Dependence of Vertical Variability of Microphysical Properties of Southern Ocean Stratus Clouds on Environmental Conditions Observed during the SOCRATES Field Campaign: Preliminary Results

Persistent low boundary layer clouds over the Southern Ocean (SO) have a high occurrence of supercooled liquid water (SLW) at cloud top and have a large impact on radiative fluxes, and hence on cloud feedbacks necessary for understanding climate change. It has been shown that several factors, such as cloud-aerosol interactions, primary and secondary ice nucleation, radiative cooling, turbulence, and surface heat flux, influence the formation and maintenance of those clouds. Unlike high-latitude regions in northern hemisphere, there have been a limited number of observations of such clouds in the southern hemisphere.

The Southern Ocean Cloud Radiation Aerosol Transport Experimental Study (SOCRATES) was conducted in the region near Hobart, Australia (42-62 °S and 140-156 °E) between 15 Jan. – 26 Feb., 2018 focusing on cloud and aerosol interactions in clean environmental conditions. During SOCRATES the National Science Foundation/National Center for Atmospheric Research Gulfstream-V (G-V) aircraft conducted 15 research flights. The cloud droplet probe (CDP), 2D-S (Stereo) probe, two-dimensional optical array cloud probe (2DC), precipitation imaging probe (PIP), and Particle Habit Imaging and Polar Scattering (PHIPS) probe measured cloud particle size distributions. Cloud bulk mass was measured by a King probe, a counterflow virtual impactor (CVI), aerosol concentrations were sampled by an ultra-high sensitivity aerosol spectrometer (UHSAS), and cloud condensation nuclei (CCN) and ice nucleating particle (INP) were also measured. The presence of SLW within cloud was indicated by a Rosemount icing detector (RICE).

The thermodynamic phase of each cloud measurement is first determined using the shape of the cloud droplet size distributions, RICE probe voltage change, particle images, PHIPS angular light scattering functions, and bulk cloud mass measured by the King probe and CVI. 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) and sea surface temperature is examined using these data. All cloud profiles sampled when the G-V aircraft ascended or descended through cloud during routine sawtooth maneuvers are used in this analysis. Characteristics of concentrations, size distributions, and shapes of cloud particles are quantified as functions of normalized cloud height and cloud thickness as part of this analysis. Correlations between, cloud, aerosol, CCN, INP, and environmental conditions are determined. Implications for processes occurring in SO boundary layer clouds are discussed.