Resolution Dependence of Southern Ocean Mixed-Phase Clouds in ICON

Extratropical low-level mixed-phase clouds (MPCs) are difficult to represent in global climate models and generate substantial uncertainty in global climate projections. In this study we evaluate the simulated properties of Southern Ocean (SO) boundary layer MPCs for August 2016 in the ICOsahedral Nonhydrostatic (ICON) model. The bulk of the simulations are part of the DYnamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domain (DYAMOND) initiative. The analysis shows that previous and current versions of ICON overestimate cloud ice occurrence in low-level clouds across all latitudes in the SO. Furthermore, cloud seeding from upper-level ice clouds into low-level supercooled liquid layers is found to strongly impact MPC occurrence in ICON. Like many other global climate models, ICON underestimates the reflectivity of SO boundary layer clouds. We can show that this effect is resolution dependent and largely due to an underestimation in cloud fraction, rather than optical depth. Additional sensitivity experiments show a pronounced sensitivity of the Wegener-Bergeron Findeisen (WBF) process with respect to temporal discretisation. Long integration intervals overestimate WBF growth due to the artificially prolonged co-existence of ice and water within the MPC regime. Furthermore, grid-imposed phase homogeneity will likely yield an overestimation in WBF growth rates in simulations performed at the scale of traditional climate models and likely at the convection-permitting scale also. In addition, WBF growth is likely overestimated due to the high bias in low-level cloud ice occurrence. Changes with respect to cloud ice detrainment from shallow convection are of secondary importance for SO MPC statistics.