Supercooled liquid clouds are ubiquitous over the Southern Ocean (SO),; even to temperatures below -20 °C, and comprise a large fraction of the; marine boundary layer (MBL) clouds. Earth system models and reanalysis; products have struggled to reproduce the observed cloud phase; distribution and occurrence of cloud ice in the region. Recent; simulations found the microphysical representation of ice nucleation and; growth has a large impact on these properties, however, measurements of; SO ice nucleating particles (INPs) to validate simulations are sparse.; This study presents measurements of INPs from simultaneous aircraft and; ship campaigns conducted over the SO in austral summer 2018, which; include the first in situ observations in and above cloud in the region.; Our results confirm recent observations that INP concentrations are; uniformly lower than measurements made in the late 1960s. While INP; concentrations below and above cloud are similar, higher ice nucleation; efficiency above cloud supports model inferences that the dominant INP; composition varies with height. Model parameterizations based solely on; aerosol properties capture the mean relationship between INP; concentration and temperature but not the observed variability, which is; likely related to the only modest correlations observed between INPs and; environmental or aerosol metrics. An updated parameterization for marine; INPs is proposed, which reduces bias relative to existing methods by; including wind speed as an additional variable. Direct and indirect; inference of marine INP size suggests MBL INPs, at least those in the; sub-2.5 μm range, are dominated by particles with diameters smaller than; 500 nm.
