Weather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation, and radiative processes, and their interactions. Projects between 2016 and 2018 used in situ probes, radar, lidar, and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN), and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF-NCAR G-V aircraft flying north-south gradients south of Tasmania, at Macquarie Island, and on the R/V Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons. Results show largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multilayered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of dynamics and turbulence that likely drive heterogeneity of cloud phase. Satellite retrievals confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.

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Observations of clouds, aerosols, precipitation, and surface radiation over the southern ocean

. Number
concentrations of ice-nucleating particles ( NINP ) in the Arctic
were derived from ground-based filter samples. Examined samples had been
collected in Alert (Nunavut, northern Canadian archipelago on Ellesmere
Island), Utqiaġvik, formerly known as Barrow (Alaska), Ny-Alesund
(Svalbard), and at the Villum Research Station (VRS; northern Greenland). For
the former two stations, examined filters span a full yearly cycle. For VRS,
10 weekly samples, mostly from different months of one year, were included.
Samples from Ny-Alesund were collected during the months from March until
September of one year. At all four stations, highest concentrations were
found in the summer months from roughly June to September. For those stations
with sufficient data coverage, an annual cycle can be seen. The spectra of
 NINP observed at the highest temperatures, i.e., those obtained
for summer months, showed the presence of INPs that nucleate ice up to
 −5 ∘ C. Although the nature of these highly ice-active INPs could
not be determined in this study, it often has been described in the
literature that ice activity observed at such high temperatures originates
from the presence of ice-active material of biogenic origin. Spectra observed
at the lowest temperatures, i.e., those derived for winter months, were on
the lower end of the respective values from the literature on Arctic INPs or
INPs from midlatitude continental sites, to which a comparison is presented
herein. An analysis concerning the origin of INPs that were ice active at
high temperatures was carried out using back trajectories and satellite
information. Both terrestrial locations in the Arctic and the adjacent sea
were found to be possible source areas for highly active INPs.

/pdf/annual-variability-of-ice-nucleating-particle-concentrations-50bhy0vecp.pdf

Annual variability of ice-nucleating particle concentrations at different Arctic locations

Overview of biological ice nucleating particles in the atmosphere.

. Shallow clouds covering vast areas of the world's middle- and high-latitude oceans play a key role in dampening the global temperature
rise associated with CO 2 . These clouds, which contain both ice and
supercooled water, respond to a warming world by transitioning to a state
with more liquid water and a greater albedo, resulting in a negative
“cloud-phase” climate feedback component. Here we argue that the magnitude
of the negative cloud-phase feedback component depends on the amount and
nature of the small fraction of aerosol particles that can nucleate ice
crystals. We propose that a concerted research effort is required to reduce
substantial uncertainties related to the poorly understood
sources, concentration, seasonal cycles and nature of these ice-nucleating
particles (INPs) and their rudimentary treatment in climate models. The
topic is important because many climate models may have overestimated the
magnitude of the cloud-phase feedback, and those with better representation
of shallow oceanic clouds predict a substantially larger climate warming. We
make the case that understanding the present-day INP population in shallow
clouds in the cold sector of cyclone systems is particularly critical for
defining present-day cloud phase and therefore how the clouds respond to
warming. We also need to develop a predictive capability for future INP
emissions and sinks in a warmer world with less ice and snow and potentially
stronger INP sources.

/pdf/opinion-cloud-phase-climate-feedback-and-the-importance-of-4ue5mm1gka.pdf

Opinion: Cloud-phase climate feedback and the importance of ice-nucleating particles

Uncertainty in radiative forcing caused by aerosol–cloud interactions is about twice as large as for CO2 and remains the least well understood anthropogenic contribution to climate change. A major cause of uncertainty is the poorly quantified state of aerosols in the pristine preindustrial atmosphere, which defines the baseline against which anthropogenic effects are calculated. The Southern Ocean is one of the few remaining near-pristine aerosol environments on Earth, but there are very few measurements to help evaluate models. The Antarctic Circumnavigation Expedition: Study of Preindustrial-like Aerosols and their Climate Effects (ACE-SPACE) took place between December 2016 and March 2017 and covered the entire Southern Ocean region (Indian, Pacific, and Atlantic Oceans; length of ship track >33,000 km) including previously unexplored areas. In situ measurements covered aerosol characteristics [e.g., chemical composition, size distributions, and cloud condensation nuclei (CCN) number concentrations], trace gases, and meteorological variables. Remote sensing observations of cloud properties, the physical and microbial ocean state, and back trajectory analyses are used to interpret the in situ data. The contribution of sea spray to CCN in the westerly wind belt can be larger than 50%. The abundance of methanesulfonic acid indicates local and regional microbial influence on CCN abundance in Antarctic coastal waters and in the open ocean. We use the in situ data to evaluate simulated CCN concentrations from a global aerosol model. The extensive, available ACE-SPACE dataset (https://zenodo.org/communities/spi-ace?page=1&size=20) provides an unprecedented opportunity to evaluate models and to reduce the uncertainty in radiative forcing associated with the natural processes of aerosol emission, formation, transport, and processing occurring over the pristine Southern Ocean.

/pdf/overview-of-the-antarctic-circumnavigation-expedition-study-2pteimam8c.pdf

Overview of the Antarctic circumnavigation expedition: study of preindustrial-like aerosols and their climate effects (ACE-SPACE)

A likely important feature of the poorly understood aerosol‐cloud interactions over the Southern Ocean (SO) is the dominant role of sea spray aerosol, versus terrestrial aerosol. Ice nucleating particles (INPs), or particles required for heterogeneous ice nucleation, present over the SO have not been studied in several decades. In this study, boundary layer aerosol properties and immersion freezing INP number concentrations (nINPs) were measured during a ship campaign that occurred south of Australia (down to 53 °S) in March‐April 2016. Ocean surface chlorophyll‐a concentrations ranged from 0.11‐1.77 mg m‐3 and nINPs were a factor of 100 lower than historical surveys, ranging from 0.38 to 4.6 m‐3 at ‐20 °C. The INP population included organic heat‐stable material, with contributions from heat‐labile material. Lower INP source potentials of SO seawater samples compared to Arctic seawater were consistent with lower ice nucleating site densities in this study compared to north Atlantic air masses.

/pdf/observations-of-ice-nucleating-particles-over-southern-ocean-2p1u7wd05k.pdf

Observations of ice nucleating particles over Southern Ocean waters

Abstract. This study uses ship-based weather radar observations collected from research vessel Investigator to evaluate the Australian weather radar network calibration monitoring technique that uses spaceborne radar observations from the NASA Global Precipitation Mission (GPM). Quantitative operational applications such as rainfall and hail nowcasting require a calibration accuracy of ±1 dB for radars of the Australian network covering capital cities. Seven ground-based radars along the western coast of Australia and the ship-based OceanPOL radar are first calibrated independently using GPM radar overpasses over a 3-month period. The calibration difference between the OceanPOL radar (used as a moving reference for the second step of the study) and each of the seven operational radars is then estimated using collocated, gridded, radar
observations to quantify the accuracy of the GPM technique. For all seven
radars the calibration difference with the ship radar lies within ±0.5 dB, therefore fulfilling the 1 dB requirement. This result validates the concept of using the GPM spaceborne radar observations to calibrate national weather radar networks (provided that the spaceborne radar maintains a high calibration accuracy). The analysis of the day-to-day and hourly variability of calibration differences between the OceanPOL and Darwin (Berrimah) radars also demonstrates that quantitative comparisons of gridded radar observations can accurately track daily and hourly calibration differences between pairs of operational radars with overlapping coverage (daily and hourly standard deviations of ∼ 0.3 and ∼ 1 dB, respectively).


W. Ponsonby

Papers

Observations of ice nucleating particles over Southern Ocean waters

Three-way calibration checks using ground-based, ship-based, and spaceborne radars