In-situ Observations and Large Eddy Simulations of Southern Ocean Boundary Layer Clouds

• Main Author: Atlas, Rachel
• Other Authors: Bretherton, Christopher
• Format: Thesis
• Language: English
• Published: 2019
• Subjects: boundary layer structure; climate model biases; large eddy simulation; low clouds; mixed-phase microphysics; southern ocean; Atmospheric sciences; Climate change; Southern Ocean
• Online Access: http://hdl.handle.net/1773/44027

Thesis (Master's)--University of Washington, 2019 Southern Ocean boundary layer clouds affect global albedo and oceanic heat uptake. Most climate models and reanalyses underestimate cloudiness in the Southern Ocean, which biases seas surface temperatures and tropospheric winds, and likely influences the global atmospheric circulation and oceanic heat uptake. This robust and persistent model bias reveals gaps in our understanding of the physical controls on the formation and evolution of low clouds in the Southern Ocean, compared to more well-studied regions. The physics of Southern Ocean boundary layer clouds are uncertain due, in part, to a lack of in-situ observations in the region. Here, I use recent state-of-the-art measurements from the SOCRATES aircraft campaign and cloud resolving simulations, to investigate the influence of synoptic dynamics, boundary layer structure and microphysical properties on Southern Ocean boundary layer clouds. I developed a technique for simulating boundary layer clouds in the synoptically active Southern Ocean with a large eddy simulation (LES) and I set up five modelling case studies from SOCRATES observations. I find that the LES realistically represents diverse boundary layer structures but produces clouds with persistently low liquid water paths. CAM6 persistently underestimates droplet concentrations and cloud driven turbulence.