The influence of primary nucleation and rime splintering on ice number concentrations in southern ocean cumuli

Persistent cloud cover over the Southern Ocean exerts a powerful influence on the global radiative balance and climate projections. Satellite-based cloud phase studies suggest these clouds contain supercooled liquid water and a lack of ice, which may promote their longevity compared to those produced in GCMs. This study uses two cumulus-sampling research flights from the recent summertime Southern Ocean, Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) field campaign in February 2018. An analysis of the radar and microphysical data collected by the aircraft instrumentation show that only about one third of the clouds contain ice; those having large amounts of ice (over 100 per Liter) tend to have multiple updrafts, and also possess the necessary characteristics for secondary ice production by rime splintering (graupel, cloud droplets exceeding 25 μm diameter, and occurrence within the -3 to -9 C temperature range). High-resolution, 3D idealized numerical simulations of a cumulus cloud undergoing primary ice nucleation with the observed numbers of ice-nucleating aerosol particles fail to reproduce the observed high ice number concentrations. Only through the mechanism of multiple, successive thermals in the simulations, combined with the rime splintering process, are the high ice number concentrations able to be reproduced. Sensitivity studies demonstrate the sensitivity of the calculations to the number of ice-nucleating particles and the effectiveness of the rime splintering process