A veteran researcher revs up an ASR project studying the influence of aerosols on the cloud radiative effect
Clouds are the most visible outward sign of the Earth’s water cycle. Tiny solid and liquid particles called aerosols make clouds possible.
How these two atmospheric realities interact to shade or simmer the planet’s surface is the hub of the research world in which Graham Feingold lives.
Feingold, a native of South Africa and a product of graduate work in Israel, leads the Clouds, Aerosol, and Climate Program at the Earth System Research Laboratories, an arm of the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado. He calls the mile-high compact city in the foothills of the Rocky Mountains “a mecca for atmospheric sciences.”
Boulder is a long way from the arid hillside in Israel where, as a graduate student in the 1980s, Feingold set up his first instrument with an eye to measuring the size of raindrops. Many challenges in atmospheric science in place back then still exist today.
One involves shallow liquid clouds, the puffy and small formations within a few kilometers of the surface of the Earth. They contain no ice crystals and prompt modest precipitation yet play an important climatological role in the transport of surface heat and the distribution of water and radiative energy.
Despite their significance, such clouds are poorly represented in predictive models of the Earth’s climate system and remain a significant source of uncertainty.
Feingold and a few collaborators are starting a new investigation into that challenge of predictive uncertainty. “Evaluating Biases in Aerosol-Cloud Interaction Metrics using ARM Data and Models” is one of 31 projects funded in July 2020 by the Atmospheric System Research (ASR) program at the U.S. Department of Energy (DOE).
“ARM” refers to the Atmospheric Radiation Measurement Program, a DOE user facility that operates fixed and mobile observatories in climate-critical regions around the world.
Clouds and the Energy Budget
The scale of Feingold’s ASR project can be summed up with a few questions about shallow liquid clouds, which despite their influence are typically just a few kilometers wide and have lifespans measured in fractions of an hour.
How might such clouds change in a warmer world? Clouds, after all, account for two-thirds of the Earth’s albedo, a measure of reflectivity. Even a fractional change in cloud cover can swing the global energy budget.
Moreover, how are shallow liquid clouds influenced by atmospheric aerosols? And how are they affected by the thermodynamic structure of the atmosphere? (The effect of aerosols on cloud radiative effect is a theme in Feingold’s work.)
Matters of scale in both space and time exacerbate an already difficult set of questions, says Feingold. Aerosol-cloud interactions occur at the scale of centimeters, for instance. Meanwhile, cloud fields are organized and distributed over hundreds to thousands of kilometers.
He offers an example: Aerosol-cloud interactions at small scales can alter the radiative character of a whole cloud system. They can change the way condensed water is spatially distributed, therefore changing the albedo of a cloud―how bright or dark it is.
All these make quantifying aerosol-cloud interactions critically important, says Feingold, “if we are to have more confidence in our projections of climate change.”
Micro and Macro Scales
To get those quantifications, researchers have to understand how meteorological conditions relate to important cloud radiative properties, including albedo.
A major challenge is identifying the relationship between meteorological conditions and important cloud radiative properties. That requires understanding how aerosol particles modify cloud fields at both a micro and macro scale. How, for instance, do perturbations―changes―in aerosols affect cloud albedo?
In his new project, Feingold and his co-investigators will combine surface observations from ARM sites with satellite remote sensing, reanalysis, regional modeling, and large-eddy simulations. The aim, he says, is to better understand the model parameters that control cloud albedo.
One research strategy is to identify the meteorological conditions in shallow marine cloud systems that make clouds vulnerable to aerosol perturbations. Hence, some of the project’s data will come from ARM’s Eastern North Atlantic atmospheric observatory in the Azores, and from a 2017―to―2018 ARM campaign called Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA).
Feingold will draw from two other main resources: the ERA5 reanalysis dataset maintained by the European Centre for Medium-Range Weather Forecasts and large eddy simulations using the System for Atmospheric Modeling (SAM) developed at Stony Brook University in New York.
Studying cloud states
Feingold is no stranger to ASR projects―nor funding from ARM, which for him goes back to 2001. Through the years, he says, “the scope and topics have evolved as the questions have changed, and as my group has grown. The work has not been done alone: I have benefitted from wonderful collaborations with colleagues like Allison McComiskey, Jan Kazil, and Tak Yamaguchi, to a name a few.”
Feingold pointed to a few key studies, beginning with a 2003 paper he led that looked at changes in aerosol loading in ice-free, non-precipitating clouds. It reported the first measurements of aerosol indirect effects at a continental U.S. site using ground-based remote sensors. Included were cloud radar, microwave radiometers, and a Raman lidar.
Aerosol indirect effects are a recurring theme in his work and are less well known than the direct scattering of solar radiation by aerosols. These indirect effects influence radiation balance and hydrology by impacting cloud microphysical processes.
The ways aerosols affect clouds are illustrated by two 2014 studies. One was on the role of heat fluxes and aerosol in selecting cloud states. Another, which appeared in Proceedings of the National Academy of Sciences with Feingold as lead author, investigated the importance of aerosol and clouds co-variability in detecting the aerosol-cloud radiative effect.
In this context, co-variability is a way to determine hard-to-estimate aerosol indirect effects by looking at whether the meteorological conditions that are conducive to cloud formation are also those that carry significant aerosol perturbations.
Recent ASR projects
On the ASR side, Feingold’s 2012-2015 ASR project used data from an ARM research aircraft to look at how aerosol particles influence albedo and rain formation in marine stratocumulus clouds over the southeastern Pacific Ocean.
In another ASR project, from 2016 to 2019, Feingold led an investigation on aerosol-cloud radiative effect that pivoted on large-eddy simulations (LES) coupled with observations from ARM’s Southern Great Plains (SGP) atmospheric observatory in Oklahoma.
Among the papers to come out of that was a 2016 study in Atmospheric Chemistry and Physics on the influence of aerosol on cloud radiative effect. (For this continental effect, the authors reported, that influence was weak.) The paper drew from 14 years of SGP ground-based measurements on low clouds, aerosol, and meteorological properties.
A July 2020 study on quantifying the radiative effect of cloud-aerosol interactions used model output and measurements from the LES ARM Symbiotic Simulation and Observation Workflow (LASSO) program. Feingold was among the authors, who looked at how meteorological variability masks the strength of the relationship between cloud drop number concentration and the cloud radiative effect.
LASSO also formed the basis of an analysis of the relationship between cloud field properties and surface shortwave radiation. The work was outlined in a March 2020 study on surface solar irradiance, which Feingold co-authored.
Following a Passion
Feingold grew up in Johannesburg, South Africa, where he remembers being impressed by dramatic summer storms that piled storm clouds on the horizon.
He was attracted early on by the natural world and by the idea that science could help quantify natural processes. In high school, he was inspired by a physical geography teacher who supplemented the course with reams of challenging material copied from college textbooks.
After graduating, Feingold spent two months on a kibbutz in Israel. It was his introduction to what he calls “a young and vibrant country” by then barely three decades old. After just a year of engineering studies at the University of Witwatersrand in Johannesburg, Feingold decamped to Tel Aviv University (BS 1982, MS 1985, PhD 1989).
He gave up on engineering to “follow my passion,” he says, by studying geophysics and atmospheric science―physical phenomena that represent the kind of “science right in front of your eyes.”
Impressed by his accomplishments and captivated by his charisma, Feingold studied with atmospheric scientist Zev Levin, whom he credits for his launch into a lifetime career―one that began with a study of raindrops.
Anchoring his master’s thesis was a new algorithm for simulating the size distributions of raindrops, which aimed to “improve the commonly used Marshall-Palmer distribution,” wrote Levin in an email.
Feingold’s doctoral dissertation put on display his first prolonged application of numerical models informed by observations. He also further developed the algorithm from his master’s work. In the end, wrote Levin, Feingold developed “a cloud model called the Multi-Moment Method of Tel Aviv University (MMM-TAU). This method is increasingly being used by research groups around the world.”
Early on, Levin steered Feingold’s broad interest in the environment as an undergraduate into the atmospheric sciences, he wrote, and calls Feingold “by far one of my best students and one who remains my friend for life.”
The Cloud Tops
By 1990, after a few years as a cloud physics research assistant at Tel Aviv, Feingold moved to Colorado for a series of posts that led him to NOAA in 1997.
Since then, research and writing predominate, marked by papers, invited talks, and book chapters. Feingold is the author of 180 peer-reviewed papers; his first, co-authored with Levin, was published in 1986.
There have been forays abroad at teaching posts and to deliver guest lectures in China, Italy, and Israel. And, of course, there were field campaigns―in Barbados, Chile, California, Oklahoma, Texas, Tennessee, and New England.
Feingold returned to Barbados in January and February 2020 for his most recent fieldwork as part of the Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC). It was sponsored by NOAA to study air-sea interactions and to improve climate and weather predictions. He took to the skies over the tropical North Atlantic Ocean in the agency’s Lockheed WP-3D Orion research aircraft.
Instruments were also mounted on unmanned aerial vehicles, remote-control ocean vehicles, and aboard NOAA’s Ronald H. Brown. It is part of a longer-term international effort to study shallow convective clouds over the world’s oceans.
Of one airborne foray, Feingold says, “we were skimming the cloud tops.”
# # #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.