Pivoting on a recent ASR project, a veteran Oklahoma researcher investigates cloud boundary layer processes in Earth’s high latitudes
In late June 2024, University of Oklahoma cloud physicist Greg McFarquhar briefly celebrated the publication of his latest paper. It was not a big celebration. He is a veteran of more than 30 years of atmospheric research and has close to 250 peer-reviewed papers to his credit.
But it was a big celebration for the study’s lead author, doctoral student Zeqian “Hazel” Xia. She is close to finishing her PhD under McFarquhar’s direction. Another PhD student, Qing Niu, also a touch away from her degree, was the lead author of McFarquhar’s second most recent paper in early May 2024.
Both publications have a few things in common.
They were funded by the Atmospheric System Research (ASR) program at the U.S. Department of Energy (DOE). Since 2012, McFarquhar has been the principal investigator (PI) of three ASR projects.
Both students are connected to McFarquhar’s latest ASR project, a 2020-2023 effort that is still underway, thanks to a no-cost extension.
And both early-career scientists, in their papers, depended on data from campaigns operated by Atmospheric Radiation Measurement (ARM), a user facility overseen by the DOE’s Office of Science.
Lasting, Long-Term ARM data
McFarquhar has a long history with ARM, too. Just in the years 2004 and 2010, when he was at the University of Illinois Urbana-Champaign, he participated in six ARM field campaigns, all involving cloud systems.
The first, in 2004, was the Mixed-Phase Arctic Cloud Experiment (M-PACE). It involved three Alaska instrument sites and a remote, in-country radiosonde launching point.
The most recent ARM campaign was Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS). McFarquhar led this 2017-2018 shipborne campaign. It deployed one of three ARM Mobile Facilities aboard Aurora Australis, a supply ship that ran regular routes between Hobart, Australia, and research stations in Antarctica.
He and his Oklahoma research group are still analyzing prolific data from MARCUS and turning out a flurry of peer-reviewed studies.
Rich Data from Stormy Seas
McFarquhar’s latest ASR project draws data from MARCUS as well as two other ARM campaigns. One is the Macquarie Island Cloud and Radiation Experiment (MICRE), led by PI Roger Marchand of the University of Washington. This 2016-2018 effort, detailed in a previous ASR feature, deployed ground instruments on this remote island in the Southern Ocean. Long-duration observations swept up data on clouds, precipitation, surface radiation, and aerosols—tiny atmospheric particles that make clouds, rain, and snow possible.
McFarquhar is a long-time student of the Southern Ocean. This vast and wild sea, which influences global climate and ocean circulations, flows completely around the Southern Hemisphere, uninterrupted by land.
Why care about the Southern Ocean?
He is the senior author of an ASR-funded May 2024 paper that begins with a truism: The wide variability in global climate predictions can be linked to deficiencies in cloud-cover simulations on the Southern Ocean, where marine cloud and aerosol properties are less studied than their oceanic counterparts in the Northern Hemisphere.
The third ARM campaign providing data for McFarquhar’s ASR project is the ASR-funded Cold-Air Outbreaks in the Boundary Layer Experiment (COMBLE). For six months in 2019 and 2020, COMBLE took place on the other side of the world, in the Norwegian Sea between Norway and the Arctic’s ice edge. The sea influences cloud conditions in the North Atlantic Ocean.
Starting on cold continents or at the edge of vast ice sheets, cold-air outbreaks (CAOs) form street-like decks of shallow cumulus clouds in the polar marine boundary layer. Regimes of such clouds, spawned when cold air races over warmer ocean water, play a vital role in the Earth’s energy balance.
“In broad terms,” says McFarquhar, “the ASR project is pulling data from three high-latitude field campaigns to expand what we know about cloud, aerosol, and boundary layer processes in some of the chilliest and most remote polar regions.”
To add gravity to the ASR project: Those cold regions are the ones most deeply impacted by a warming world.
Comparing Cold-Air Outbreaks
Data from MARCUS and COMBLE (along with ASR funding) are at work in the June 2024 paper led by Xia. It compares CAO observations made during the two campaigns to investigate the macrophysics of boundary layer clouds formed by CAOs in both the Norwegian Sea and the Southern Ocean.
Even if CAOs in both places are parametrized to assume the same environmental questions, Xia and McFarquhar ask, are there differences?
“The fundamental answer is—there are differences,” he says, “which means that we still don’t fully understand what controls cloud properties in cold-air outbreaks.”
However, the Xia paper did find that CAOs are more intense over the North Atlantic because of a larger temperature differential between the moving air masses and the ocean beneath them.
On the other hand, the boundary layer inversion was more robust over the Southern Ocean. Such inversions can alter the ocean-atmosphere energy balance by creating updrafts that move heat, moisture, and aerosols from the surface to the sky.
“A lot of this means we really need more data,” says McFarquhar, noting that a follow-up paper on CAO microphysics is in the works.
Recently, he took a step in the direction of getting more data as co-PI of the Cold Air Outbreak Experiment in the Sub-Arctic Region (CAESAR), which wrapped up in April 2024 after six weeks in Kiruna, Sweden.
The aerial campaign, funded by the National Science Foundation, studied CAOs from aboard an NSF-NCAR C-130 aircraft. Data will complement ground measurements from COMBLE.
The combined data, says McFarquhar, who served as both a CAESAR flight and ground scientist, “is going to be very, very effective in learning more about cold-air outbreaks,” including more details on cloud properties and solar radiation budgets.
Surface, Ship, and Air Needs
Getting more atmospheric measurements is a steady refrain from McFarquhar, whose career bridges the worlds of observations and modeling.
That refrain applies to another research interest of his: biogenic and sea salt aerosols in the Southern Ocean.
“We don’t know much about them,” says McFarquhar, “which is why we need more air-, ship-, and land-based observations.”
One target is to a get more process-oriented understanding of these particles. Another target, he says, is to see “how they vary over the antarctic ice sheet, the marginal ice zone, and out over the Southern Ocean.”
Ship-based measurements can delve into oceanic properties, he says, including “aerosols right above the ocean layer,” along with the fluxes of sensible and latent heat.
A March 2024 workshop, jointly organized by ARM and ASR, discussed the feasibility of using commercial ships to observe marine aerosols and clouds. The meeting, followed by a June 2024 report, also identified opportunities, scientific priorities, and logistical challenges.
On the other hand, says McFarquhar, ground observations provide “a much longer temporal and spatial” view of aerosol life cycles, which are best interpreted by comparisons with complementary data from ships and research aircraft.
“A major part of my job is just to make sure I am representing adequately the work that all our scientists are doing.”
– Greg McFarquhar
“A major part of my job is just to make sure I am representing adequately the work that all our scientists are doing.”
– Greg McFarquhar
CAPE-k Considerations
To that point, McFarquhar is keeping an eye on the progress of a largely ground-based ARM field campaign in coastal Tasmania, on the edge of the Southern Ocean. The Cloud And Precipitation Experiment at kennaook (CAPE-k) started in April 2024 and will continue through September 2025.
ARM and Australian instruments at this decades-old air quality measurement station are gathering data on low-cloud and precipitation properties, their seasonality, and aerosol-cloud-precipitation interactions.
“I’m heavily aware of what’s going on there,” says McFarquhar of CAPE-k.
He has no direct role in the campaign but does harbor aspirations towards future airborne campaigns over the Southern Ocean. Such campaigns could complement observations underway at CAPE-k. For now, at least, ship-borne instruments will be deployed during COAST-k, a three-week ship-borne operation slated to take place in May 2025.
On a grander scale, McFarquhar is among many international scientists who have joined the Partnerships for Investigations of Clouds and the biogeoChemistry of the Atmosphere in Antarctica and the Southern Ocean (PICCAASO).
This collaborative global initiative focuses on the links between marine biogeochemistry and the atmosphere of the Southern Ocean, whose winds, scarcely ever passing over landforms, generate some of the most pristine air on Earth.
McFarquhar was coauthor of an April 2023 PICCAASO overview paper. It outlines how in the next five years the initiative will track more than 20 intensive research campaigns in Antarctica and the Southern Ocean.
A New Algorithm
Aerosols were the focus of the May 2024 paper led by Niu and funded by McFarquhar’s ongoing ASR project. This investigation of aerosol patterns in the Southern Ocean’s marine boundary layer depended on austral spring and summer MARCUS data from ARM’s ship-borne aerosol observing system.
The aim was to understand how aerosol properties vary between southern and northern portions of the Southern Ocean, a divide Niu set at latitude 62 degrees South. On either side were what McFarquhar called the “South” and the “North” Southern Ocean.
In the South, closer to Antarctica, concentrations of cloud condensation nuclei were higher, and seasonal variations were higher than in the North.
The paper reported other insights, including data on atmospherically rare ice nucleating particles over the Southern Ocean, which primarily originated from organic and biological sources.
McFarquhar says the study also described and employed “a new machine learning algorithm that removes ship-stack contamination of the aerosol observing system,” a problem in retrieving aerosol data from MARCUS.
Previous studies had to filter out up to 92% of the data, he says. “We only had to filter out 68% of the data. Now we’re at the point where the algorithm can retrieve 32% of the aerosol stack data,” a boon to future users of MARCUS aerosol measurements.
Next in line for Niu is an investigation of cloud condensation nuclei bias in the popular Community Atmospheric Model, version 6 (CAM6). She has already found that some representations of atmospheric processes (parameterizations) in CAM6 are inconsistent with observations made during MARCUS.
Southern Ocean Physical Processes
McFarquhar’s cornucopia-like ASR project has also spun off other fruitful projects.
One involves looking at MARCUS data for insights on hygroscopicity—the varying ability, in this case, for aerosols to take up moisture. That includes investigating aerosol size distributions and size ranges.
McFarquhar and his research group, once again under the ASR funding umbrella, are also studying the radiative properties of Southern Ocean aerosols observed during MARCUS. Such data can “provide indirect information about chemical composition,” he says, which can, in turn, “help us understand more about Southern Ocean physical processes.”
Another ASR-related project delves into the differences between those physical processes in the North and South Southern Oceans. Researchers have already observed that there is more biogeochemical activity in the South, due in part to higher concentrations of dimethylsulfide from marine phytoplankton.
On the other hand, in the North, between about 60 and 50 degrees latitude, there are greater concentrations of aerosolized sea salts. These are whipped into the air by breaking waves and the high-velocity winds that accompany mid-latitude cyclonic systems.
To better identify these differences in physical processes, says McFarquhar, “we are working on another paper.”
Severe Weather’s Societal Impacts
McFarquhar, a native of suburban Toronto, Canada, wrote his first paper at age 19. He was then a rising junior at the University of Toronto (B.Sc., Mathematics and Physics, 1987). However, the paper (on modeling the width and speed of the boundary layer between snow and rain precipitation) was not published until he was 21.
By then he was starting his master’s and doctoral work at Toronto, where he earned successive atmospheric physics degrees in 1989 (M.Sc.) and 1993 (PhD). McFarquhar’s undergraduate background was in pure physics: the hard-to-see, interiorized world of leptons, muons, and neutrinos. However, he says, that kind of knowledge set him up for a lifetime of studying cloud physics, including the intricacies of thermodynamics and heat transfer.
The narrative of his trajectory from pure physics to cloud physics is detailed in an earlier ASR scientist profile. But suffice it to say here, that after career stints in southern California, Colorado, and Illinois, McFarquhar landed in Oklahoma in 2017 to direct what was then the university’s Cooperative Institute for Mesoscale Meteorological Studies (CIMMS).
In 2021, CIMMS was absorbed into a new entity, the Cooperative Institute for Severe and High-Impact Weather Research and Operations (CIWRO), a joint project of the University of Oklahoma and the National Oceanic and Atmospheric Administration.
“We’re not just doing research,” says McFarquhar. “The goal is also to take some of our research and transform it into operational products” that protect against impacts from a wide range of severe weather, including floods and heat waves.
Oklahoma is just the place for it: a place of ground-ripping tornadoes, record hail, epic thunderstorms, and straight-line winds that can cause extensive damage.
Those kinds of threats are why, this summer, McFarquhar is trying to train his dog to head for the storm shelter when the routine tornado sirens go off at noon every Saturday. Instead, he says, “she goes out into the backyard and starts howling.”
ASR Spinoffs
CIWRO has five research themes: radar and observations, research and development, mesoscale storm forecasting, mesoscale modeling research, and societal impacts. McFarquhar provides the program with scientific and management leadership.
“A major part of my job,” he says, “is just to make sure I am representing adequately the work that all our scientists are doing.”
CIWRO has about 225 scientists on hand. Some of them are involved in other ASR projects.
In July 2023, ASR awarded project funding to research scientist Yongjie Huang Yongjie Huang to examine surface, aerosol, and meteorological controls on metropolitan convective clouds on a subtropical coast. Huang is using data from a 2021-2022 ARM field campaign in the Houston, Texas, region called Tracking Aerosol Convection Interactions Experiment (TRACER).
“We’re using a combination of the TRACER observations and model simulations to investigate things like thermodynamic controls on the life cycles of convective clouds,” says McFarquhar.
Huang is also looking at how increases in aerosol concentrations may enhance convection through the invigoration effect and even shorten the lifetime of convection through stronger evaporative cooling.
In another corner of the CIWRO shop, research scientist Andrew Dzambo is leading an ASR project that leverages ARM data to investigate surface, aerosol, and meteorological controls on arctic boundary layer clouds.
The project, described in a 2023 ASR profile of Dzambo, has an observational mission, overseen by Dzambo, and a modeling mission led by Kamal Kant Chandrakar at the National Center for Atmospheric Research in Colorado. Overall, the aim is to develop a large-scale, particle-based cloud microphysics scheme.
“A natural evolution in someone’s (CIWRO) career,” says McFarquhar, “is to promote scientists in our group and give them a chance to be a project PI.”
He Shall Return
So far, McFarquhar, who has not played golf in 35 years, is resisting Dzambo’s attempts to get him out on a local course.
Not that McFarquhar is inactive. He is a travel buff who took a recent springtime trip to Spain with his wife and one of their children. (The couple have one son and one daughter.) McFarquhar also hikes, plays ice hockey (O Canada!), and runs half-marathons.
At the same time, McFarquhar runs through ideas for future deployments. He has notions involving Korea and Greece, as well as airborne campaigns in the Southern Ocean.
“The key thing is that we need more observations” of this climate-critical polar region, he says, “whether they be ship-based or air-based in a variety of environmental conditions.”
Australian atmospheric chemist Melita Keywood, who has a role in CAPE-k, called the Southern Ocean a region that, despite a flurry of recent attention (including PICCAASO), remains “the most under-observed in the world.”
McFarquhar agrees that observations of cloud and aerosol effects in the region have been “sparse and infrequent.” Despite their importance to global oceanic and weather dynamics, Southern Ocean atmospheric realities are poorly represented in earth systems models.
During his career studying cloud physics, McFarquhar has investigated the elusive properties of snow bands in wintertime cyclones, freezing drizzle, hurricanes, and biomass burning. But he keeps returning to the mystery of low clouds over the Southern Ocean.
That return, McFarquhar hopes, will be in person through a future campaign—or at least during a sabbatical planned for two years from now.
He says: “I certainly intend to go back.”
# # #Author: Corydon Ireland, Staff Writer, Pacific Northwest National Laboratory
This work was supported by the U.S. Department of Energy’s Office of Science, through the Biological and Environmental Research program as part of the Atmospheric System Research program.