ARM field campaign gathers data that will be used to examine the life cycle of shallow convective clouds

What controls the initiation, maintenance, and distribution of shallow convective clouds? The just-finished ARM campaign Holistic Interactions of Shallow Clouds, Aerosols, and Land-Ecosystems, or HI-SCALE, collected data that will help answer that question. Making use of ARM’s Southern Great Plains (SGP) atmospheric observatory and the ARM Aerial Facility (AAF), HI-SCALE looked at a wide range of factors that could influence cloud development.

The Southern Great Plains observatory is ARM's premier measurement site.
The Southern Great Plains observatory is ARM's premier measurement site.

Why Clouds?

Clouds play an important role in a number of natural processes, from the water cycle to how much energy the Earth absorbs from the sun.

Despite this, clouds remain something of a mystery to atmospheric researchers. For example, scientists know that size and composition of aerosols determine if the particles are suitable for forming cloud droplets, but they don’t know all of the processes that influence how particle size changes over time.

Another problem is that once shallow convective clouds have formed, it’s hard to tell what makes some grow into thunderstorms while others remain small. Researchers want to know more about clouds to improve predictions on all scales, from tomorrow’s weather to the next century’s climate.

Measuring Many Variables

HI-SCALE gathered data that will provide a closer look at the life cycle of shallow convective clouds. In two intensive operational periods—one from April to May and a second from August through September—the campaign made use of the SGP’s standard equipment as well as the AAF’s Gulfstream-159 (G-1) aircraft, loaded with instruments from ARM and collaborators like the University of Washington, Brookhaven National Laboratory, and the Environmental Molecular Sciences Laboratory (EMSL).

Measurements covered the standard meteorological information like temperature, humidity, and winds, and more specialized trace gas, aerosol, and cloud measurements like the number and size of water droplets. Several collaborators from EMSL, University California – Riverside, University of Colorado, Augsburg College, Aerosol Dynamics Inc., and Aerodyne Inc. also collected specialized aerosol measurements at ARM’s SGP site.

Instruments filled the cabin of the ARM Aerial Facility G-1.
Instruments filled the cabin of the ARM Aerial Facility G-1.

Aircraft in the Sky, Instruments on the Ground

Even though they were studying the atmosphere, the HI-SCALE researchers looked down at the ground, too. Principal investigator Jerome Fast, Pacific Northwest National Laboratory, said that it’s important to recognize the role the land plays in driving atmospheric processes. “A lot of atmospheric research treats the ground simply as a boundary condition,” meaning a line is drawn and anything on the other side isn’t considered. “The same happens with soil science and hydrology, just in the other direction.”

Tying the two disciplines closer together could lead to breakthroughs in understanding variability in cloud and land surface properties. “They interact in so many ways that people don’t even think about,” Fast said. Even just across a few acres of farmland, there can be major differences because of the interaction. “If it’s cloudy over here but not over there, then there’s a difference in sunlight reaching the ground and providing energy to plants. If it’s raining over here but not over there, soil moisture levels are going to be different. If the plants are green and growing or brown and dry, that will affect what sort of particle precursors are being emitted into the air.”

The split operating period of HI-SCALE was meant to capture some of that variation. By taking measurements in both the spring growing season and later during the fall, the campaign’s data will show what effects changes in land vegetation have on cloud formation. The SGP observatory was an ideal location, surrounded by agricultural fields and a variety of land use.

The G-1 is prepped before a flight.
The G-1 is prepped before a flight.

HI-SCALE’s High Temperatures

Flying a plane in the hot Oklahoma summer had its own challenges, however. Higher-than-average temperatures made it difficult to keep the cabin cool enough for the instruments to function at peak performance, and also required the plane to carry less weight. “Less weight means less gas, which means shorter flights and less overall data collection,” Fast explained. To collect the necessary data, they flew two flights on some days.

Even with the heat, Fast is happy with the campaign results. “We sampled what we set out to sample and will be providing a coupling of ground and aircraft based data that will lead to a better understanding of how land-atmosphere interactions and boundary layer properties influence the evolution of populations of shallow clouds and aerosol particles.”

These measurements will be useful both for understanding the mechanisms by which clouds form and for incorporating those mechanisms into atmospheric models. After being processed and checked for quality, HI-SCALE’s results will be available in the ARM Data Archive, where researchers can access all of ARM’s data.

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The ARM Climate Research Facility is a national scientific user facility funded through the U.S. Department of Energy's Office of Science. The ARM Facility is operated by nine Department of Energy national laboratories including the Pacific Northwest National Laboratory, which leads management of the ARM Aerial Facility, and Argonne National Laboratory, which manages ARM's Southern Great Plains atmospheric observatory.