October 4, 2013
Climate Models Help Researchers Understand Atmospheric Electricity
April Flowers for redOrbit.com - Your Universe Online
A detectable electrification of the air is caused by electrical currents born from thunderstorms. These currents flow through the atmosphere around the globe, even in places with no thunderstorm activity.
Until recently, however, science has not had a good explanation for how conductivity varies throughout the atmosphere and how that may affect the path of the electrical currents.
A new study led by the University of Colorado Boulder added a layer to a climate model created by colleagues at the National Center for Atmospheric Research (NCAR) to create a global electric circuit model.
The atmosphere is generally less conductive over the equator and above Southeast Asia, according to the model, and more conductive closer to the poles. The findings, published in the Journal of Geophysical Research, revealed that this is so even though the atmosphere's conductivity changes seasonally and with the weather.
Scientists have been researching atmospheric electrification since the 1750s, when researchers such as Benjamin Franklin were trying to better understand the nature of lightning. From the Kew Observatory near London, scientists in the 1800s measured changes in the atmosphere's electric field. In the 1900s, an all-wooden ship named the Carnegie was built without any magnetic materials. The ship crisscrossed the ocean while taking atmospheric electricity measurements that are still referenced today.
Obtaining a global picture of atmospheric conductivity, however, has been difficult because the atmosphere's ability to channel electricity is not static. A continuous bombardment of galactic rays add ions -- which allow current to move through the air -- to the upper atmosphere. The lower atmosphere ions are added through radioactive decay. The ions in the atmosphere can be dispelled in a variety of ways, however.
"They can recombine, to some degree, but they also attach themselves to aerosols and water droplets," said Andreas Baumgaertner, a research associate in CU-Boulder's aerospace engineering sciences department. "Once they are attached to a heavy particle, like a water droplet, then you've lost the ability for it to conduct a current."
As moisture-laden clouds move through an area, the amount of water droplets in the atmosphere varies and the quantity of aerosols varies depending on their source. Tailpipes, smokestacks and erupting volcanoes pump aerosols into the atmosphere as well.
Baumgaertner worked with CU Boulder Professor Jeffrey Thayer, director of the Colorado Center for Astrodynamics Research; Ryan Neely, an atmospheric scientist at NCAR; and Greg Lucas, a CU-Boulder doctoral student in aerospace engineering sciences on the current study. The team developed the idea of using NCAR's existing Community Earth System Model to create a global picture of conductivity. The NCAR model was ideal because it already took into account both water vapor and aerosols.
Equations that represent how many ions are produced by cosmic rays from space and by radioactive decay through radon emissions from the Earth's surface were added to the model, as well as equations for how those ions react in the atmosphere. This resulted in 2,000 lines of code that allowed the team to create the first global picture of conductivity and how it evolves over time.
The team discovered that over the course of a year, the atmosphere was on average less able to conduct electricity above areas of the globe that also have high emissions of aerosols, especially in Southeast Asia. They also found that the atmosphere above the equator, in general, was less conductive mainly due to fewer galactic cosmic rays there than at the poles. The conductivity of the atmosphere as a whole varied with the seasons, according to the findings, and was generally less conductive in June and July than in December and January.
The team is now working to feed data into the model on the frequency and location of storms to better understand how the current provided by lightning actually moves.
"The next step is to incorporate the distribution of thunderstorms," Lucas said. "Currents generally travel upwards above thunderstorms distributed around the equator and return down over the poles, away from the thunderstorms. Part of the future work is going to be determining what influence those thunderstorms have on the global system."