Scientific investigations, data analysis, and model improvements unfold a year after the end of a Colorado field campaign on mountain hydrology
Since operations began in 1992, the Atmospheric Radiation Measurement (ARM), a U.S. Department of Energy (DOE) user facility, has sponsored 1401 field campaigns. These expeditions out in nature, awarded after a competitive process, gather intensive batches of data on clouds, precipitation, and other phenomena.
If a field campaign is large, by virtue of the number of instruments and researchers, a pattern emerges during and after observations: new data analysis, new scientific insights, new research collaborations, and a new bloom of early-career scientists, sensing adventure and opportunity.
Another constant in ARM campaigns is funding from the DOE’s Atmospheric System Research (ASR) program.
Take for instance ARM’s Surface Atmosphere Integrated Field Laboratory (SAIL) campaign, which wrapped up 21 months of observations just a year ago, in June 2023.
During its mission to study atmospheric influences on mountain hydrology, SAIL deployed a suite of instruments near Crested Butte, Colorado, including 50-plus instruments in a containerized observatory called an ARM Mobile Facility. Since finishing its observations phase, SAIL has spun off an impressive 41 publications: 27 technical reports and 14 peer-reviewed journal articles. Of the journal articles, 10 were supported by ASR.
SAIL’s principal investigator, Lawrence Berkeley National Laboratory (LBNL) climate scientist Daniel Feldman, was funded by ASR too. So was the SAIL campaign overview paper he led, published in December 2023 by the Bulletin of the American Meteorological Society (BAMS).
A Global Threat to Water Resources
SAIL collected continuous, all-season measurements of the evolving surface and atmospheric conditions that control hydrometeorology, hydroclimatology, and hydrology within Colorado’s East River Watershed.
The watershed, a dot on the Colorado map, takes up only 300 square kilometers (186 square miles). But it is a significant source of water for the Colorado River, where about 70% of flow originates as runoff from mountain snowfall. In turn, the Colorado supplies 40 million people downstream in seven U.S. states and Mexico.
Globally, the kind of terrain studied during SAIL—peaks, steep slopes, scattered forest, and deep valleys—constitutes less than a quarter of Earth’s land surface. Yet mountain rivers, streams, and groundwater supply 60% to 90% of the world’s fresh water, giving this terrain a wildly outsized impact on society.
Meanwhile, water supplies from mountainous watersheds are increasingly at risk. Earth’s higher elevations are warming faster than lower elevations, and levels of water-storing snowpack are declining. In the future, seasonal snow in a warming world will likely create less and less water downstream and such “snow drought” will be more frequent and severe.
A 2023 letter in Environmental Research Letters, led by University of California, Berkeley, PhD candidate Marianne Cowherd, combines historical data and modern simulations to “pinpoint snow drought as an emerging global threat to water resources.”
Cowherd on Measuring Future Snowpack
Snow drought is a threat to mountain terrain. Yet real-time measurements of the changing atmospheric and surface processes remain scant in hard-to-access mountain areas.
SAIL stepped in to fill that knowledge gap.
Two of the most recent ASR-supported papers illustrate the importance of early-career scientists to the research aftermath of large ARM campaigns. Both are by early-career scientists: Cowherd is one. William “Will” Rudisill, a hydrologist and postdoctoral researcher at LBNL) in California is another.
Cowherd was lead author of a paper published in mid-June 2024 in Communications Earth & Environment that “looks at how automated snowpack measurement sites will work under future climates,” she says.
It outlines a scheme that salvages the utility of around 900 existing snowpack measurement sites. Such “snow pillow” stations measure snow-water equivalent but may be less useful “as climate change shifts snow to higher elevations,” says Cowherd. “We use artificial intelligence models to produce accurate (snowpack) estimates that work even in new climates.”
The paper also acknowledges ASR funding for co-authors Feldman and Utkarsh Mital, an LBNL earth scientist, and Cowherd’s early-career support from DOE’s Computational Science Graduate Fellowship (CSGF) program.
Rudisill on Correcting Cold Bias
Rudisill, who earned his PhD in 2022, led a 2024 study on cold bias in models of the atmosphere in mountainous terrain. Until this study, based on temperature readings at a height of 2 meters (6.5 feet), the pervasiveness of cold bias had not been systematically identified in the literature.
Rudisill compared observations from SAIL and elsewhere with regional climate model simulations of mountainous watersheds across the world.
The authors, including Feldman and two other LBNL scientists, examined how well current models estimate mountain temperatures, which is important for predicting whether it will rain or snow. After looking at 44 studies of mountain ranges worldwide, the authors found that each study overestimated how cold temperatures were by several degrees or more, especially on mountain peaks and ridges.
Previous studies of cold bias pointed to snow albedo (reflectivity) as a culprit. But Rudisill says the most compelling finding in the paper is that there is no one prominent cause, no “single variable (in a model) we can tweak and fix.”
However, the many causes of cold biases seem to be “quite heavily tied to topography,” he adds. In turn, this means there is a need for more studies of complex wind circulations in mountainous terrain and how snowpack exchanges energy with the atmosphere.
With measurements from a heavily instrumented, long-duration mountain field campaign such as SAIL, the authors conclude, cold bias and similar inaccuracies can be “uprooted” from models.
A Haul of Data
Feldman calls SAIL’s haul of data “the largest and best archive ever collected on mountain hydrology in North America.”
The numbers are indeed big. There are 227 terabytes of quality-checked SAIL campaign measurements on hand, says ARM Chief Data and Computing Officer Giri Prakash. “By far, that’s the most gathered from the last four ARM Mobile Facility deployments.”
That many terabytes are equivalent to nearly 15 billion document pages—enough to fill almost 300,000 filing cabinets. (Remember those?)
As of mid-June 2024, SAIL has 166 data products, 233 datastreams, and 398 primary measurements, which are freely accessible from ARM’s Data Discovery portal.
“Those numbers are not static,” says Feldman. Researchers are still adding their own observational and model products to the archive. Measurements there cover critical information bases: clouds, aerosols, solar and thermal energy, precipitation, temperature, pressure, humidity, and wind.
“The data is so dense,” says Rudisill, “you could have an entire career just looking at it.”
Partners in the Field
The trove of mountain hydrology data collected during SAIL benefited from the efforts of a long list of other institutions.
First, SAIL had a strong and legendary local partner to provide sites for some instruments, along with decades of know-how on mountain snow and hydrology: the venerable Rocky Mountain Biological Laboratory in Gothic, Colorado.
Another nearby partner was the Aspen Global Change Institute. A four-hour car ride from the SAIL study site, it provided researchers and technicians with deep knowledge of regional conditions.
Meanwhile, other institutions funded collocated field campaigns whose science missions complemented SAIL’s.
The National Oceanic and Atmospheric Administration was the primary supporter of the Study of Precipitation, the Lower Atmosphere and Surface for Hydrometeorology (SPLASH), which ran from fall 2021 through summer 2023. Gijs de Boer of the University of Colorado Boulder was SPLASH’s principal investigator.
A May 2023 paper led by de Boer outlines the campaign’s surface sensors (networked across five sites), uncrewed aerial systems, and freely available data sets.
The National Science Foundation’s Sublimation of Snow (SOS) campaign, led by University of Washington snow hydrologist Jessica Lundquist, published its own overview paper in April 2024. SOS data are also available for free.
Another major SAIL collaborator was DOE’s longstanding Watershed Function Science Focus Area, led by LBNL biogeophysicist Ken Williams to collect surface and subsurface observations in the East River Watershed.
As Feldman often says, this kind of research provided SAIL with “a bedrock-to-atmosphere” perspective on hydrology in complex mountain terrain.
The Mystery of Missing Water
Feldman says SAIL data may help solve some universal questions about mountain hydrology.
One such question was among the early motivations for the campaign: Where does the missing water go?
The East River Watershed’s recent hydrological history shows snowpack reserves that are not historically low. But it is evident that not all that snowpack melts and shows up as fresh water downstream.
SAIL scientists are investigating the causes of missing water, including precipitation, evaporation, transpiration, and sublimation processes.
Sublimation occurs when snow bypasses the melting stage and dissipates in the atmosphere as vapor. This process has been particularly difficult to measure in the past.
Scientists have been able to constrain this process in models by using data from SAIL, SPLASH, and SOS.
Modeling work is still in its early stages, says Feldman, “but there are clear areas where SAIL research is making headway.”
One is in basic radiative processes, including the representation of the terrain effects on longwave radiation. This is lacking in most atmospheric process and earth system models. So is the representation of surface air temperature, its process controls, and its impacts on surface processes.
“Scientists will never be able to measure everything, everywhere, all at one time,” says Feldman. “So, modelers are key to taking lessons learned about processes and using them to accurately predict when mountain water will be available downstream and how much there will be.”
SAIL and Aerosols
SAIL co-investigator Allison Aiken, an aerosol scientist at Los Alamos National Laboratory in New Mexico, recently submitted a paper on scientists’ quest for real-time, single-particle bioaerosol data in high mountain terrain.
Single particle bioaerosols include bacteria, pollen, and fungi emitted from plants and the soil microbiome. They influence clouds, precipitation, and surface hydrology.
The authors found that this class of bioaerosols made up a significant concentration of the total particles measured at SAIL in the spring and summer of 2022.
The paper captures the diurnal cycles of the particles and their weather-related changes in type and abundance. The heavier the rain, the authors found, the higher the concentration of bioaerosols.
“To our knowledge,” says Aiken, “these are the first measurements of this type at a high-altitude site in the United States and North America.”
She and her team are writing a paper that reports SAIL-enabled insights on supermicron particles—those larger than 1 micron in diameter. They will submit the study this summer.
Aiken also contributed to some of the dozen or so smaller campaigns that deployed guest instruments in the SAIL study area. One was SAIL Supermicron Bioaerosol, funded by ASR, which Aiken led from April 2022 through June 2023. It characterized and quantified supermicron particle events and bioaerosol activity in real time.
Another was the ASR-funded SAIL Aerosol Vertical Profiles (SAIL-AVP), which relied on ARM tethered balloon system flights in 2022 and 2023.
In all, says Aiken, there are “lots more data we hope to keep working on for years to come.”
Keeping in Touch
Growing waves and ways of collaboration show that “we have not retreated back to our little research groups” a year after SAIL observations have ended, says Feldman, but instead are explicitly growing a new research community.
Feldman, Aiken, Rudisill, and others have maintained strong science ties through biweekly SAIL/SPLASH teleconferences, which began before official data were collected. Topics have included snowpack estimation, canopy effects, radiative surface energy budgets, blowing snow, and the history of East River Watershed precipitation.
The most recent presentation, on June 12, 2024, was by Zezhen “Jay” Cheng, a chemist at DOE’s Environmental Molecular Sciences Laboratory (EMSL) in Washington state. He and others looked at the chemistry, shape, and warming effects of wildfire aerosols (measured by Aiken’s instruments) that blew through the SAIL site in September 2022 from a forest blaze in Idaho.
At EMSL, analysis of the data revealed the surprising, widespread occurrence of tarballs in soot particles.
This work, says Feldman, “highlighted the value of SAIL’s multidisciplinary science and capabilities.”
During and after each teleconference, as researchers recognize new convergences of field data, ideas for collaboration pop up.
Getting SAIL exposure at conferences and workshops helps too.
The campaign inspired presentations at the Joint ARM User Facility/ASR Principal Investigators Meeting (August 2023), a SAIL/SPLASH/SOS science workshop (November 2023), the American Geophysical Union Fall Meeting (December 2023), and the American Meteorological Society (AMS) Annual Meeting (January 2024).
At the annual Western Snow Conference in April 2024 in Corvallis, Oregon, says Feldman, presentations on the East River Watershed took up nearly a third of the meeting.
In July 2024 in Boise, Idaho, and online, the AMS will host the 21st Conference on Mountain Meteorology. SAIL science, says Feldman, “will be a major part of it.”
Breakout Groups
Among the latest developments in the SAIL collaborations saga is a new aerosol discussion series initiated by Aiken, Feldman, and four others. It is part of an interdisciplinary project to link dust transport, deposition on snow, and changes in surface radiation to changes in albedo.
The series is also “the kind of organic, curiosity-driven exploration that supports improvements in predictive models,” says Feldman. In this case, it’s an exploration of how surface albedo affects the timing of snowmelt and therefore the output of water in mountainous terrain.
In the same vein, Aiken joined with Sonia Kreidenweis at Colorado State University and Jim Smith at the University of California, Irvine, to start a new SAIL Aerosol Science Monthly Telecon. (They are soliciting new participants.)
Its first meeting, in June 2024, drew 20 participants from universities and national labs, “including students, postdocs, and staff,” says Aiken. “Some have already submitted papers while others are just getting started on their SAIL science.”
‘They Are So Bright’
SAIL enjoyed some novel exposure in May 2024 at the ARM Open Science Summer School in Cleveland, Ohio. The campaign was one of four featured tracks intended to teach undergraduates through postdoctoral researchers about ARM data and open-source software.
Virtually, Feldman taught the track on SAIL data, which became a project that focused on linking aerosol deposition and surface albedo in mountainous terrain.
“The students and postdocs who participated in the summer school,” he says of his students before pausing. “Let me put my sunglasses on here. They are so bright.”
Team SAIL, with Feldman as a guide, “dove very quickly into how aerosols impact albedo,” he says, “getting to sophisticated levels of analysis in a matter of hours using advanced, open-source tools developed by ARM.”
All four summer school teams used collaboration and modeling tools that emerged within the last decade, including Slack and Jupyter Notebooks. These tools helped the group “really accelerate,” says Feldman, compared to the days of downloading data and writing large pieces of software from scratch to analyze data.
“When students don’t have to reinvent the wheel,” he says, “they can and do move forward far and fast.”
The Next Generation
The research community growing around SAIL, SPLASH, and SOS data even features its own next generation.
Rudisill is one case. Along the way, DOE’s Office of Science Graduate Student Research (SCGSR) program supported a paper he led in 2023 on cold-season precipitation.
“I cannot imagine a project that more aligned with my interests and aspirations for science than SAIL,” he says.
Cowherd is also building her early career, in part, through SAIL-related research, such as her published work on global snow drought and snowpack estimation.
She is part of an ongoing collaboration with Feldman and Ethan Gutmann at the National Center for Atmospheric Research in Colorado, funded by the National Science Foundation. They are using terrestrial scanning lidar and weather data to investigate “bedforms” on the snow surface.
These peak-like features, says Cowherd, “increase the surface area of the snowpack and create bumpiness that can affect turbulence in the air above the snow. We want to know how large they are and how they are moving or changing shape.”
Two other early-career researchers, Daniel “Danny” Hogan and Eli Schwat, are PhD students working on SOS with Lundquist at the University of Washington. The helped co-author the April 2024 SOS campaign overview paper and from January through mid-March 2023 undertook research in the East River Watershed by digging snow pits to collect data on snow temperature, density, and morphology.
Interns Step In and Step Up
Internships are another way for scientists just starting out to learn about SAIL and its data.
An intern on Aiken’s team, Elijah Valverde, an applied mathematics student at San Francisco State University, was matched through the Shin Institute’s Sustainable Research Pathways program. He is investigating trends supermicron data from SAIL-AVP.
Aiken says that Valverde will look for “events of interest,” anomalies, and opportunities to make predictions using Bayesian statistics.
At LBNL, two interns from DOE’s Science Undergraduate Laboratory Internships (SULI) program are working with Feldman in summer 2024 to contextualize SAIL data. Emily Ammeraal of Oregon State University and Felix Yu of the University of Michigan will develop a cloud climatology of the Colorado River Basin and high-resolution surface albedo products from satellite data.
Getting a view of the basin from surface instruments across complex terrain is not easy, says Feldman, “but is straightforward to measure with satellites.”
The satellite data, compared with SAIL’s ground-based measurements, he adds, may reveal little-known links between clouds and snow albedo.
Finding such insights is the central impetus of SAIL, its curious data users, and its growing collaborations—a convergence of factors that may unravel the hydrological puzzles wrapped so tightly in mountainous terrain.
Solving puzzles appeals to Feldman.
“I like mysteries,” he says. “They give us an opportunity to explore.”
# # #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.