An Evaluation of Cloud‐Precipitation Structures in Mixed‐Phase Stratocumuli Over the Southern Ocean in Kilometer‐Scale ICON Simulations During CAPRICORN.

A persistent shortwave radiative bias of Southern Ocean (SO) clouds in climate models is strongly associated with incorrect cloud phase representation, which impacts precipitation. Measurements characterizing precipitation in low‐level mixed‐phase clouds, which frequently form over the SO, are rare, and our understanding of precipitation efficacy within these clouds remains limited. The simulated surface precipitation bias has an indirect effect on determining global climate sensitivity and a direct impact on the hydrological cycle. This study investigates the representation of low clouds, cloud variability, and precipitation statistics over the SO in real‐case Icosahedral Nonhydrostatic (ICON) simulations at the kilometer scale. The simulations are contrasted with 48 hr of continuous shipborne observations of open and closed‐cell stratocumuli, south of Tasmania. Our simulations show the significance of heavily rimed particle formation, their in‐cloud growth, and subcloud melting to capture the observed cloud‐precipitation vertical structure. In addition, supercooled drizzle formation impacts the vertical structure and precipitation statistics. ICON captures the observed intermittency of precipitation even at a standard vertical resolution of 200 m in the boundary layer but only captures the observed sparse distribution of intense precipitation (>1 mm hr−1) when the maximum vertical resolution is reduced to 100 m. However, the simulations of the 2‐day accumulated precipitation and the radiative effect are largely insensitive to the vertical resolution. The cloud reflectivity of the broken cloud deck is underestimated due to negative biases in cloud optical depth. Plain Language Summary: Stratocumulus (Sc) clouds cover a large portion of the Southern Ocean (SO), where they substantially cool the ocean surface. Our understanding of the complex physics of these clouds, which include both liquid and ice remains incomplete. Accordingly, the representation of these clouds in global climate and weather models remains biased. In particular, their timing, frequency of occurrence, cloud phase and distribution, cloud cover, and precipitation characteristics are still associated with uncertainties. This results in biases in the energy balance over the SO and global equilibrium climate sensitivity. We use measurements from the Clouds, Aerosols, Precipitation, Radiation, and atmospherIc Composition Over the southeRn oceaN (CAPRICORN) voyage south of Tasmania, to evaluate the representation of broken cloud fields, the dominant ice processes, and the precipitation characteristics in high‐resolution numerical simulations. Our results suggest that, in addition to capturing the observed discrete cloud events, graupel‐like particle formation, its growth in the cloud layer, and subsequent melting in the subcloud layer are critical processes in accurately representing the SO broken low cloud fields and precipitation characteristics during CAPRICORN. Additionally, compared to observations, the simulated clouds are too reflective. Key Points: In‐cloud heavy riming, subcloud melting, and supercooled processes are vital to capture observed SO low‐cloud‐precipitation structureA finer vertical grid spacing of 100 m or less is needed to capture the frequency of observed intense surface precipitation (>1 mm hr−1) eventsWhile the simulated precipitation statistics agree with observations, the clouds are associated with a negative radiative bias [ABSTRACT FROM AUTHOR]