Ice, Ice, Maybe? A Process System Assessment of Southern Ocean Aerosol-Cloud Interactions

Clouds over the Southern Ocean are critical to accurately representing Earth’s radiative properties, yet continue to challenge Earth System Models due to complex micro-scale processes that influence regional-scale radiation. Subgrid-scale processes, including turbulence, cloud droplet activation, droplet collision-coalescence, ice nucleation, secondary ice production, and ice growth, are represented in coarse resolution models with parameterizations. It is well-documented that coupled Earth System model simulation results are highly sensitive to changes in these subgrid processes and that both structural and parameter uncertainties remain large. 

In this talk, we will discuss a process system approach for interrogating the representation of model microphysical processes in the Community Atmosphere Model version 6 (CAM6). Instrument simulators that translate model output into “observable” quantities were developed based on the sampling and measurement capabilities of the field instruments and “deployed” during several SO field campaigns using a specified dynamics configuration of the CAM6. Assessments revealed a low bias in cloud droplet number concentrations (CDNC), consistent with a low bias in cloud condensation nuclei (CCN) from missing sulfate aerosol. Model predictions of SO ice nucleating particles (INPs) are skillful in the boundary layer, but are much more variable aloft. In a series of simulations aimed at determining the needed INP predictive skill for accurately representing SO clouds, we find little to no sensitivity in CAM6 clouds to changes in ice nucleation. Analysis of these simulations reveal that ice formation in CAM6 SO clouds is unrealistically dominated by heterogeneous freezing of supercooled rain and is linked to the CDNC-CCN bias chain. Efforts to address these biases will also be discussed.