Vertical variability of microphysical properties of Southern Ocean stratus clouds observed during the SOCRATES field campaign

Ubiquitous low boundary layer clouds over the Southern Ocean (SO) have a high occurrence of supercooled liquid water (SLW). Several factors, such as cloud-aerosol interactions, primary ice nucleation, secondary ice processes, radiative cooling, turbulence, and surface heat flux, influence their formation and maintenance. Unlike high-latitudes in the northern hemisphere, there have been limited observations of such clouds in the southern hemisphere.

The Southern Ocean Cloud Radiation Aerosol Transport Experimental Study (SOCRATES) conducted in the region south of Hobart, Australia between 15 Jan. - 26 Feb., 2018 focused on cloud-aerosol interactions in pristine conditions. The National Science Foundation/National Center for Atmospheric Research Gulfstream-V (G-V) aircraft was equipped with a cloud droplet probe, 2D-S (Stereo) probe, two-dimensional optical array cloud probe, precipitation imaging probe, and Particle Habit Imaging and Polar Scattering (PHIPS) probe to measure cloud particle size distributions. Bulk mass was measured by a King probe and a counterflow virtual impactor (CVI). Aerosol number concentrations were sampled by an ultra-high sensitivity aerosol spectrometer, and cloud condensation nuclei (CCN) and ice nucleating particles (INPs) were measured. The presence of SLW was indicated by a Rosemount icing detector (RICE).

Thermodynamic phase is determined using the shape of cloud droplet size distributions, RICE voltage change, particle images, PHIPS single-particle angular scattering functions, and bulk cloud mass. The dependence of the distributions of the size, shape and phase of cloud particles on aerosol, INP and CCN concentrations above and below cloud, overlying free troposphere aerosol concentrations, temperature, humidity, wind speed, meteorology (e.g., location in cold sector, stratiform/cumuliform cloud, open or closed cells) and sea surface temperature is examined for all ascents and descents of the G-V through cloud. Characteristics of concentrations, size distributions, and shapes of cloud particles are quantified as functions of normalized cloud height and cloud thickness. Correlations between, cloud, aerosol, CCN, INP, and environmental conditions are determined. Implications for processes occurring in SO boundary layer clouds are discussed.