Important ice processes in Southern Ocean mixed-phase clouds are missed by GCMs
Submitter
Liu, Xiaohong — Texas A&M University
Area of research
General Circulation and Single Column Models/Parameterizations
Journal Reference
Science
Global climate models (GCMs) are challenged by difficulties in simulating cloud phase and cloud radiative effect over the Southern Ocean (SO). Some of the new-generation GCMs predict an excess of liquid and insufficient ice in mixed-phase clouds. This misrepresentation of cloud phase in GCMs leads to weaker negative cloud feedback over the SO and a higher climate sensitivity. In this study, we conducted an integrated model-observational study focusing on a crucial source of uncertainty in GCMs: ice formation and evolution over the pristine SO. This study highlights the importance of accurately representing the cloud phase over the pristine remote SO by considering the ice nucleation of sea spray organic aerosols (SSOAs) and secondary ice production (SIP), which are currently missing in most GCM cloud microphysics parameterizations.
Impact
Through an integrated model-observational approach, researchers have emphasized the crucial role of considering the ice nucleation of SSOA and SIP in accurately representing the cloud phase over the pristine remote Southern Ocean (SO). This study represents the first quantitative assessment of the relative contribution of primary ice nucleation and secondary ice production to ice particle formation in SO mixed-phase clouds. The findings of this study underscore the significance of accurately representing the cloud phase over the pristine remote SO. By incorporating the ice nucleation of SSOA and SIP, which are currently absent in most GCM cloud microphysics parameterizations, we can improve the fidelity of modeling cloud properties and their impacts on the future climate projection.
Summary
This study addresses a significant uncertainty in the Community Earth System Model version 2 (CESM2) regarding cloud phase, specifically focusing on ice formation in pristine SO clouds. By comparing model simulations against data from the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES), the researchers aimed to shed light on this key uncertainty. The results indicate that SSOAs are the most important ice nucleating particles (INPs) over the SO, with concentrations approximately one order of magnitude higher than those of dust INPs. Moreover, the study reveals that SIP is the dominant mechanism of ice production in moderately cold clouds with temperatures above -20°C. The inclusion of SIP in the model significantly enhances the in-cloud ice number concentrations by 1-3 orders of magnitude, leading to improved agreement with observations. Overall, this study highlights the necessity of considering both the ice nucleation of SSOA and the SIP processes, which are currently missing in most GCM cloud microphysics parameterizations.