with some important caveats that we explore. Interestingly, we do not see evidence of large radiation biases in these constrained simulations compared to the observations, perhaps suggesting that cloud microphysical processes alone are not responsible for known biases in many large scale models. Further, a detailed comparison of cloud microphysics schemes, one a bulk one-moment scheme, and the other a two-moment scheme with prediction of mass and number, indicates that several key processes create differences between the schemes, but can be harmonized to a remarkable degree. The fall speed of precipitation matters quite a bit to the comparisons with observed water paths and surface radiative fluxes. With the two-moment scheme, we are also able to test the sensitivity of the results to input of liquid Cloud Condensation Nuclei (CCN) and Ice Nuclei (IN). The cloud properties and resulting radiative effects are found to be quite sensitive to the CCN and IN concentrations.