Thunderstorms and the Upper Atmosphere
Scientists at the National Center for Atmospheric Research (NCAR) and other organizations are targeting thunderstorms in Alabama, Colorado, and Oklahoma this spring to discover what happens when clouds suck air up from Earth’s surface many miles into the atmosphere. Thunderstorms result from the rapid upward movement of warm, moist air. They can occur inside warm, moist air masses and at fronts. As the warm, moist air moves upward, it cools, condenses, and forms cumulonimbus clouds that can reach heights of over 20 kilometers. The Deep Convective Clouds and Chemistry (DC3) experiment, which begins the middle of this month, will explore the influence of thunderstorms on air just beneath the stratosphere, a little-explored region that influences Earth’s climate and weather patterns. Scientists will use three research aircraft, mobile radars, lightning mapping arrays, and other tools to pull together a comprehensive picture.
"We tend to associate thunderstorms with heavy rain and lightning, but they also shake things up at the top of cloud level," says NCAR scientist Chris Cantrell, a DC3 principal investigator. "Their impacts high in the atmosphere have effects on climate that last long after the storm dissipates."
Past field projects have focused on either the details of thunderstorms but with limited data on the atmospheric chemistry behind them, or on the chemistry but with little detail about the storms themselves. DC3 is the first to take a comprehensive look at the chemistry and thunderstorm details, including air movement, cloud physics, and electrical activity.
One of the key goals of DC3 is exploring the role of thunderstorms in forming upper-atmosphere ozone, a greenhouse gas that has a particularly strong warming effect high in the atmosphere.
"When thunderstorms form, air near the ground has nowhere to go but up," says NCAR scientist Mary Barth, a principal investigator on the project. "Suddenly you have an air mass at high altitude that’s full of chemicals that can produce ozone."
Ozone in the upper atmosphere plays an important role in climate change by trapping significant amounts of energy from the Sun. However, ozone is difficult to track.
Low level ozone (or tropospheric ozone) is an atmospheric pollutant. It is not emitted directly by car engines or by industrial operations, but formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers down wind.
Updrafts within thunderstorm clouds range from about 20 to 100 miles (about 30-160 kilometers) per hour, so air arrives at the top of the troposphere, about 6 to 10 miles (10-16 kilometers) up, with its pollutants relatively intact.
"In the midlatitudes, the tropopause is like a wall," says Barth. "The air bumps into it and spreads out."
The DC3 scientists will fly through these plumes to collect data as a storm is under way. Then they’ll fly through the same air mass the next day, using its distinctive chemical signature to see how it’s changed over time.
"We are pretty sure lightning is the largest natural source of nitric oxide," says NOAA National Severe Storms Laboratory scientist Don MacGorman. "It is important to know the naturally occurring contribution."
Scientists at each of the sites will combine data from radars with Doppler capabilities (for wind information) and polarimetric capabilities (for wind and cloud particle information) with lightning mapping arrays to better understand both how storms produce lightning as well as how to use lightning mapping data to improve storm forecasts and warnings.
For further information see Thunderstorms.
Thunderstorm image by Bob Henson via NCAR.