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Upper Atmosphere Temperatures Hold Some Surprises

A sensitive laser radar (lidar) system first deployed over Okinawa, Japan, to observe meteor trails during the 1998 Leonid meteor shower has now been used to probe temperatures in the upper atmosphere over both geographic poles.

As reported in the April 1 issue of Geophysical Research Letters, scientists at the University of Illinois used a specially designed lidar system to obtain the first measurements of upper atmosphere temperatures, iron densities and polar mesospheric clouds over the North and South poles.

"Measuring temperature profiles over the poles is essential for validating global circulation models and for providing a baseline for assessing the impact of global warming over the coming decades," said team leader Chester Gardner, a UI professor of electrical and computer engineering. "Until now, we were limited to measurements taken with balloon-borne sensors to altitudes of less than 20 miles."

In collaboration with scientists at The Aerospace Corp. and the National Center for Atmospheric Research, Gardner and his UI colleagues -- professor George Papen, research scientist Xinzhao Chu and graduate student Weilin Pan -- developed a more robust lidar system for measuring temperature profiles from the middle of the stratosphere (about 20 miles up) to the lower thermosphere at the edge of space (about 70 miles above Earth).

The system uses two powerful lasers operating in the near ultraviolet region of the spectrum and two telescopes to detect the laser pulses reflected from the atmosphere.

The researchers use two techniques for determining temperature. For altitudes up to 50 miles, the amount of laser light reflected from air molecules is measured and used to derive the temperature profile. For higher altitudes, scattering of the laser beams from iron atoms -- deposited in the upper atmosphere by meteoric ablation -- is measured.

In June 1999, the lidar system was flown over the North Pole to obtain temperature and iron density measurements during the Arctic Mesopause Temperature Study. Six months later, the instrument was taken to the Amundsen-Scott South Pole Station where it is now being used to measure the atmosphere temperature structure throughout the year.

The National Science Foundation provided funding for the two measurement campaigns.

"Temperature profiles obtained in the thermosphere over the North Pole on June 21, 1999, and in the mesopause region over the South Pole on Jan. 27, 2000, agreed closely with model predictions," Gardner said. "Significant departures from the model were observed during the austral fall, however. On May 8, 2000, for example, the lower mesosphere was about 20 degrees [Celsius] warmer and the upper mesosphere was about 20 degrees [Celsius] cooler than predicted."

Gardner and his colleagues also measured the heights of polar mesospheric clouds that formed over each of the poles during mid-summer. Unlike the lower atmosphere, the upper atmosphere is colder during summer than in winter. Polar mesospheric clouds form over the summertime polar caps when temperatures fall below minus 125 degrees Celsius.

These clouds are the highest on Earth, forming at an altitude of about 52 miles. Their brightness and geographic extent have been increasing during the past four decades. It is thought that these changes may be related to increasing levels of atmospheric carbon dioxide and methane, which, in the upper atmosphere, lead to cooler temperatures and increasing levels of water vapor.

Surprisingly, the altitudes of the polar mesospheric clouds over the South Pole were consistently one to two miles higher than those over the North Pole.

"Higher polar mesospheric clouds may be an indication of stronger upwelling in the summer mesosphere over Antarctica compared with the North polar cap," Gardner said. "Stronger upwelling would result in a cooler mesopause region." - By James E. Kloeppel

[Contact: Harvey Leifert, James E. Kloeppel]






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