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Photonics on ice!

Photonics Spectra
Dec 2011
Lidar systems help unravel atmospheric mysteries over Antarctica


Summer comes to the icy vistas of the Antarctic shores. The winds die down a bit, and the temperature climbs (sometimes to above freezing). Most remarkable is the burgeoning population of the unusual bipeds, all puffed up in their striking, yet warmth-providing coats.

Yes, a swarm of scientists has arrived for a new season of vital research.

Antarctica, a land to which no one nation can lay claim, is home to a thousand or so scientists – along with some tourists and others – during the summer months of November through February. In some ways, it is like a small interdisciplinary science conference, with attendees focused on their own work but always moving from one conversation to the next, with new viewpoints and ideas to mull over and problems to help sort out.

Much of the science that occurs on Antarctica revolves around biology – penguins, seals, gulls and whales being favorite research targets. Studying the environment also has a lot of traction on the continent, exemplified not just by the large number of experiments and tests performed with weather balloons, but by the number of light detection and ranging (lidar)-based projects occurring year-round.

Lidar is perfect for studying the atmosphere. A laser beam directed straight into a clear sky not only can gauge the temperature tens of kilometers above, but also can record data on the presence, composition and motion of particulate matter wafting along the various atmospheric layers.


Fish-eye image of lidar, aurora australis and the Milky Way galaxy. © Nick Roden, Davis Station.


Iron, sodium, ice particles and more throughout the assorted layers of the sky tell their individual stories. Scientists look to atmospheric lidar data to determine temperature and wind patterns, to track global climate changes and fluctuating disturbances in the air, and to ascertain the possible origins of the materials found there. Each type of particulate is traceable by lidar systems on the ground.

Lidar, which can reach much farther into the upper atmosphere than can weather balloons, helps elucidate not only the chemical and thermal changes that are ongoing within the layers, but also those within the boundaries between them.

One such lidar project is operated by Xinzhao Chu and her group at the University of Colorado at Boulder. The team split much of its time between Boulder and McMurdo Station, a US-run base that – at about 78° south latitude – is about halfway between the South Pole and the Antarctic Circle. Its position provides the opportunity to take long looks at a region of the atmosphere that isn’t explored by many other instruments.

The Chu investigators set up the first iron lidar system at the South Pole Station in 1999 and upgraded and installed it at McMurdo Station in 2010. The system features frequency-doubled alexandrite lasers made by Light Age Inc. of Somerset, N.J. Operating at 372 and 374 nm, the new equipment offers tunability of about 40 nm around 750 nm – ideal for reaching into the mesosphere and beyond. Members of the group have spent both summers and winters at the station to run the system and maximize the amount of atmospheric data collected.

“My student Zhibin Yu recently became the first of the lidar team to overwinter at the station. Wentao Huang is there now for the summer. I will be returning around Christmastime. We will be there for at least two or three more years, depending on funding,” Chu said.

The researchers are looking at magnetic iron lidar studies. They recently discovered that their system can scan for iron as high up as 155 km, which Chu said could result in an entirely new research field: the study of atmospheric chemistry, dynamics and thermal structures in the 100- to 200-km range. Their paper on this discovery is in press at Geophysical Research Letters.

“Something unique is happening here,” she said. “We are very excited by the McMurdo campaign.”

Another prominent lidar project is at Davis Station, which is run by the Australian Antarctic Division (AAD). The station itself has been in operation since the 1950s, but its lidar system has been operational only since 2001. The station’s current system, which comprises a 30-W Nd:YAG laser operating at 532 nm, collects atmospheric measurements from about 5 to 85 km.


The lidar system at Davis Station in the Antarctic collects data on atmospheric composition and dynamics. © Ian Phillips, Davis Station.

“Generally, [the lidar] is operated for about eight hours continuously during the autumn-winter-spring nighttime (because there’s a better signal-to-noise ratio at night),” said AAD researcher Simon Alexander. “During the summer, when the sun never sets for a few months, we use a Fabry-Perot spectrometer to remove the background sunlight to allow us to continue making observations.”

From the lidar backscatter signal, Alexander and his colleagues obtain temperature, aerosol and cloud information.

“During winter, in the cold Antarctic atmosphere, polar stratospheric clouds form at altitudes of 15 to 25 km,” he said. “We study the microphysics of these clouds with the lidar because they are a necessary component in the annual cycle of Antarctic ozone destruction.”

Other AAD research topics include the internal dynamics and small-scale atmospheric processes that occur inside thin cirrus clouds at an altitude of around 10 km, the seasonal variations of temperatures in the middle atmosphere (30 to 80 km), the small-scale changes in temperature that occur within a single night’s observation, and polar mesospheric clouds (80 to 85 km) that form in the very cold summertime upper mesosphere.

Both Chu and Alexander acknowledge the basic fact of working in Antarctica: You have to appreciate solitude, especially if you are one of the few who live and work there over the harsh and lonely winter season.

Lidar work is generally solitary because of the shift-work nature, Alexander said. During a typical winter week, a team member at Davis might perform three or four nights’ worth of observations, plus be responsible for various laser maintenance and administrative tasks.

“During winter, there are usually no other scientists on station,” he said. “There is a winter engineer who helps maintain other atmospheric instruments, and there are also some meteorological observers and technicians who launch weather balloons throughout winter. The lidar scientist will interact with these people on work-related matters.”

Despite the solitary nature of data collection and shift work, Alexander said, the lidar scientist maintains an active role in station life, taking part in some of the duties shared by everyone who works there, such as kitchen work, station cleaning, fire team preparations, and search and rescue operations. Chu added that the various researchers at McMurdo hold occasional talks that allow them to describe their work to everyone else at the station.

Perhaps not too surprisingly, living and working in Antarctica is an enticement for young researchers, although lidar scientists stay down there continuously for 12 or 18 months, so it is not suitable for everyone, Alexander noted.

“Being in Antarctica for at least 12 months means you get to experience all of the seasons there, which are all very different, and it is a unique experience,” he said. “It is great to be able to see with your own eyes some of the things which you are observing with the lidar, such as the polar stratospheric clouds in winter.”


GLOSSARY
lidar
An acronym of light detection and ranging, describing systems that use a light beam in place of conventional microwave beams for atmospheric monitoring, tracking and detection functions. Ladar, an acronym of laser detection and ranging, uses laser light for detection of speed, altitude, direction and range; it is often called laser radar.
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