A ‘Historically Successful’ International ARM Deployment in Finland Yields Rich Data From Novel Multi-Instrument Measurements
From the air, the Hyytiälä Forestry Research Station in southern Finland is surrounded by green boreal forest streaked with slender blue lakes.
At Hyytiälä one day in late December 2013, Nicki Hickmon stepped onto a snow-covered clearing to make a preliminary placement of instrument containers for a field campaign called Biogenic Aerosols – Effects on Clouds and Climate (BAECC). Sponsored by the ARM Climate Research Facility, it went on to run from February 1 to September 13, 2014.
BAECC was designed to obtain important details on processes related to aerosol, cloud, and snow formation that are not currently well understood or well represented in earth system models.
ARM’s contribution to BAECC was the second ARM Mobile Facility (AMF2)—37 instruments packed into eight seatainers, each 10 feet high.
Around Hickmon loomed the forest. A freezing drizzle pelted down from a dark sky. When a single ray of sun pierced the tree cover, “I moved to stand in it,” says Hickmon, who is now the ARM associate director for operations, and who was then the AMF2 site manager. “I hadn’t seen the sun in so many days.”
Despite the darkness early on, everyone involved in BAECC is sunny about the results, which include something like 20 papers so far. The campaign was designed to better understand the role of organic vapors and other natural emissions (such as pollen) on the formation of clouds. (A mission-equivalent campaign, Green Ocean Amazon, took place in the Amazon rainforest from 2014 to 2015.)
BAECC also provided critical new data on the microphysical processes of snow, phenomena that have a great impact on global water resources.
The Right Place
Finland hosted BAECC under the direction of principal investigator Tuukka Petäjä, a young atmospheric physicist from the University of Helsinki.
The BAECC campaign was located at the Station for Measuring Ecosystem-Atmosphere Relations II (SMEAR II), which is colocated with the Hyytiälä Forestry Research Station. In a 2016 paper summarizing the BAECC campaign, Petäjä described SMEAR II as “one of the world’s most comprehensive surface in situ observation sites in a boreal forest environment.”
For 19 years before BAECC, the Finns had been using the Hyytiälä site to record in situ observations of snowfall, biogenic aerosols, and other phenomena focused on interactions between the forest and the atmosphere.
Since BAECC, because of Finnish investments and upgrades, the Hyytiälä Forestry Research Station has become “a fully equipped cloud profiling station,” says Dmitri Moisseev, a cloud and precipitation physicist at the University of Helsinki who also leads the university’s Radar Meteorology Group. “Now, measurements similar to AMF2 are carried out continuously. BAECC had a tremendous impact on our research.”
That upgrade of instruments grew right out of the 2014 U.S.-Finland campaign.
“BAECC was the first experiment where we tried to make the link between biogenic aerosols and clouds,” says Moisseev. “And because of BAECC, we have decided to continue these kinds of observations.”
BAECC spurred a flurry of papers, cemented European-American connections in the atmospheric sciences, gave young researchers in seven nations a fast start on studying snow and ice microphysics, and assured follow-up campaigns.
It also inspired the start of U.S.-European Union summer and winter training workshops on the observation and modeling of clouds and precipitation. The second ARM Summer Training and Science Applications event takes place July 14-21, 2018, at ARM’s Southern Great Plains (SGP) atmospheric observatory.
These educational opportunities were also inspired by Hyytiälä itself, which for the past two decades has hosted intensive field courses twice a year.
Challenges in the Field
At the Hyytiälä station, the initial installation of AMF2 took place during an outbreak of arctic air so intense that temperatures dipped to minus-26 degrees Celsius (about 15 degrees below zero Fahrenheit).
Of the weather during installation, says Michael Ritsche: “It rained, then it snowed, then it froze.” During BAECC, Ritsche was the AMF2 technical operations manager.
Technicians wrestled with cold-stiffened cables and used heat guns to melt ice on instrument platforms. Grounding and support spikes snapped off in the icy ground.
Working in the cold “is kind of my own fault,” says Annakaisa von Lerber, a research scientist at the Finnish Meteorological institute (FMI). “I chose my field to be snowfall microphysics.”
The instruments, she adds, proved to be “robust and reliable” despite the cold.
The AMF2 team moved the instruments on a cargo vessel that traveled from Long Beach, California, to Finland under a tight schedule—all the while dealing with the movement of individual instruments, repairs, and calibrations.
Ritsche, now the facility manager of ARM’s SGP atmospheric observatory, acknowledged challenges other than the weather.
For one, operational requirements call for enough open ground to mount radar corner-reflector towers, yet there is little open ground in a tall-tree boreal forest setting. To solve this, the SMEAR II team erected a tower; in another case they placed a reflector atop their existing 127-meter tower.
In the face of these obstacles and others, SMEAR II staff followed the lead of Hyytiälä site manager Janne Levula, who according to ARM personnel went beyond the role of gracious site host.
The tall trees were also an impediment for the Ka/X-Band Scanning ARM Cloud Radar (Ka/X-SACR). To allow the radar to scan the base of clouds at what scientists call the “interest level,” the AMF2 team engineered a solution: Hitch three containers solidly together, then stack the radar container on top.
A Culture of Working Together
“BAECC was remarkable, a great thing to see,” says Stony Brook University atmospheric scientist Pavlos Kollias, who has a joint appointment at Brookhaven National Laboratory. “It was a historically successful ARM story.”
The campaign was also an international deployment that cemented European-American collaborations, he says—a “perfect storm” of capabilities that matched NASA and Finnish assets on the ground with ARM’s sky-pointing radar arrays.
BAECC also inspired researchers from seven nations—five of them in Europe.
ARM had an earlier deployment in Germany and in 2020 will roll out a campaign in Norway, Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE).
Data from BAECC drew researchers from Finland, Germany, the United Kingdom, Austria, Italy, Canada, and the United States. Many of them wrote multinational collaborative papers.
Among those officially joining with ARM during BAECC were the University of Helsinki, FMI, NASA’s Global Precipitation Measurement Ground Validation Autonomous Parsivel Unit, Massachusetts-based Aerodyne Research, Colorado State University, and several European research institutes.
“There is a good culture of working together,” says Kollias.
Petäjä agreed, calling the American-Finnish collaboration “very easy and constructive.”
The Power of Young Scientists
Kollias, a self-described “radar guy,” was also pleased so many young scientists got involved. “They grew through the program,” he says, “and started their careers with BAECC.”
Two of his former postdoctoral students at McGill University in Canada used BAECC data to unravel cloud processes. Heike Kalesse is now at the Leibniz Institute for Tropospheric Research in Leipzig, Germany. Stefan Kneifel is at the University of Cologne in Germany.
Kalesse was lead author of a paper on the “fingerprints” of a riming event as observed on cloud radars. Riming is a coating process that occurs as snowflakes fall through the liquid bath of the atmosphere, gaining mass and falling faster as they strip water out of the air and increase their density.
Riming is an important and little understood activity in the hydrological process that has great implications for warming or cooling the Earth. Having less liquid makes clouds less of an insulator, for one, which opens the sky to cooling.
“The interplay between the liquid and ice phase of cloud particles, and how they interact, is a very tantalizing problem for us,” says Kollias, who was a co-author on the Kalesse paper. “In fact, solid precipitation is really, I would say, impenetrable” to scientists—but radars can help.
Kneifel, the other Kollias protégé, was lead author of a paper on the advantage of using the triple-frequency radar sets available during BAECC, which allows for better validation of present and future snow-scattering models.
Such measurements also make it possible “to retrieve the size distribution of precipitation,” says Jennifer Comstock, ARM’s engineering and process manager. She is an expert on the physical processes that influence cloud life cycles.
Von Lerber, also a young scientist, collected data on falling ice particles during BAECC and then wrote a paper on the connection between snow properties and radar observations. She used three instruments to assemble a time series that elucidated the microphysical parameters of falling snow.
Along with Kollias, Moisseev has helped mentor young PhDs interested in remote sensing, cloud radars, the dynamics of snowfall, and the microphysics of mixed-phase and ice clouds. He was also a co-author on von Lerber’s paper.
A Good Foundation
The main legacies of BAECC were to prove the “operational stability and performance” of ARM radar systems in adverse weather, says Kollias, and to create a “blueprint” for future deployments of multiwavelength radar systems for microphysical studies of ice and snow.
Another paper to come out of BAECC, by young Scottish lead author Victoria A. Sinclair of the University of Helsinki, was based on using dual-polarization radar observations. In this case, the coincident radars were a Ka/X-SACR and a Finnish C-band polarimetric radar, which measures the drop size of precipitation, including drizzle, hail, and graupel (snow pellets).
By putting these devices in tandem, Sinclair and her co-authors (including Moisseev and von Lerber) identified signatures of secondary ice production in precipitating clouds.
Moisseev agreed on the power of the colocated radars. He also believes that BAECC data will continue to be useful in testing and improving radar-based retrievals of cloud properties.
“BAECC was the first experiment where multifrequency cloud and precipitation radar observations were combined with comprehensive surface observations of falling snow microphysics,” says Moisseev.
During BAECC, these coordinated radars were focused for the first time on ice microphysics in a boreal environment.
“We were performing the equivalent of an MRI (magnetic resonance image) on snow and ice events,” says Kollias, “and identifying some of the fundamental properties of (cloud-forming) particles. That’s not possible with one radar.”
More is coming, perhaps even a “BAECC II,” says Petäjä. “The BAECC field work and data provide a good foundation to plan further actions.”
This spring he is hosting a campaign in Hyytiälä to measure ice nucleating particles with colleagues from Finland, Germany, and the United Kingdom.
“BAECC brought together at one site individuals working on similar efforts in different parts of the world,” says Hickmon. “It connected scientists, site staff, and programs—purely for the advancement of science.”# # #
The ARM Climate Research Facility is a DOE Office of Science user facility. The ARM Facility is operated by nine DOE national laboratories.