Evaluation of Ice and Mixed-Phase Cloud Properties in Three Climate Model Simulations using Airborne Observations over the Southern Ocean

Biases and uncertainties of cloud coverage and reflected shortwave radiation in global climate model simulations have been found to be related to phase partitioning between ice and supercooled liquid water. In this study, a comparative analysis of ice and supercooled water distributions and aerosol indirect effects is conducted between the NCAR Community Atmospheric Model version 5 and 6 (CAM5 and CAM6), the U.S. Department of Energy (DOE) Energy Exascale Earth System Model (E3SM) and the NSF Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) airborne observation. Compared with the 100-km spatially averaged observations, CAM6, as the descendant of CAM5, shows an improvement by allowing supercooled liquid water below -15C. While having similar average IWC and LWC as the observed at 0 to -25C, CAM6 overestimates LWC and underestimates IWC at -40 to -25C by 1 2 orders of magnitude. On the other hand, E3SM also overestimates LWC below -20C by 1 2 orders of magnitude. In addition, E3SM underestimates (overestimates) IWC below (above) -15C by 0.5 1 orders of magnitude. This leads to lower ice phase frequencies in E3SM between -35C and -20C, but higher ice phase frequencies above -20C. In terms of the aerosol indirect effects on cloud microphysical properties, the 100-km scale observations show slight increases of LWC, Nliq, IWC and Nice with increasing aerosol number concentrations between -20C and -10C. In contrast, no significant aerosol indirect effect is seen for IWC and Overall, compared with the CMIP5 models, improvements are seen in CESM2/CAM6 and E3SM/EAMv1 for allowing more supercooled liquid water, yet a deficiency of IWC has been found in simulations over the Southern Ocean, which requires further investigation to the model parameterizations.