Evidence for secondary ice production in Southern Ocean open cellular convection

Ice particles present at temperatures warmer than -9°C were encountered in unexpectedly high number concentrations (up to 54 L-1) by an instrumented aircraft over the Southern Ocean (SO), off the southwest coast of Tasmania, Australia, on 07 September 2013. The sampled clouds were precipitating, characterized by mixed-phase, open-cellular shallow convection. These clouds were present within a large-scale environment characterized by cold air advection, in a pristine air mass for over 72 hours. Using a Cloud and Aerosol Spectrometer, aerosol particles (diameters > 0.6 μm) size and number concentrations were measured and ice nucleating particle (INP) number concentrations were estimated with a recognized ice nuclei parameterization scheme. The estimated INP number concentrations were in the range of 10-5-10-1 L-1 at temperature above -9°C, which is up to three orders of magnitude less than the ice number concentrations typically observed. The high ice number concentrations are largely consistent with the theoretical values when ice crystals are produced via a splinter production. The evidence suggests that secondary ice processes(likely the Hallett-Mossop mechanism) were playing a key role in generating the high ice number concentrations observed. Satellite observations from an A-Train overpass in the neighborhood during the flight period reveal a qualitatively consistent story, with patchy,mixed-phase (but predominantly supercooled liquid water) clouds observed at cloud-top temperatures around -6°C. Using back trajectory calculations, these clouds are tracked over 23 and 46 hours with A-Train observations. The presence of these clouds is found to be common over the SO during this period of time. This suggests that the ice particles present in a relatively warm temperature range could potentially be commonplace, within the widespread (up to thousands of kilometers) shallow convective cloud fields over the SO. These clouds may have important implication for the energy budget and precipitation production over this climatically important region.