The Southern Ocean (SO) surrounding Antarctica and consisting of parts of the southern Atlantic, Pacific and Indian Oceans, is one of the cloudiest places on Earth. The fractional cover of low clouds (below 3-km altitude) prevalent in the warm and cold sectors of frequent extratropical cyclones reaches nearly 80% year-round (Mace et al. 2009; IPCC 2013). Relative to more easily sampled locations, there is a dearth of in situ observations of aerosols, clouds, and precipitation over the SO, especially south of 60°S. This makes it difficult to evaluate remote sensing retrieval products. General circulation models (GCMs) have difficulty with simulating the present-day aerosol, cloud coverage and cloud phase over the SO, with implications for anthropogenic aerosol impacts and cloud feedbacks on climate (e.g., Trenberth and Fasullo 2010; Tan et al. 2016), two key uncertainties in interpreting the historical climate record and projecting future climate change.
Numerical weather prediction (NWP) and GCMs have struggled to correctly simulate the radiative budget over the SO due to low cloud biases. Most Coupled Model Intercomparison Project phase 5 (CMIP5) models predict too much shortwave (SW) radiation absorbed over the SO region (Bodas-Salcedo et al. 2014, 2016; Naud et al. 2014), with impacts on ocean temperature, the Southern Hemisphere (SH) jet (Ceppi et al. 2014), Antarctic sea ice trends (Flato et al. 2013) and tropical rainfall (Hwang and Frierson 2013). Comparisons with satellite data indicate that model radiative biases are due primarily to a lack of low- and midlevel clouds in the cold sectors of cyclones (e.g., Flato et al. 2013; Bodas-Salcedo et al. 2014). It was hypothesized on the basis of limited observations, mainly from satellites, that GCMs might be glaciating what are in reality persistent supercooled liquid clouds. Indeed, GCM simulations in which convective parameterizations have been forced to produce greater amounts of supercooled liquid water (SLW) have reduced SW biases (Kay et al. 2016).
A related motivating issue is the apparent paucity of ice nucleating particles (INPs) over the SO (Bigg 1973; Burrows et al. 2013), due to it being far removed from any continental air sources; INP parameterizations are based mostly on Northern Hemisphere (NH) observations. Satellite retrievals of cloud-top phase indicate that SLW is more prevalent over the SO than at equivalent latitudes in the NH (Choi et al. 2010; Hu et al. 2010; Morrison et al. 2011). This could be because SO supercooled clouds are starved for INPs, as hypothesized by Kanitz et al. (2011) and Vergara-Temprado et al. (2018).
A final overarching question is how droplet concentrations are regulated in SO boundary layer (BL) clouds in a synoptically active environment with high winds over a biologically productive ocean. The SO is a biologically unique marine aerosol environment, its pristine nature is as close to preindustrial conditions as exists on Earth, and thus represents a natural laboratory to study anthropogenic aerosol indirect radiative forcing (Carslaw et al. 2013; Ghan et al. 2013). Hoose et al. (2009) showed that GCMs with prognostic aerosols tended to simulate SO clouds with too few droplets compared to satellite observations, making them overly susceptible to human aerosol perturbations. One hypothesis is that these models underestimate marine biogenic production of cloud condensation nuclei (CCN). Satellite retrievals and some previous field observations show the SO has a strong summertime maximum in cloud droplet concentration Nc (Boers et al. 1996, 1998), CCN (Ayers and Gras 1991), and aerosol concentrations Na (Sciare et al. 2009) correlated with phytoplankton productivity. Quinn et al. (2017) found that except for the high southern latitudes, sea spray contributes less than 30% to the total CCN. However, observations in the Aerosol Characterization Experiment 1 (ACE-1) campaign suggested that copious sulfate aerosols can be produced in the outflow of shallow precipitating cumulus clouds from nucleation of marine biogenic gases (Hudson et al. 1998; Clarke et al. 1998; Russell et al. 1998).
Thus, there is a clear need for observations to help better model the natural aerosol life cycle and mixed-phase BL cloud over the SO. Prior to the campaigns described here, cloud and aerosol measurements over the SO included those listed in Table 1. But, further observations on cloud and aerosol concentrations over cold waters poleward of 60°S are critical for understanding cloud processes over the SO. To understand the transition of aerosols to CCN over the remote oceans, it is necessary to quantify particle sources and sinks as well as processes related to their aging, including the role of new particle formation in the free troposphere, generation from breaking waves over the ocean, generation of biogenic particles from gas phase oceanic emissions, the role of precipitation scavenging, and the effects of updrafts and dynamics on clouds.
Previous field campaigns and data collection activities over the SO.
Climate model evaluation, and much current knowledge of SO clouds, aerosols, precipitation, and surface radiation properties is based on satellite retrievals. Satellite studies have found that cloud-top SLW is more frequent over the SO (Hu et al. 2010; Choi et al. 2010; Huang et al. 2012a,b; Kanitz et al. 2011; Morrison et al. 2011; Protat et al. 2014; Huang et al. 2015a, 2016) and Antarctic (Grosvenor et al. 2012) than over the NH, but there are significant variations between satellite retrieval products in the frequency of cloud-top SLW (Delanoë and Hogan 2010; Huang et al. 2015a) and these retrievals tell us little about the phase of condensate below cloud top. However, potential errors in cloud retrievals, particularly those related to large solar zenith angles (Grosvenor and Wood 2014) and three-dimensional effects (Wolters et al. 2010; Zeng et al. 2012; Cho et al. 2015) remain a concern. Additional ground-based and airborne remote sensing, and airborne in situ measurements, are therefore needed to evaluate satellite retrievals.
A 2014 community workshop at the University of Washington discussed these issues, recognizing the need for a large international multiagency effort to improve the understanding of clouds, aerosols, precipitation and their interactions over the SO (Marchand et al. 2014). The workshop served as a motivation for the proposals of separate, but integrated, projects to various funding agencies in the United States and Australia. These four collaborative projects were 1) the Clouds Aerosols Precipitation Radiation and atmospheric Composition over the Southern Ocean (CAPRICORN) I and II research voyages of the Research Vessel (R/V) Investigator, led by the Australian Bureau of Meteorology (BoM), that made extensive in situ and remote sensing measurements in 2016 and 2018, respectively; 2) the 2017–18 Measurements of Aerosols, Radiation and Clouds over the Southern Ocean (MARCUS) project, during which the United States Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program Mobile Facility 2 (AMF2) was deployed on the Australian icebreaker Research Supply Vessel (RSV) Aurora Australis (AA) as it made resupply voyages to Australian Antarctic bases; 3) the 2016–18 Macquarie Island Cloud Radiation Experiment (MICRE) acquiring surface in situ and remote sensing observations using equipment from DOE ARM, the BoM, and the Australian Antarctic Division (AAD); and 4) the 2018 Southern Ocean Cloud Radiation and Aerosol Transport Experimental Study (SOCRATES) using the NSF–NCAR G-V aircraft to sample clouds, aerosols, and precipitation from Hobart, Australia, to within approximately 650 km of the Antarctic coast. Although each project was a separate effort and no formal steering committee coordinated the projects, many investigators served on the advisory board of several of the projects and there was much collaboration between the campaigns. There was one integrated planning workshop (2017 Boulder) and two integrated data workshops after the completion of the projects (2018 Boulder, 2019 Hobart). Data have been freely exchanged among participants, and a special collection of papers in the Journal of Geophysical Research/Geophysical Research Letters covering all four projects has been established and is expected to grow substantially over the next few years. This collaboration is essential to maximize the projects’ impacts. Synergistically these data provide the best available measurements of the BL and free troposphere structure, together with vertical distributions of liquid and mixed-phase clouds and aerosols properties, over cold SO waters where SLW and mixed-phase BL clouds are frequent.