PNNL

This thunderstorm in Eastern Colorado is typical of a large convective cloud that spreads out over the Earth for many miles. Image courtesy of the DC3 field campaign. Enlarge Image.

Results: Inside tall, turbulent storm clouds, atmospheric particles hardly seem to be the lead story of storms. But it's often unwise to overlook the small stuff.

Scientists at Pacific Northwest National Laboratory followed the journey of atmospheric particles within anvil-shaped storm kings called convective clouds, and analyzed how well the model could simulate the particles' travels.

The researchers devised a new modeling approach that tackles a couple of particle problems: how they are moved to the upper troposphere (about 5-6 miles above Earth), and how efficiently they leave the atmosphere inside droplets, rain, or snow.

"Our study showed the importance of secondary activation. That's when particles form droplets above rather than just at the base of the cloud," said Dr. Qing Yang, atmospheric scientist and lead author of the study. "The wet removal of aerosol particles in deep convective storms will consequently affect the lifetime of the particles in the atmosphere and their radiative effects on the climate."

Why It Matters: Aerosol particles, those tiny bits of soot, dust, or pollution suspended in the atmosphere, affect the climate by absorbing and scattering the sun's radiant energy. They also influence how reflective clouds are (the albedo effect), how long the cloud lasts, and precipitation.

These effects partially hinge on how concentrated the particles are and how they are vertically distributed within the clouds. Both of these effects depend on how many or how few of the particles are removed by rain or snow, and how efficiently they are moved from lower levels to the upper troposphere by the force of convection.

This study shows a new method to make those important calculations, so that climate models better describe the elusive environment and effects of convective storm clouds. An accurate picture of the climate relies on better understanding and representing atmospheric particles in models, and their important actions within tall and turbulent storm clouds.

Methods: The PNNL-led research team developed a new passive-tracer-based transport analysis framework. They used vertical profiles of several slow reacting non-soluble trace gases as a proxy for aerosol particles in their model. They employed a popular and effective regional process model called the Weather Research and Forecasting model.

They fed the model with observed gas and aerosol vertical profile measurements taken in deep convective storms during the Deep Convective Clouds and Chemistry (DC3) field campaign over a broad swath of the U.S. plains states, including northeastern Colorado, central Oklahoma, and northern Alabama.

Structure of a Storm Cloud During the DC3 field campaign, researchers gathered data to help understand turbulent convective storms. These tall clouds develop from rising moist air that carries gases, dust particles, and chemicals from lower levels, creating an upward air movement that then cools and condenses, creating many tiny droplets and ice crystals. As the clouds grow taller and the droplets/crystals grow larger, eventually some of them fall out of the cloud (wet removal) in different forms, depending on the temperature. The graphic shows the different measurement techniques used to study these clouds. Graphic courtesy of the DC3 campaign website.

The results emphasize the importance of secondary activation, that is, aerosol wet removal above the cloud base. The study offers a new framework that can be extended to different types of storms and could be used to evaluate convective transport and wet removal in global models.

What's Next? The researchers will apply a similar model design to different types of storms over other regions to gain more comprehensive understanding of the variability of convective transport and scavenging.

Acknowledgments

Sponsors: The research was supported by the  Department of Energy's Office of Science, Office of Biological and Environmental Research Atmospheric System Research program. Additional support provided by the NASA Postdoctoral Program, and the National Science Foundation.

User Facility: ARM Climate Research Facility

Research Team: Qing Yang, Richard Easter, Jerome Fast, Steve Ghan, Hailong Wang, Larry Berg, ManishKumar Shrivastava, Balwinder Singh, and Jiwen Fan, PNNL; Pedro Campuzano-Jost, Jose Jimenez, and Megan Bela, University of Colorado Boulder; Hugh Morrison and Eric Apel, National Center for Atmospheric Research; Conrad Ziegler, NOAA National Severe Storms Laboratory; Glenn Diskin, NASA Langley Research Center; Tomas Mikoviny, Oak Ridge Associated Universities; and Armin Wisthaler, University of Innsbruck, Austria.

Research Area: Climate & Earth Systems Science

Reference: Yang Q, RC Easter, P Campuzano-Jost, JL Jimenez, JD Fast, SJ Ghan, H Wang, LK Berg, MC Barth, Y Liu, MB Shrivastava, B Singh, H Morrison, J Fan, CL Ziegler, M Bela, E Apel, GS Diskin, T Mikoviny, and A Wisthaler. 2015. "Aerosol Transport and Wet Scavenging in Deep Convective Clouds - A Case Study and Model Evaluation using a Multiple Passive Tracer Analysis Approach." Journal of Geophysical Research: Atmospheres 120(16): 8448-8468. DOI: 10.1002/2015JD023647