Looking back on climate gear both rugged and precise

This is the second article about a recently published monograph on the first 25 years of the ARM Climate
Research Facility.

Three scanning microwave radiometers (left) undergo testing in the instrument field at the Southern Great Plains site's Central Facility before becoming part of the ARM suite of instrumentation.
Three scanning microwave radiometers (left) undergo testing in the instrument field at the Southern Great Plains site's Central Facility before becoming part of the ARM suite of instrumentation.
During its more than two data-rich decades, the Atmospheric Radiation Measurement (ARM) Climate Research Facility has provided science with many enduring gifts, including an organizational template for climate research across the world. High on the list of ARM gifts is also an approach to instrumentation that is comprehensive, coordinated, and flexibly mobile.

Starting with its first fixed field location in 1992, ARM has grown into the world’s premier climate observations facility. Its scientists and technicians at nine national laboratories oversee nearly one hundred instrument types that operate at three fixed sites in Oklahoma, Alaska, and the Azores; that are packaged into three mobile facilities for what has now been 16 campaigns; and that are put aboard a variety of aerial platforms.

By design, most ARM instruments observe the atmosphere continuously, day and night, which was a radical idea for a research program in its day. Not all ARM instruments collect data continuously, but they all work collectively on a common mission to enrich climate science data. They reside in a multitude of platforms—on the ground, aboard planes, on tethered balloons, at sea, and tucked within tiny radio-controlled unmanned aerospace systems (UAS).

The history of ARM instrumentation is a tale of coordinated devices designed to continuously gather, record, and stream high-quality climate data. (A data archive run by ARM checks, stores, and distributes every byte of data, which is another story.) Instruments run like a thread through all 30 chapters of a recently published monograph detailing how ARM came to be.

The online document, written by ARM veterans and published this spring by the American Meteorological Society, covers how ARM evolved operationally and scientifically; how it expanded its fixed and mobile sites; what it has contributed to climate change science; how it inspired international research analogs; what its data targets have been, from spectral radiation and aerosol optical properties to radiative fluxes and water vapor profiles; and ultimately what its impact has been on regional and large-scale climate models.

Every chapter, in some way, relates to the instruments that investigate the complex puzzle of Earth’s climate: the various radars, lidars, interferometers, disdrometers, radiometers, gauges, probes, profilers, monitors, and imagers in the ARM toolbox.

Getting There

The millimeter wavelength cloud radar was a key instrument included at all the sites. It is pictured here at the Southern Great Plains in 2002.
The millimeter wavelength cloud radar was a key instrument included at all the sites. It is pictured here at the Southern Great Plains in 2002.

ARM and its instruments first went to work in the spring of 1992. They came to be because of growing concern over climate change that began two decades before. Of special concern were questions about the impact of increasing levels of carbon dioxide in the atmosphere. In the 1970s, the U.S. Department of Energy (DOE) commissioned a set of six reports to probe the uncertainties in that era’s general circulation models. Predictions of greenhouse warming by the models, for one, spanned a troublingly wide range.

In the 1980s, after a major intercomparison study, the DOE concluded that cloud-radiative feedback was the single largest determinant of climate responses to human activity. That eventually led to the creation of ARM in 1990, the same year of the first report by the Intergovernmental Panel on Climate Change. That report also noted a huge knowledge gap regarding the role of clouds in climate change.

Since 1996, the ARM Southern Great Plains site has maintained one of the few operational Raman lidars in the world, providing one of the most requested ARM data sets.
Since 1996, the ARM Southern Great Plains site has maintained one of the few operational Raman lidars in the world, providing one of the most requested ARM data sets.
The “cloud problem” everyone acknowledged led to ARM’s first operational decision: It had to be a facility that operated continuously. Up until then, periodic field campaigns were typically a few weeks to perhaps a couple of months long. As a result, the data collected were quite literally at the mercy of the weather. This method led to some excellent results, but it did not result in a climatically representative data set.

Over these short terms, scientists deployed instruments, gathered data, and tested hypotheses with observations. Such research forays relied on (and helped develop) many fine instruments, but few were ready for what Gerald Stokes described in the monograph as ARM’s “24/7 operational paradigm.” (He led the program-plan writing team and in 1990 became the ARM’s first Chief Scientist. Today, he is a visiting professor at Stony Brook University.)

Data would now have to be gathered unceasingly 24 hours a day all year, every year. It was “uncharted territory,” wrote Stokes.

Instruments had to change. Instead of needing on-site control by researchers over a short campaign, instruments had to run continuously and with less direct oversight. Monograph co-editor Dave Turner recalled one early lidar system “that required several PhDs to run.” (He was the monograph co-lead editor, an early ARM researcher, and is now a physical scientist with the National Severe Storms Laboratory in Oklahoma, a branch of the National Oceanic and Atmospheric Administration.)

To streamline the process, ARM named a “mentor” for each type of instrument. He or she is responsible for field deployments, for determining maintenance procedures for the on-site engineering staff, for data quality, and for keeping an eye on each device remotely. Many of each type of instrument may be deployed at several sites. On-site technicians deal with day-to-day instrument calibration and repair.

With new requirements for continuous data, devices got more robust.

“A large number of instruments were developed or improved,” said Ted Cress, ARM Technical Director from 1990 to 2004. “There was a fundamental gain in the source of field instruments.”

Set up at the Pacific Northwest National Laboratory, the first ARM Mobile Facility prepares for its first deployment to Point Reyes, California.
Set up at the Pacific Northwest National Laboratory, the first ARM Mobile Facility prepares for its first deployment to Point Reyes, California.

By 1998, ARM sites were fully instrumented. Each included at least a cloud radar, a cloud lidar, and temperature and humidity sensors attached to balloons.

But fixed instruments are not everything. Some critical climate measurements cannot be made continuously. So the campaign model survived in what ARM came to call “intensive operational periods,” or IOPs. The first ARM-funded IOP was in 1993.

“Shorter deployments of specialized instruments augment the continuously operating baseline ARM instruments, and give you a better overall data set,” said ARM Technical Director Jim Mather of Pacific Northwest National Laboratory. The kind of 24/7, continuous-data science that ARM normally does, he added, captures the range of conditions that exist over a year, five years, or 10 years.

The ARM Aerial Facility operated Gulfstream-159 (G-1) research aircraft sits in a hanger during the Carbonaceous Aerosols and Radiative Effects Study (CARES), ready for its complement of instruments.
The ARM Aerial Facility operated Gulfstream-159 (G-1) research aircraft sits in a hanger during the Carbonaceous Aerosols and Radiative Effects Study (CARES), ready for its complement of instruments.

“Nature never behaves the way you want it to,” said Mather of short-term campaigns that can’t take the long view ARM does. “We are not constrained that way. We see everything. You don’t want to see one of those clouds, you want to see thousands.”

One value of such long-term data sets is that they help put campaign data sets into perspective. For instance, did a field campaign sample “normal” conditions, or something unique?

With that long view in mind, other instrument strategies came into play. In 1994, ARM began a series of IOPs designed to support modeling studies. In 2005, ARM created the first ARM Mobile Facility (AMF), designed for campaigns around the world for six months to a year. (There are now three mobile facilities.)

ARM also took instrumentation into the air: In 1993, it launched the Unmanned Aerospace Vehicle (UAV) program. Throughout the 1900s and 2000s, ARM also carried out a number of manned aircraft missions, often in partnership with other agencies, before creating in 2007 the ARM Aerial Facility, which included components from the UAV program.

One of three X-band scanning precipitation radars at ARM’s Southern Great plains site funded through the American Recovery and Reinvestment Act. They provide measurements that identify precipitation type, estimate rainfall rates, and estimate the velocity of wind fields.
One of three X-band scanning precipitation radars at ARM’s Southern Great plains site funded through the American Recovery and Reinvestment Act. They provide measurements that identify precipitation type, estimate rainfall rates, and estimate the velocity of wind fields.

In 2009, two things happened to improve and expand ARM instrumentation. The American Recovery and Reinvestment Act provided an opportunity to add a multi-frequency suite of radars, lidars, aerosol instruments, and aircraft probes. ARM also upgraded many of its standard instrument suites. Observations from these new instruments gave scientists access to a broader set of climate-relevant parameters, wrote Mather in a report at the time, as well as “a description of the full 3D cloud field and its temporal evolution.”

In the same year, ARM joined its science goals and organizational structure with the DOE’s Atmospheric System Research program. The merger provided additional research funding, broadened the scope of scientists using ARM data, and brought in the G-1 aircraft (though its instrumentation came from Recovery Act funds).

In all, said Turner, these augmented capabilities finally allowed ARM instruments to gain more than a limited “soda straw” view of the atmosphere. The result, he added, was something closer to a “3D view of the clouds.”

A Signature Instrument

At least 10 of the 30 chapters in the ARM Monograph are about, or significantly describe, ARM instruments. Around a hundred different types of instruments are deployed by the ARM Facility now, but a few of those instruments stand out, according to monograph authors.

Those include the Microwave Radiometer, which measures the microwave emissions of water vapor and liquid water molecules; the Raman Lidar, which uses a pulsed laser to profile water vapor, clouds, and aerosols in the atmosphere; and the Millimeter Wavelength Cloud Radar (MMCR), which detects cloud particles, captures the vertical motions of these particles, and measures the upper and lower boundaries of clouds. The MMCR’s observations are so valuable that each ARM site has this instrument. The ARM Program was the first in the world to continuously operate a Raman lidar and MMCR. Using Recovery Act funds, the MMCRs were replaced with Ka ARM Zenith Radars (KAZRs), which utilize a new digital receiver that provides higher spatial and temporal resolution than the MMCR.

An AERI on display at the University of Wisconsin-Madison, where the valuable ARM instrument was designed and purpose-built before its first deployment in 1993.
An AERI on display at the University of Wisconsin-Madison, where the valuable ARM instrument was designed and purpose-built before its first deployment in 1993.

Another well-known (and unique) instrument used at ARM sites and mobile facilities is the Atmospheric Emitted Radiance Interferometer (AERI). First deployed at ARM’s Southern Great Plains (SGP) site in 1993, this spectroscopic device was designed and purpose-built for ARM at the University of Wisconsin-Madison. It detects infrared radiance that propagates downward from the atmosphere. Broadband instruments can do that too, but only with a very coarse spectral resolution, said Jonathan Gero, ARM’s instrument mentor and a research scientist at the University of Wisconsin-Madison. (He oversees all nine AERI instruments in ARM.)

Like a fine pair of glasses, the AERI sees the infrared spectrum in fine detail, not in the “blurs and shapes” of broadband instruments,” said Gero. “Once you have the right glasses you can see more details.” AERI looks upward to take data snapshots every 20 seconds from different layers of the atmosphere.

“This instrument looks at the different colors of light in the invisible spectrum,” Gero explained. “It can measure extremely fine variations.” Different trace gases, including carbon dioxide, have distinct spectral signatures. So do particulates, aerosols, and clouds. AERI’s spectral resolution is good at reading these signatures accurately. And because it collects data continuously, it can record the presence and amount of all these climate factors over time.

So far there are 20 years of data from the AERI. These have been used for a wide range of studies, including those that improved the accuracy of infrared radiative transfer models; that derived cloud properties; that measured water vapor and temperature profiles throughout the lower atmosphere; and that verified the impact of rising carbon dioxide levels on the Earth’s surface radiation balance.

AERI is an example of a signature ARM instrument in action, and of what power continuous data can have over time. Improving the infrared radiative transfer model alone, based on AERI observations, “is pretty important work that has found its way into most climate models now,” said Mather. “It gets a lot of people excited.”

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The ARM Climate Research Facility is a national scientific user facility funded through the U.S. Department of Energy's Office of Science. The ARM Facility is operated by nine Department of Energy national laboratories.