A highly cited scientist is unraveling the puzzle of mixed-phase clouds and improving how aerosols are represented in earth system models

For good reasons, aerosols are difficult to understand. Unlike other trace gases, which can persist in the atmosphere for years or even centuries, the lifespans of aerosols are measured in days.

In his campus office, Liu pauses during modeling work. Using global and regional models, he has investigated the effects on clouds and climate of aerosols from mineral dust, wildfires, and air pollution. Photo is courtesy of the University of Wyoming.
In his campus office, Liu pauses during modeling work. Using global and regional models, he has investigated the effects on clouds and climate of aerosols from mineral dust, wildfires, and air pollution. Photo is courtesy of the University of Wyoming.

To complicate things even more, aerosols scatter in large spatial variations, come in a wide variety of sizes, contain multiple chemical species, and they change—“age,” in science parlance—in complicated physical and chemical ways.

The complexity of aerosol behavior ratchets up considerably in mixed-phase clouds, where liquid and ice coexist in temperatures below zero.

Such clouds, abundant in the Earth’s frigid high latitudes and above the Southern Ocean, are difficult to represent accurately in the earth system models designed to predict climate change.

Atmospheric scientist Xiaohong Liu is trying to help by leading a three-year research project funded by the Atmospheric System Research (ASR) program of the U.S. Department of Energy (DOE). The goal is to improve representations of mixed-phase clouds and aerosol interactions at high latitudes.

He is the endowed chair in climate science at the University of Wyoming and a longtime investigator of the complex and consequential interactions of clouds, aerosols, precipitation, and radiation. Broadly, his research asks: What are the effects of those interactions on climate?

Liu and his research group develop multi-scale models of clouds and aerosols. They also improve cloud and aerosol parameterizations—streamlined mathematical representations—of clouds and aerosols in earth system models.

Even after decades of intense study, aerosol-cloud interactions remain the greatest sources of uncertainties in models designed to predict future earth system change.

A Hand from ARM

Decades ago, as a graduate student, Liu studied warm clouds. By contrast, he says today, “mixed-phase clouds are a complex example.”

Liu and his co-investigator—Zhien Wang, a remote sensing expert at the University of Colorado, Boulder—are six months into the ASR project, which wraps up in 2021.

They are focused on improvements to Community Atmosphere Model version 6 (CAM6), which Liu helped develop at the National Center for Atmospheric Research (NCAR). It was released in 2018.

Some data for Liu’s ASR project come from instruments aboard the Aurora Australis in the Southern Ocean during the 2016-2017 Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) ARM field campaign.

The Liu-Wang ASR project will compare CAM6 to observational data from the DOE’s Atmospheric Radiation Measurement (ARM) user facility. In the Earth’s most critical climate zones, ARM manages fixed and portable atmospheric observatories, where continuous measurements are processed, evaluated, and then stored in a permanent archive.

Beyond what ARM provides from its North Slope of Alaska atmospheric observatory and its campaigns in the Southern Hemisphere, says Liu, there is not much data available for studying the mixed-phase clouds that occur at high latitudes.

The project will tap data from four ARM field campaigns: the 2004 Mixed Phase Arctic Cloud Experiment (M-PACE); the 2008 Indirect and Semi-Direct Aerosol Campaign (ISDAC); the 2016 ARM West Antarctic Radiation Experiment (AWARE); and Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS), which took place in 2016 and 2017.

Most earth system models incorporate treatments of aerosols and their effect on climate, but can still get things wrong.

For instance, says Liu, without the right validation by observational data, models can misrepresent the ice crystal generation and growth processes in mixed-phase clouds, which in turn can affect predictions of sea ice loss in the Arctic.

“These kinds of processes are not well understood,” he says.

The 2018-2021 ASR project is a natural next step in Liu’s longtime passion for improving the kinds of models that steer climate predictions.

He was an important member of the team developing CAM5 and CAM6 at NCAR, and is also part of the DOE’s Energy Exascale Earth System Model (E3SM) project, a computationally advanced earth system model intended to improve predictions on both weather and climate scale.

‘A Very Wide Science’

Liu, the son of an army officer in Mao-era China, was born in the 1960s. His boyhood home was in Anhui, a province in eastern China known for the Huangshan (Yellow Mountain) range. Its peculiar granite peaks were long celebrated in traditional Chinese paintings.

“When I grew up, China was very poor,” says Liu. It was the time of the Cultural Revolution. Food was scarce and universities were shuttered.

He was still a boy when the family moved to a town near Shanghai, in Jiangsu province, where he received his K-12 education.

Still very young, Liu was attracted to the sciences—an attraction he says is a reflection of Chinese cultural preference for physics, chemistry, and mathematics.

In middle school, a teacher advised him that one interesting way to pursue physics was through meteorology—the course of study Liu chose when entering Nanjing University, one of the top universities in China.

“I’m pretty lucky,” he says. “I went to college in 1982,” just as China was gradually redeveloping its education system.

He went on to earn three degrees at Nanjing University: B.S. (highest honors, 1986), M.S. (1989), and PhD (1992).

From the beginning, Liu’s course of study was atmospheric science, a field he describes as having some of everything, especially in the realm of aerosols—physics, chemistry, mathematics, and even biology.

“It’s a very wide science,” says Liu. “I like that. You can touch on everything. I think it’s perfect for me.”

His PhD dissertation was on aerosol activation in warm clouds. At that time, aerosol-cloud studies were a sleepy corner of atmospheric sciences, says Liu, though just a few years later—with a burgeoning interest in the science of climate change—“it was a very hot topic.”

He was an Alexander von Humboldt Research Fellow in Germany from 1996 to 1997—his first travel to the West—and in 1998 was a visiting scientist at the University of Washington. After that, for two years, Liu did research in atmospheric physics at the Chinese Academy of Sciences in Beijing.

Travel to the West to pursue science was a generational dream come true, says Liu. He remembers that when China opened to the outside world, there was a fervor amongst his peers to study in the West.

Deep Dives into Modeling

Liu (center right, in blue sweater) poses in 2016 with his research group at the University of Wyoming. Photo is courtesy of Liu.
Liu (center right, in blue sweater) poses in 2016 with his research group at the University of Wyoming. Photo is courtesy of Liu.

The first science stop for Liu in the United States was the University of Michigan, from 2000 to 2006, where he worked with applied mathematician and aerosols researcher Joyce Penner. It was there he first dealt with earth system models.

From 2006 to 2013, Liu worked at Pacific Northwest National Laboratory (PNNL) in Richland, Washington.

“The national lab is a very good environment for people to grow,” says Liu of PNNL. “It has a very strong aerosols and clouds team—very diverse.”

At PNNL, Liu was the lead author on what he considers his most influential and memorable paper—a 2012 study in Geoscientific Model Development written with PNNL colleagues and others on representing aerosols in CAM5, the predecessor to CAM6.

“We developed an aerosols model,” he says, “and it’s highly cited.”

Liu has 170 peer-reviewed papers to his credit. From 2014 through 2018 he appeared on an annual list of highly-cited researchers published by Clarivate Analytics, and formerly assembled by Thomson-Reuters.

He calls the University of Wyoming a place that is “good for research,” though it is “a little different career path” compared to PNNL.

That’s partly because he has teaching duties now, and also because the university town of Laramie, Wyoming (population 30,000), can seem isolated. Still, says Liu of Wyoming, “I like the quiet.”

For him, is there life outside science?

“I swim,” he says of an exercise habit started 25 years ago in China. “It keeps me energetic—for research.”

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This work was supported by the U.S. Department of Energy's Office of Science, Office of Biological and Environmental Research as part of the Atmospheric System Research Program.