Atmospher Sci & Global Chg
Research Highlights

October 2016

The Lower Atmosphere's Mixing Messages

Researchers pick apart processes causing layers of climate-concerning particles to vary in the Mid-Atlantic coastal atmosphere

On the shores of Cape Cod, DOE’s ARM Climate Research Facility set up a mobile site to capture ground-based data in the clouds and atmosphere at the edge of an area where “clean” and “polluted” air particles mix and provide interesting data for scientists trying to understand the way these particles influence the energy balance of the planet. Scientists in this paper were comparing their ground-based and aircraft captured data with model representations of the atmosphere’s particle activity. Their detailed, high-resolution model (top) and the global model (bottom) can provide information to further explain how different processes are affecting our atmosphere at important interfaces, and how well those can be explained in climate models for understanding the future. Images courtesy of the ARM Climate Research Facility and the authors.. Enlarge Image.

Scientists have observed vertical layers of tiny, virtually unseen particles, shifting and mixing over the lower level of the atmosphere. Knowing how these layers disrupt and waylay the light energy traveling from Sun to Earth, and reflected from Earth to space, helps determine Earth's energy balance. Previous attempts to simulate these layers and their effects have revealed the true difficulty in nailing down what role they perform for the climate.

Results: In a study analyzing particle layers observed over the mid-Atlantic US coast, researchers at Pacific Northwest National Laboratory (PNNL) and collaborators unraveled processes that contribute to their vertical motion and distribution, to better represent them in a climate model. They used a detailed regional model that revealed strong vertical motions affecting the layers and concentrations of particles in the free troposphere, above the Earth's direct influence and boundary layer.

While the high-resolution regional model produced stronger vertical motions and captured the structure of the layers and concentrations in the free troposphere, the team warned that using regionally refined domains does not entirely solve the misrepresentation of these particles in a global context.

"Our higher resolution model produced stronger vertical air motions, sending more particles to the free troposphere above the boundary layer," said Dr. Jerome Fast, lead author and atmospheric scientist at PNNL. "And these results correlated better with the ground-based lidar data we were capturing during the field campaign."

Why It Matters: Miniscule atmospheric particles called aerosols come in all shapes, sizes, compositions and characteristics. Some absorb light energy, others deflect it. Some mix with other kinds of aerosols to make new kinds, forming all new characteristics. Some are natural—from trees and foliage, dust or sea salt particles—and others are human-caused—from burning coal, gas, and wood, for instance. Certain locations might harbor a particular set of aerosols, and because of the air dynamics of the region they tend to congregate in layers. Some travel the airwaves until they either fall out with precipitation or get lofted higher in the atmosphere.

What causes them to congregate in layers? Do they block or facilitate energy traveling from the Sun toward Earth? These are some of the questions that interest scientists looking into these layers, especially in climate sensitive areas where natural particles are mixing with human-produced particles and the atmospheric dynamics cause interactions among them. Ultimately, scientists want to unravel these conditions so that they can more accurately simulate them for better projections of climate and weather conditions.

Methods: The PNNL-led research team used extensive in situ and remote-sensing measurement data to identify the atmospheric processes responsible for the structure and composition of the aerosol layers.

The data was collected during the 2012 Two Column Aerosol Project (TCAP) in the Mid-Atlantic region off the coast of Cape Cod. The goal of the TCAP campaign was to sample aerosol microphysical properties in two columns; one fixed column near the Cape Cod National Seashore's Highlands Center on the eastern shore of Cape Cod, Massachusetts and another movable column several hundred kilometers over the Atlantic Ocean. Some of the observations were collected onboard a research aircraft managed for the Department of Energy's Atmospheric Radiation Measurements (ARM) Climate Research Facility and the ARM Aerial Facility (AAF). The Gulfstream-1, or G-1, holds dozens of complex instruments to gather atmospheric particles and data, some of which are analyzed in real-time.

The team observed aerosol layers on every G-1 flight conducted by the research aircraft, although the altitude, thickness, and aerosol concentrations varied day by day. A key challenge was to understand the reason for this variability in the layers, particularly those located in the free troposphere several kilometers above the surface. They then sought to identify the source of these aerosols.

The research found that using a higher resolution regional model, the Weather Research and Forecasting model (WRF), produced more aerosol mass in the free troposphere than a coarser resolution global climate model, the Community Atmosphere Model, or CAM5. This showed that the fraction of aerosol optical thickness in the free troposphere was more consistent with lidar measurements.

Simulated aerosol layers in the free troposphere were largely the result of mean vertical motions that transported aerosol particles from the top of the boundary layer to higher altitudes. The vertical displacement and the time period associated with upward transport in the troposphere depend on the strength of the synoptic (large-scale) system and whether relatively high boundary layer aerosol concentrations are present where convergence occurs. While a parameterization of subgrid scale convective clouds modulated the concentrations of aerosols aloft, it did not significantly change the overall altitude and depth of the layers.

What's Next? A similar study will be performed using the TCAP data collected during February 2013 so that the summer and winter periods can be compared in terms of understanding the processes contributing to aerosol layers in the vicinity of Cape Cod.

Acknowledgments

Sponsors: The work was supported by the Department of Energy's (DOE) Office of Science, Office of Biological and Environmental Research as part of the Atmospheric System Research Program. The work was also supported by the National Science Foundation and NASA.

User Facilities: ARM Climate Research Facility, ARM Aerial Facility, ARM Mobile Facilities, Environmental Molecular Sciences Laboratory, EMSL

Research Team: Jerome D. Fast, Larry K. Berg, Kai Zhang, Richard C. Easter, Ying Liu, John E. Shilling, Manish Shrivastava, Jason M. Tomlinson, Jacqueline Wilson, Rahul A. Zaveri and Alla Zelenyuk, PNNL; Richard A. Ferrare, Johnathan W. Hair and Chris A. Hostetler, NASA Langley Research Center; Ivan Orgega and Rainer Volkamer, Colorado University Boulder; and Arthur Sedlacek III, Brookhaven National Laboratory

Research Area: Climate and Earth Systems Science

Reference:  Fast JD, LK Berg, K Zhang, RC Easter, RA Ferrare, JW Hair, CA Hostetler, Y Liu, I Ortega, A Sedlacek III, JE Shilling, M Shrivastava, SR Springston, JM Tomlinson, R Volkamer, J Wilson, RA Zaveri and A Zelenyuk. 2016. "Model Representations of Aerosol Layers Transported from North America over the Atlantic Ocean During the Two-Column Aerosol Project." Journal of Geophysical Research: Atmospheres 121(16): 9814-9848. DOI: 10.1002/2016JD025248