Cracking the Puzzle of Ice in Clouds

 
Published: 23 October 2019

Hugh Morrison leads an Atmospheric System Research project on parameter variability in modeling ice microphysical processes

Hugh Morrison enjoys a quiet moment at his desk at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.

Not that long ago, while a graduate student in atmospheric science, Hugh Morrison glided from class to class on a skateboard.

These days, 15 years past getting a PhD, he keeps the board on a peg in his garage. Otherwise, says Morrison, “I risk bodily injury.”

But Morrison—a senior scientist at the National Center for Atmospheric Research (NCAR)—is still gliding towards knowledge, clicking along as an author and balancing two main interests: cloud microphysics and (increasingly) the dynamics of deep convective storms.

Regarding the first research interest, he asks: How can we learn more about all the processes that affect ice particles in the atmosphere? How do they grow, aggregate, rime, and form deposits on water vapor?

On that score, Morrison is the principal investigator on a project for the U.S. Department of Energy’s (DOE) Atmospheric System Research (ASR) program. The idea is to improve earth system models by accounting for the generally ignored effects of parameter variability in ice microphysical processes.

As for the second main research interest, Morrison wonders: How do we more precisely describe the dynamics of strong thunderstorms—the strong vertical updrafts, for instance, that affect weather—and the microphysical processes that drive such storms?

And how do we learn more about the towering anvil clouds that block solar radiation from above and longwave radiation from below?

In both of these research interests, says Morrison, “A lot of things are not well understood.”

‘A Lot of Stuff We Care About’

Polar-region clouds were an early interest for Morrison. In the fall of 2004, he traveled to ARM’s North Slope of Alaska atmospheric observatory for the Mixed-Phase Arctic Cloud Experiment (M-PACE). The field campaign deployed a nephelometer, above left, and a microwave radiometer profiler, right.

For his work on cloud processes and properties, Morrison was recently named co-chair of ASR’s Convective Processes working group—one of four groups focused on the program’s research priorities regarding uncertainties in climate prediction. (The others relate to aerosol, boundary-layer, and high-latitude processes.)

Deep convective processes, and the dynamical character of the thunderstorms that may result, “have a strong influence on a lot of stuff we care about,” says Morrison.

Understanding the strong updrafts in convective systems is important just “from a practical weather standpoint,” he says, and on a global-climate scale “for understanding the larger-scale circulation of the atmosphere.”

Then there is the issue of anvil clouds. Convective systems influence how wide and deep they get, factors that impact the Earth’s radiative balance.

Meanwhile, the processes that drive vertical winds within convective systems are not well understood. Nor are storm-cloud microphysical processes, which determine—individually and as bulk properties—the shape, density, mass, and size of the particles within clouds.

The microphysics is complex, says Morrison. Consider, for instance, that within even a relatively small cloud there can be 1015 (1,000,000,000,000,000—or 1 quadrillion) cloud droplets.

Ice Microphysics

Morrison recently developed a theoretical model describing horizontal and vertical winds in thunderstorm clouds at various heights, as illustrated in a paper currently under review. “R” is the cloud radius.

Morrison and his co-investigators in the 2016-2019 ASR project are specifically interested in how to better represent ice microphysical processes in models. They are developing and testing a parameterization scheme for cloud and climate models. (The project, under a no-cost extension for a year, will wrap up in the summer of 2020.)

He and the other ASR-funded researchers on his team are using observational data from DOE’s Atmospheric Radiation Measurement (ARM) user facility, which maintains fixed and portable atmospheric observatories in climate-critical regions around the world. The observations, they say, will constrain the microphysical framework on which they are working.

In real-world storm dynamics, the range of ice-particle characteristics varies widely. Yet, until now, the complexities of mass, concentration, shape, densities, fall speeds, and other characteristics—which influence the reflectivity and scattering properties of clouds—have appeared as constants in models.

Morrison and his team are trying to correct this by accounting for the effects of parameter variability.

One paper has already emerged from the project—a study led by then-University of Illinois PhD student Joseph Finlon, now a postdoctoral researcher at the University of Washington. Its focus is on storm-region observations made by airborne and surface instruments during ARM’s 2011 Midlatitude Continental Convective Clouds Experiment (MC3E) field campaign in Oklahoma.

A second, accepted recently but not yet published, is led by University of Utah PhD student McKenna Stanford. It focuses on the modeling side. The Morrison team’s new approach to models account for the variability in mass, size, and other ice-particle properties swept up in observations—especially those made by aircraft flying within clouds.

The Woods and the Weather

Morrison grew up in northern Minnesota. “We were outside all the time,” he says, and it was just part of his boyhood to awaken to the natural world, which included an interest in thunderstorms. “I’ve always loved a good convective storm.” (To this day, Morrison often checks radar images for weather patterns.)

“I’ve always loved a good convective storm.”

At the same time, starting in elementary school, he acquired a deep interest in mathematics and worked through a series of what he calls “science phases”—serial fascinations for rocks and minerals, chemistry, and more. Then came the University of Minnesota (B.S. in geography, 1997). Morrison took premed courses (his father and brother are physicians). But what to do with those math skills and science fascinations and science phases?

You guessed it: skateboarding.

Morrison moved to Boulder, Colorado, to board around town with friends and to snowboard when there was powder. He supported himself by delivering pizza.

“It probably wasn’t a good long-term plan,” Morrison says now. “I needed to figure out what I wanted to do.”

Considering his abiding interests in the sciences and meteorology, he soon found a good way to leverage his background in math and geography. In 1998, Morrison enrolled in the University of Colorado, Boulder’s program in astrophysical, planetary, and atmospheric sciences (MS 2000, PhD 2003).

Arctic Clouds

Not long before, Morrison’s mentor and eventual dissertation advisor, now-retired climatologist Judith Curry, had returned from the Surface Heat Budget of the Arctic Ocean (SHEBA) field campaign. In part, the year-long drift expedition was motivated by earth system models that were already showing “arctic amplification,” the near doubling of warming trends in the rest of the world.

That convergence of circumstances cast the young graduate student into polar research.

“I wasn’t dead-set on working on clouds then,” says Morrison. “But there were all these (SHEBA) data there, and I became interested in how important clouds were to the climate system—especially arctic clouds.”

Motivated by SHEBA, he used his dissertation to develop a new approach to modeling clouds and then apply it to the Arctic.

Not much was known about arctic clouds then. Morrison was also drawn to the emerging fact that arctic clouds were full of supercooled water. That alters the radiative properties of such clouds, which in turn has a strong effect on the surface radiation budget and the fate of sea ice.

“These clouds,” says Morrison, “sit at the nexus of a lot of important things.”

Morrison with his 3-year-old daughter Victoria. “Yes,” he says. “There is life outside of science.”

Convection Calls

After graduating, Morrison was a postdoctoral research associate at both Boulder and NCAR, and was briefly a non-resident research scientist at the Georgia Institute of Technology.

In the fall of 2004, as part of polar-region research, he traveled to ARM’s North Slope of Alaska atmospheric observatory for ARM’s Mixed-Phase Arctic Cloud Experiment (M-PACE) field campaign.

By 2007, he was a scientist at NCAR’s Mesoscale and Microscale Meteorology Division.

At the time, NCAR was leading the development of the Weather Forecasting and Research Model (WRF), a region-scale numerical weather prediction model. Morrison wanted to get his own cloud-modeling scheme into it. (He did.)

At the same time, WRF (“WARF,” to those in the know) led Morrison more and more into research on deep convection.

Starting in November 2019, he will take that interest with him to Sydney, Australia, for six and a half months, where he will work with convective dynamics expert Steven Sherwood at the University of New South Wales.

With Morrison, of course, will be his wife Mila and his daughter Victoria.

“Yes, there is life outside of science,” he says of family life and his steady interests in biking, running, and hiking. “I’m lucky.”

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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.