Convection permitting simulations of mixed-phase clouds over the Southern Ocean

Atmospheric and climate models exhibit large radiative flux biases over the Southern Ocean, largely due to deficiencies in representing supercooled liquid and mixed-phase low-level clouds. These biases propagate into errors in sea surface temperature, sea ice, and large-scale circulation. We evaluate a convection-permitting configuration of the Met Office Unified Model using aircraft and satellite observations in February 2023 and aircraft, ship-borne and satellite observations in November 2024 collected during the two Southern Ocean Clouds field campaigns.

 

From the first campaign the analysis focuses on three mixed-phase cloud properties: ice nucleating particle (INP) concentrations, droplet number concentration, and the spatial mixing of liquid and ice. Using lower temperature-dependent INP concentrations, that are consistent with observations, reduces ice mass and number, increases cloud liquid water, and decreases the net surface cloud radiative effect by up to 14 W m⁻². Reducing droplet number concentrations to observed campaign averages produces a comparable but oppositely signed radiative impact (up to 22 W m⁻²), indicating compensating errors. Changes in phase partitioning also strongly affect radiation, with impacts of up to 31 W m⁻². These results demonstrate that all three of these mixed-phase processes are critical for accurately simulating Southern Ocean clouds and their radiative effects.

 

We also present analysis from the second field campaign, providing an independent evaluation of the Met Office Unified Model and assessing its sensitivity to aerosol-aware parameterisations and secondary ice processes.