Constraints on Southern Ocean Mesoscale Cellular Convective Cell Growth

Mesoscale cellular convection (MCC), which can be found in- and outside marine cold air outbreaks (MCAOs) over the Southern Ocean (SO), has been shown to influence the cloud radiative effect and potentially shortwave cloud feedbacks. While MCC morphology and cell-size scaling have been studied extensively in the subtropics and North Atlantic MCAOs, far less is known about how these relationships behave in the SO, where mixed-phase clouds dominate. In this study, we investigate the physical controls on MCC cell size and its variability during SO MCAOs based on collocated active and passive remote sensing products and reanalysis fields.

Specifically we combine MODIS retrievals of liquid water path and 0.86 μm reflectance for MCC classification and cell identification, ERA5 reanalysis for dynamical and thermodynamic fields, and DARDAR-v2 radar–lidar profiles to determine cloud-top height, cloud-top temperature, and cloud phase. Image segmentation applied to 200 × 200 km² scenes along DARDAR overpasses yields a catalogue of 19,500 MCC cells, 86% of which are supercooled—a clear reflection of the high prevalence of mixed-phase clouds in the SO.

Contrary to established behaviour in shallow NH boundary layers, we find no evidence of a constant aspect-ratio regime and no systematic deepening of the BL during MCAO evolution. Open and closed cells exhibit similar median diameters (~36–37 km), although open cells display a longer tail toward larger sizes. Thermodynamic and dynamic conditions—including stability parameter M, BL depth, and surface forcing—show minimal influence on cell-size variability. Approximately half of all mixed-phase open cells occur within MCAO regimes defined by M > –5 K, yet cell diameter remains largely insensitive to the strength of the outbreak.

Backward trajectory analysis indicates that time since cold air mass formation may play a more decisive role: larger cells tend to reside in older, more mature MCAO air masses. Our findings suggest that, in the SO, MCC cell growth is primarily constrained by air-mass age rather than boundary-layer deepening or thermodynamic forcing.