LiDARs See Layers (and More): The Kellyville Upper Atmosphere ARCLITE Project

In grade school, most of us were taught that Earth’s atmosphere was layered, like a parfait (as Shrek’s donkey would say).  We memorized the names of these layers and probably thought of each as a discrete entity unrelated to the others – Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.

Turns out, the atmosphere is a lot more dynamic and complicated than a parfait (or even an onion).  Professor Jeffrey Thayer’s (University of Colorado, Aerospace Engineering Sciences Department) ARCLITE (Arctic LiDAR Technology) NSF-funded project uses remote sensing techniques to figure out just how much more complicated the atmosphere really is. Located at the Sondrestrom Upper Atmosphere Research Facility (affectionately known as Kellyville after SRI’s John Kelly, a renowned incoherent scatter radar expert who established the facility), ARCLITE uses cutting-edge technology for studying Earth’s middle and upper atmosphere above the Arctic.

“We don’t yet have whole atmosphere models to describe how matter and energy are transferred across atmospheric layers. LiDAR technology allows us to study the atmosphere across its layers including the region between 35 and 80 km, which is difficult to study using other meteorological methods like weather balloons or satellites,” explains Thayer. “We study the Arctic atmosphere because there are processes unique to the polar regions which may tell us something about climate change.”

Operated by CPS partner SRI International for the National Science Foundation and the Danish Meteorological Institute, Kellyville is located north of the Arctic Circle near Kangerlussuaq, Greenland. The facility is centered on a 32-meter steerable dish antenna. Relocated from Alaska in 1982, the radar was designed to measure parameters characterizing the aurora borealis. The large radar system drove Kellyville’s original infrastructure toward upper atmosphere research, which continues today. Thayer established the ARCLITE project at Kellyville in 1992 and served as principal investigator of the Kellyville facility for seven years (1998-2004), which continues to develop with NSF funding.

“Kellyville currently demands a year-round staff of four or five people for facility and instrument maintenance, various associated engineering projects, and ongoing data operations for about 20 universities at any one time,” explains Thayer. “Over the last several years, we have been upgrading the ARCLITE system to measure more atmospheric parameters. Eventually, the system will be more autonomous to make routine measurements from the upper troposphere and stratosphere through the mesosphere (5-90 km above Earth’s surface) with remote operations. We already have an application that allows us to control our LiDAR at Summit Station using an iPhone. We’re trying to move in that direction with our Kellyville instruments as well.”

Sondrestrom houses four slightly different LiDARs. Three “green beam” and one “yellow beam” LiDARs transmit inch-wide green and yellow laser light straight up into the atmosphere. A sensitive Newtonian telescope sees and records the properties of the light reflected back to the Earth’s surface in much the same way as your eyes see (and your brain records) dust in the air in front of a slide projector or flashlight. In this way, Thayer’s group extracts a wide range of information about the middle and upper atmosphere such as density, temperature, aerosol content, and amount of the water vapor.

But Thayer says LiDARs have their limitations as well as advantages.

“Satellites can look at the upper atmosphere, but only in certain configurations. Similarly, for our purposes of looking at the middle and upper atmosphere, LiDAR systems work only in clear weather. We can measure as frequently as weather permits since the LiDARs can’t see through clouds. We schedule about 5-8 hours of observation time each week. Once in a while we get lucky and have longer observation periods in winter, often once Disko Bay freezes over and the winter weather pattern stabilizes. We provide really detailed measurements of the Earth’s atmosphere, just not as frequently as we would like."

The green beam with the most energy characterizes aerosols, ice clouds, fine particles like smoke, dust, and volcanic ash in the atmosphere all the way to the edge of space. Two others use polarized light to examine aerosols.  The yellow beam ‘resonates’ with sodium atoms in the atmosphere and is used to study the 90-120 km region of the atmosphere where meteorites ablate and leave a shell of calcium, sodium, iron, and magnesium atoms.

Several atmospheric phenomena are unique to the polar regions and Thayer’s group studies all of them. During the winter, a powerful circulation called the Arctic polar vortex occurs in the stratosphere (~20-50km above Earth’s surface). Dynamical changes in the vortex alters the entire polar system and can modify atmospheric chemistry contributing to ozone depletion in the northern hemisphere.

The Thayer team also studies the Aurora Borealis, a natural northern latitude light show resulting from the collision of charged particles with Earth’s magnetic field about 100km above the surface of the earth. Scientists study not only how aurorae form, but also how the aurorae, in turn, might affect the middle atmosphere.

Thayer has been a part of a science investigation to study polar mesospheric clouds using LiDAR. These clouds, also called noctilucent clouds, occur only in the polar regions during the summer. They are the highest clouds in Earth’s atmosphere, forming at about 83 km.  Typically, noctilucent clouds are difficult to see except at twilight as the sun reflects on them from below the horizon.  Noctilucent clouds, Thayer says, may be harbingers of climate change, but their role remains unclear.

“We see clouds because of water vapor and cold temperatures in the atmosphere. It’s tough to get water through the troposphere, where we live, to the middle atmosphere because temperatures at the tropopause are very cold and cause the water to freeze out and fall back to earth.  So we don’t expect to see clouds as high as 83 km. But water can be created in the middle atmosphere, the mesosphere, by the presence of methane. At higher altitudes ultraviolet light breaks down the methane and forms water vapor which leads to cloud formation. If we see more clouds, then we may be getting more CO2 in the upper atmosphere as well. The role of CO2 in the mesosphere actually cools the region in contrast to its warming of the troposphere. The cooler temperatures can further support polar mesospheric cloud formation and we can use the clouds' behavior as indicators of change associated with CO2 and methane. However, we still have a great deal to understand about our whole atmosphere and all of the exchange processes that occur across its layers.”—Marcy Davis