Ice, Ice, Maybe? Aerosol-Cloud Interactions over the Southern Ocean Region
The Southern Ocean (SO) is one of the cloudiest regions on Earth and serves as a critical component of Earth’s radiative balance. Coarse resolution Earth system models (ESMs) have reported that the negative cloud phase feedback, where ice-containing clouds transition to liquid clouds under warming, is especially prevalent over the SO region with important implications for global climate sensitivity. While most state-of-the-art ESMs have substantially reduced previously large biases in simulated SO absorbed shortwave radiation, there are well-documented biases and uncertainties in SO cloud properties that are not captured by the traditional top-of-atmosphere diagnostics used in ESM development. Uncertainties in microphysical processes that have climate-scale impacts drive the need for process-scale model assessment. Challenges in representing SO aerosol cloud interactions, including ice nucleation, that are addressed in this work are 1) the unique SO aerosol population primarily comprised of marine aerosol with intermittent transport of continental aerosol and 2) 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. An assessment of CAM6 simulated observations and field campaign observations revealed important biases in simulated micro-scale aerosol and cloud features in CAM6. Additionally, a series of CAM6 simulations were performed to determine contributions of specific modeled freezing processes, including ice nucleation, secondary ice production, heterogeneous freezing of rain, and precipitation processes, to the mass and number budgets in the CAM6. Results from these model experiments reveal ice formation in CAM6 is unrealistically dominated by heterogeneous freezing of rain, driven by an inability of CAM6 to simulate the supercooled drizzling clouds that are often observed in the SO region.