Investigating Secondary Ice Production in Summertime Southern Ocean Cumuli

Persistent cloud cover over the Southern Ocean exerts a powerful influence on the global radiative balance and climate projections. It has been speculated that the ubiquity of these clouds is due to the continued presence of supercooled liquid water and a lack of ice, based upon past satellite-retrieved cloud phase studies. This study uses new in-situ aircraft measurements within cumulus clouds situated in the cold sector of cyclones over the Southern Ocean. The goals are to determine if these clouds do consist mostly of supercooled liquid water as suggested by satellite measurements, and if so, possible reasons for the lack of ice and the ineffectiveness of secondary ice production.

Two research flights conducted during the recent summertime Southern Ocean, Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) field campaign (Jan-Feb 2018) sampled developing supercooled cumulus clouds near 50 degrees latitude, south of Tasmania. Cloud passes within and above the rime splintering zone (also referred to as the Hallett-Mossop zone) were made to sample the microphysical development within the clouds, and clear-air passes above the cloud tops and below the cloud bases allowed sampling of the cloud environment and the ambient aerosol, including ice-nucleating particles. The data include dropsondes, observations from the HIAPER Cloud Radar and Lidar, size distributions and bulk properties derived from in-situ cloud probes including the Particle Habit Imaging and Polar Scattering (PHIPS) probe, and standard research aircraft thermodynamic and dynamic cloud measurements.

Of particular interest is scrutiny of the data related to the cloud characteristics necessary for the rime splintering process (e.g., graupel, cloud droplets exceeding 25 micrometers diameter, and their occurrence within the -3 to -9 C temperature range) which has historically been documented in cumuli off the coast of Tasmania. Limited evidence supporting secondary ice production by rime splintering is presented. Satellite products are used to place the sampled fields of cumuli into a seasonal context by estimating the frequency of retrieved cloud phase in similar fields of clouds; inherent uncertainties due to different retrieval algorithms are noted. Future plans for high-resolution, idealized modeling of the interactions between supercooled liquid water and ice in these clouds will be discussed.