The Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES): An Observational Campaign for Determining Role of

Clouds, Aerosols and Radiation in Climate SystemTHE SOCRATES PLANNING TEAM: Greg McFarquhar (mcfarq@atmos.uiuc.edu),Roj Marchand,

Robert Wood, Chris Bretherton, Alain Protat, Patricia Quinn, Steven Siems, Simon Alexander, and Bob Weller

What is SOCRATES ?

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, and remote from most human influences. It influences the atmospheric and oceanic circulation of the entire southern hemisphere and beyond.

This poster discusses motivation and proposed plans for a set of international experiments that will take place in the Southern Ocean, south of New Zealand and Australia from 2016 through 2018. These experiments include ground-based, shipborne and aircraft measurements that together form the SO Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES).

The material shown here is taken from the SOCRATES whitepaper, which was developed following a workshop on Southern Ocean Cloud, Aerosols, Radiation, and the Air-Sea Interface that was held at the University of Washington in March 2013. The white paper and workshop presentations are available at: http://www.atmos.washington.edu/socrates/

MotivationsThe remoteness from anthropogenic and natural continental aerosol sources makes the SO a unique testbed for our understanding of cloud-aerosol interaction, both for liquid and ice clouds, and the role of marine biogenic aerosols and their precursors.

Climate models are challenged by uncertainties and biases in the simulation of SO clouds, aerosols, and air-sea exchanges which trace to poor physical understanding of these processes in this region.

These biases affect the simulated global energy budget (above), the location of tropical rainfall belts, simulation of anthropogenic indirect aerosol effects on climate, and may impact simulated global cloud feedbacks and carbon-cycle feedbacks on climate change. These biases also affect our understanding of the vast uptake of carbon dioxide into the SO. The SO surrounds Antarctica and therefore interacts closely with massive ice shelves whose stability to climate change is uncertain.

There have been sparse and infrequent observations of clouds, aerosols, precipitation, radiation and the air-sea interface in this region.

Theme 1: Documenting the synoptically-varying vertical structure of Southern Ocean boundary layers and clouds

Cyclone compositing indicates consistent patterns of insufficient reflected shortwave in the cold, dry regions of the cyclones. Figure shows bias in absorbed shortwave radiation for AMIP models from Bodas-Salcedo et al. (2013).

Hypothesis 1.1: A primary reason that most climate models simulate too little cloud in the

cold sector of mid-latitude cyclones is inadequate liquid water reaching the tops of

parameterized shallow convective clouds, due to vertical transport and microphysical biases.

Hypothesis 1.2: Over the SO, there is substantial free-tropospheric aerosol variability that is

mainly tied to synoptically-varying long-range transport from remote oceanic and continental

sources. Within the boundary layer, local processes such as surface sources and

precipitation scavenging are also important contributors to aerosol variability.

Hypothesis 1.3: Synoptic-scale aerosol variability

has measurable effects on the cloud microphysical and radiative characteristics

through its influence on cloud droplet and ice crystal concentrations.

Theme 2: Understanding seasonal and synoptic variability in Southern Ocean cloud condensation and ice nucleus concentration and the role of local biogenic sources.

Seasonal cycles of cloud drop concentration (Nd) and CCN concentration over the Southern Ocean. Nd data are derived from from MODIS (red), limited aircraft flights during winter and summer (black squares), and measurements at Cape Grim (Ayers and Gras 1991).

Hypothesis 2.1: Entrainment of biogenically-derived aerosols from the free troposphere constitutes a major source of CCN for Southern Ocean PBL clouds during summer, but sea-salt

aerosols are the dominant CCN source in winter.

Hypothesis 2.2: Biogenic particles are the dominant source of ice nuclei over the Southern Ocean

CN

Theme 3: Supercooled liquid and mixed-phase clouds

Probability of cloud containing supercooled liquid water between -40 and 0 oC, retrieved using CALIPSO depolarization measurements from DARDAR algorithm of Delanoë and Hogan (2008)

Hypothesis 3.1: Supercooled liquid clouds contribute substantially to observed cloud

reflectance over the Southern Ocean.

Hypothesis 3.2: At similar temperatures and latitudes, there are systematic differences in the sizes and concentrations of supercooled drops, as well as in the mass contents and

concentrations of ice crystals between Northern and Southern Hemisphere clouds.

Hypothesis 3.3: The activity of IN is the main modulator of the range of temperatures at

which supercooled stratus clouds are observed.

CMIP5 model clouds do not reflect enough sunlight over the SO. Ensemble mean error for CMIP5 models in shortwave radiation absorbed by the Earth System. Positive values indicate too much shortwave radiation absorbed (Trenberth and Fasullo 2010).

Hypotheses

SOCRATES hypotheses are categorize into four overlapping science themes. By design, these themes also encompass much of the aforementioned climate modelling uncertainty, but they are organized at a process level that flows into falsifiable hypotheses that SOCRATES is designed to test.

CCN@0.2%

Pres

sure

[hPa

]

CN (all particles)

Profiles of CN and CCN showing more CN, and as many CCN in the free troposphere as the boundary layer. Hudson et al. (1998) ACE-1 aircraft composites

Activities / Platforms• Ground-based observations (March 2016 - March 2018) and

routine profiling of lower atmospheric properties from Macquarie Island which will include cloud radar, lidar, passive microwave, broad-band radiative fluxes (supported by U.S. DOE, AAD, and BoM) as part of the Macquarie Island Cloud and Radiation Experiment (MICRE); and will also include unmanned aerial systems (UAS) for enhanced sampling and aerosol profiling (NSF).

Intensive but short duration (1-2 month) airborne observations (NSF G-V, NOAA P3, and NASA DC-8 or ER-2) during both summer and winter, including in situ and remote sensing observations

Intensive, short duration (1-2 month) shipboard observations (R/V Investigator, NOAA R/V Ron Brown, NSF UNOLS) during summer and winter, including atmospheric in situ and remote observations and microlayer, surface, and sub-surface seawater observations.

Analysis of cloud, aerosol and meteorological parameters retrieved from satellites. Construction and analysis of model simulations over a variety of temporal and spatial scales. Individually, each of these activities is of high scientific value, and doing them as part of a coherent

strategy will create the strong synergies needed to address SOCRATES hypotheses.

Vertical distribution of cloud top phase retrieved from MODIS operations product (Platnick et al. 2003), CALIOP (Hu et al. 2010) and DARDAR algorithm (Delanoë and Hogan 2010). From Huang et al (2014b). 

Theme 4: Advancing satellite retrievals related to clouds, precipitation, and aerosols

Hypothesis 4.2: Satellite VIS based estimates of SO liquid cloud droplet concentration and LWP

have important biases, particularly in winter, due to the persistently low sun angle and difficulty in separating the effects of cloud liquid, cloud ice,

and precipitation. These biases can be reduced by more careful consideration of horizontal

inhomogeneity and cloud phase screening.

Hypothesis 4.3: Better satellite estimates of accumulation-mode

aerosol within the SO boundary layer can be

developed with SO surface and airborne observations.

Hypothesis 4.1: Errors in current remote-sensing estimates of SO mixed-phase / multilayer cloud and precipitation properties can be reduced by using detailed in-

situ observations to constrain the retrieval assumptions.

Larger difference in satellites retrievals of precipitation rates, as well as, cloud and aerosol optical and microphysical properties are found over the SO than most other regions.

Satellite retrievals over the SO are challenging because of prevalent multi-layer / mid-level / mixed-phase clouds, low solar zenith angles, large-cloud-fractions, and low aerosol concentrations.