Scientists will gather detailed data on aerosol-cloud interactions within deep convective systems
Deep convective clouds—the kind that often pack lightning and pour rain—occur nearly everywhere in the world. They are an important feature of the atmosphere, especially in storm systems that dominate the tropics and midlatitudes.
To poets, these harbingers of storms are a durable inspiration. German writer Johann Wolfgang von Goethe saw them as “the mad thunder-cloud, (where) fierce legions clash.”
Outside the realm of verse, scientists are still struggling to know what goes on inside convective clouds—exactly how their “fierce legions” of energy and mass combine and interact.
Says Michael P. Jensen, a meteorologist at Brookhaven National Laboratory in Upton, New York: “We still don’t understand important details about how atmospheric convection works”—at least not well enough to represent convective clouds in models with acceptable accuracy.
In particular, many researchers are vexed by how aerosols influence the physics of such clouds, including the precipitation processes and vertical velocities that affect rates of cloud growth, ice formation, and lightning occurrence.
Aerosols are solid or liquid particles dispersed into the atmosphere by natural sources (such as tree cover) and artificial ones (such as cars and coal plants). Under the right conditions, aerosols help clouds form, and in this way play an important part in weather and climate patterns.
Finding out what happens inside deep convective clouds is the aim of a new field campaign for which Jensen is principal investigator.
The Tracking Aerosol Convection Interactions Experiment (TRACER) got the go-ahead in mid-October 2018 from the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) user facility.
With ARM mobile instruments in place, TRACER is scheduled to run from April 2021 to April 2022 in and around Houston, Texas.
“We want to focus on the convective core,” says Jensen. “We need very detailed measurements of the convective updraft—including the vertical winds and cloud microphysics—and how they evolve over the cloud life cycle, and under varying atmospheric conditions.”
To date, he adds, the microphysical processes within convective clouds are represented only in a limited way in the parameterizations used in models of both weather and global climate.
One related controversy is prominent today among scientists, says Jensen. Do aerosols invigorate deep convective storms? Some scientists say yes: Such particles affect the growth of cloud droplets, accelerate updrafts, cause more rain, and enhance turbulence and mixing.
Jiwen Fan, an atmospheric scientist at Pacific Northwest National Laboratory (PNNL) in Richland, Washington, argued for the rain-making power of fine aerosols as lead author of a 2018 paper in the journal Science. Other scientists doubt a connection.
Meanwhile, present climate and weather models overestimate how strong these updrafts are within convective clouds. It’s one of the model biases—offsets from observations—that TRACER is designed to correct.
“In the end,” says Jensen, “that’s what we’re trying to do. We’re trying to improve the representation of these clouds in climate models.”