MARCUS

Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS)

1 October 2017 - 1 April 2018

Lead Scientist: Greg McFarquhar

Observatory: AMF , MAR

The Southern Ocean (SO) is the stormiest place on Earth, buffeted by winds and waves that circle the ice of Antarctica, sheathed in clouds that mantle a dynamic ocean with rich ecosystems. The remote and usually pristine environment, typically removed from anthropogenic and natural continental aerosol sources, makes the SO unique for examining cloud‐aerosol interactions for liquid and ice clouds, and the role of marine biogenic aerosols, their precursors, and sea salt. There is strong seasonality in aerosol sources and sinks over the SO that are poorly understood. Weather and climate models are challenged by uncertainties and biases in the simulation of SO clouds, aerosols, and air‐sea exchanges that trace to poor physical understanding of these processes, and by cloud feedbacks (e.g., phase changes) in response to warming. Models almost universally underestimate sunlight reflected by near surface clouds, particularly in the cold sector of cyclonic storm systems, and this might be due to difficulties in representing pervasive supercooled and mixed‐phase boundary layer clouds. Motivated by these issues, a large international multi-agency effort called the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) has been proposed to improve understanding of clouds, aerosols, air‐sea exchanges and their interactions over the SO. Researchers propose to acquire comprehensive observations of aerosols including cloud condensation nuclei (CCN) and ice nucleating particles (INP) in the boundary layer, vertical distributions of macrophysical and microphysical properties of liquid and mixed‐phase clouds, and downwelling radiative fluxes over the SO during a seven‐month period centered upon the austral summer. In particular, researchers propose the Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) field campaign. The second ARM Mobile Facility (AMF2) will be installed on the Australian Antarctic supply vessel Aurora Australis (AA) as it routinely travels between Hobart, Australia, and the Antarctic, visiting the Australian Antarctic stations Mawson, Davis, and Casey. The MARCUS observations will be self‐standing and unique within SOCRATES in that they will capture the variability in aerosol and cloud properties across the SO from spring to autumn, especially in cold waters at latitudes poleward of 60 degrees south, where supercooled and mixed‐phase boundary layer clouds in the cold sector of cyclones are frequent and where past and planned SOCRATES observations are most sparse. The data to be obtained during MARCUS under a range of synoptic settings will document how temperature‐dependent distributions of cloud properties and frequency of supercooled water vary with concentrations of CCN and INPs, synoptic regime, latitude, and season (spring, summer, fall). MARCUS data will also help in understanding the sources, sinks, and variability of CCN and INPs, the increased bias of absorbed shortwave radiation in summer in models, and conditions conducive to extensive supercooled water. Specific hypotheses will be tested under four SOCRATES themes to understand:

• the synoptically varying vertical structure of SO boundary layer clouds and aerosols
• sources and sinks of the ocean's CCN and INPs, including the role of local biogenic sources over spring, summer, and fall
• mechanisms controlling supercooled liquid and mixed‐phase clouds
• advances in retrievals of clouds, precipitation, and aerosols over the SO.

Parameterization development and testing needs are integrated in MARCUS' design and in the design of the multi-agency SOCRATES project so that systematic confrontation and improvement of leading climate models with data will be possible. In addition to measurements from the Aerosol Observing System, weather balloons with sondes, micropulse lidar, microwave radiometer, marine W‐band (95 GHz) stabilized cloud radar, ceilometer, sun photometer, downwelling radiometers, and inertial navigation system, user‐supplied filter samples will be collected and subsequently processed in order to determine immersion freezing INP concentrations as a function of temperature.