Which INPs and secondary ice processes are necessary to accurately model a warm Arctic mixed-phase cloud?

The phase of low-level clouds plays a crucial role in their interaction with radiation, particularly in the Arctic.  Despite the Arctic's notably clean air, characterized by low aerosol and ice nucleating particle (INP) concentrations, cloud ice can still be observed at relatively warm sub-zero temperatures. 

We present a modeling closure analysis of an Arctic low-level mixed-phase cloud observed during the 2023 ARTofMELT (Atmospheric rivers and the onset of sea ice melt) campaign using the large eddy simulation model MIMICA. Comprehensive measurements of INPs and aerosols were taken at the surface, within, and above the cloud. By combining modeling and observations, we will explore which aerosol population was necessary to aid in the formation of ice within the cloud. 

The minimum observed in-cloud temperature was -8 °C, with ice present throughout the cloud’s lifetime. The highest temperature at which INPs were detected was -13 °C, with an INP concentration of approximately 1.6e-4 /L. Our findings indicate that observed INP concentrations alone are insufficient to produce a significant amount of ice in the model. This suggests the need for other processes like secondary ice formation and INP recycling, or a possible misrepresentation of microphysical processes in the model. By utilizing the model and including these missing processes, we aim to determine the necessary INP concentrations and properties, as well as the secondary ice mechanisms, to account for the observed ice. This includes analyzing the importance of local versus long-range transported aerosols and primary versus secondary ice production.