Sunday, January 16, 2000

A SPARCLE in their eyes

Published in the Antarctic Sun

Scientists who study gases mostly confine them to flasks in laboratories. Not Stephen Warren and Von P. Walden, atmospheric researchers from the University of Washington in Seattle. They are studying the air out on the polar plateau.

Away from the sterile, controlled environments of indoor research facilities, Warren and Walden have created their own work site right next to the Clean Air Sector. The project is called SPARCLE, the South Pole Atmospheric Radiation and Cloud Lidar Experiment.

“We’re studying processes important for climate,” Warren said. In 1985, Warren began examining how sunlight reflecting off snow affects the energy budget of Antarctica. An significant reason for the extreme cold of Antarctica is that snow reflects 83 percent of the incoming solar energy. Warren also looked at the sizes and shapes of the snow crystals themselves to learn why snow reflects sunlight the way it does.

Walden studied the other half of the energy budget, measuring the amount of infrared energy emitted by the different gases in the air, as well as by clouds. He found that even the small amount of water vapor over the plateau was responsible for two-thirds of the natural greenhouse effect here, and carbon dioxide was responsible for most of the rest.

Now they are combining their efforts in a two-pronged attack on a tough problem.

“The most important greenhouse gas, worldwide, is water vapor,” Warren said. But nobody has accurately measured how much infrared energy it is capable of absorbing at low temperatures.

This information is vital for predictions of climate change not just in Antarctica but around the world.

And conditions on the ground at the South Pole, with temperatures dropping to minus 120 F, are similar to those at high altitude, in the upper troposphere, elsewhere in the world.

Learning more about the interactions between water vapor and infrared energy helps make climate-change models more accurate. While many causes contribute to climate change,
Warren said, they come back to one place.

“They either start with radiation or involve radiation,” he said.

The team, including graduate student Penny Rowe and research meteorologist Richard Brandt,
has devised two different ways to look at water vapor.

One is using the flat expanse of the polar plateau to provide a long path of uniform air. They have an instrument that reports how much infrared energy is absorbed by water vapor in the air. But it can be reconfigured to measure how much infrared energy is emitted by the atmosphere.

Water vapor’s absorption, Walden said, is weak in parts of the infrared spectrum. So to measure it accurately requires a lot of water vapor in the air. At high temperatures, it’s easy to get lots of water vapor in a small chamber in a laboratory. But such high-temperature measurements
may not be applicable to the cold upper troposphere. At low temperatures, the only way to get sufficient water vapor is with a long distance, more than half a mile, of air. Because the plateau is featureless, the air moving across it is usually fairly uniform in terms of wind speed and direction, humidity and temperature.

The other way the team is measuring the characteristics of water vapor is with a tethered balloon. They can send different instruments up with the balloon, to more than a mile high,
and photograph ice crystals and measure humidity and temperature. Most of the water vapor in the atmosphere is in the lowest mile of air.

The tethered balloon also allows the team to take sustained measurements at fixed altitudes, which is uncommon. Usually this type of research is done from freely rising balloons or from
airplanes, which move quickly through clouds and also may alter the cloud properties.

Their observations are compared with existing models of the atmosphere and its characteristics. In collaboration with other climate modelers, the team’s new data can be incorporated into improved concepts of the climate.

The information is also useful for interpreting data from satellites and other remote-sensing devices. The devices can record observations, but to interpret that information requires a
knowledge of the processes involved, including how gases absorb radiation.

But Warren and Walden can’t observe everything at once. To complete their descriptions of atmospheric conditions, they collaborate with NASA, NOAA and local weather observers.

This summer’s research is largely a testing phase. Much of the real work will happen next summer and over the following winter of 2001. Two members of the group will winter at the
South Pole to conduct the research, which uses existing tools in new ways.

One of the instruments was originally designed to measure pollutants coming out of factory smokestacks. Now it’s in use measuring water vapor in Antarctic air.

“We’re using new technology to increase our understanding of the Antarctic continent to make better predictions of climate for this region,” said Walden.