New Unique Observations of Clouds, Aerosols and Precipitation over the Southern Ocean: An Overview of SOCRATES and MARCUS

The Southern Ocean (SO) is a unique environment for examining cloud-aerosol interactions, removed from anthropogenic and natural continental aerosol sources, most similar to pre-industrial conditions that must be understood to quantify aerosol indirect effects. But, weather and climate models are challenged by uncertainties and biases in the simulation of SO clouds, aerosols, precipitation and radiation. Thus, the 2018 Southern Ocean Cloud Radiation Aerosol Transport Experimental Study (SOCRATES) and the 2017/18 Measurements of Aerosols Radiation Clouds over the Southern Oceans (MARCUS) projects were held to make comprehensive measurements of boundary layer structure and vertical distributions of liquid and mixed-phase clouds and aerosols, including cloud condensation nuclei (CCN) and ice nucleating particles (INPs), over cold waters where supercooled and mixed-phase boundary layer clouds are frequent.

During SOCRATES, the National Science Foundation/National Center for Atmospheric Research G-V aircraft flew 15 research flights comprising 117 flight hours using in-situ probes to measure cloud and aerosol size distributions and bulk cloud mass, CCN and INP concentrations, as well as remotely retrieving cloud and aerosol properties with a cloud radar and lidar and releasing dropsondes to characterize the thermodynamic environment, principally in the cold sector of cyclonic storms. During SOCRATES, there were 3 overflights of the Australian R/V Investigator which was making measurements of ocean eddy and biological sampling, surface meteorology, aerosol, CCN and INP properties, as well as retrieving cloud and aerosol properties. For MARCUS, the Department of Energy Atmospheric Radiation Measurement Program’s Mobile Facility was installed on the icebreaker Aurora Australis as it made 4 transits between Hobart, Australia and the Australian Antarctic stations Mawson, Casey and Davis as well as Macquarie Island making in-situ measurements of aerosol size and composition, retrievals of cloud and aerosol properties, and characterizing boundary layer structure with radiosondes.

Preliminary results from SOCRATES and MARCUS will be highlighted. Notable observations include the presence of a very clean atmosphere with numerous small and few large aerosol particles above cloud suggesting regions of new particle formation, the presence of high concentrations of aerosols 500 feet above the ocean surface especially when wind speeds were intense, the frequent presence of supercooled water in clouds at temperatures as low as -33 Celsius with frequent geometrically thin and multiple layers, and the frequent presence of small-scale generating cells at cloud top believed to be responsible for production of small amounts of ice particles. Isolated ice crystals were frequently found in the presence of copious amounts of supercooled water droplets under such conditions. Further the shipborne MARCUS data are self-standing and unique in that they capture the seasonal as well as latitudinal and meteorological variability in aerosol, precipitation and cloud properties over the SO, especially south of 62 degrees south. Preliminary findings will place the SOCRATES and MARCUS cloud and aerosol data in context of air mass origin, lower tropospheric stability, wind speed and direction, location within a cyclone and relative to the oceanic polar front and sea surface temperatures. Implications for concurrent modeling studies will also be discussed.