Lasers show atmosphere differs from models

New observations of the atmosphere over Earth’s polar regions may require scientists to revamp their mathematical models of environmental conditions at high altitudes. Such refinements could lead to better predictions of the effects of global warming.

Data collected by instruments at the Atmospheric Research Observatory at the South Pole may lead to better models of the middle and upper atmosphere. Gardner/UIUC

During recent summers at the two poles–June and July 1999 in the Arctic and December 1999 and January 2000 in Antarctica–researchers from the University of Illinois at Urbana-Champaign shone ultraviolet (UV) lasers into the sky to measure the temperatures at altitudes from 30 to 105 kilometers. They found that the high-altitude temperatures matched those predicted by standard atmospheric models. However, as fall arrived at the South Pole, temperatures began to stray from predictions, says Chester S. Gardner, an electrical engineer who led the research team.

In May 2000, temperatures between 55 and 75 km up were as much as 20C higher than predicted. At heights between 75 and 95 km, temperatures were as much as 20C lower than expected. Gardner and his colleagues report their findings in two papers in the April 1 Geophysical Research Letters.

At heights from 30 to 80 km, the amount of UV-laser-light scattering from the air molecules provided a measure of the air’s density, and therefore its temperature. Above 80 km, laser-light scattering from iron atoms allowed the researchers to directly gauge the temperature of the air. The iron in the upper atmosphere comes from meteors vaporized in their descent.

The scientists also used their lasers to study polar mesospheric clouds. At altitudes between 82 and 86 km, these are Earth’s loftiest clouds. They form only at high latitudes and at atmospheric temperatures below –123C. Over the past 40 years, the frequency, brightness, and area covered by these high, diffuse clouds has increased. These changes may reflect increased concentrations of water at high altitudes due to global warming.

Gardner’s team found that the mesospheric clouds over the South Pole were, on average, at an altitude of 85.5 km at the peak of summer. This is about 2 km higher than the average for such clouds in the Arctic and may be due to strong summertime upwelling of the atmosphere over Antarctica. As the summer progressed and the upwelling waned, the clouds’ altitude fell about 64 meters each day.

Gardner notes that his team’s finding of higher altitudes for Southern Hemisphere polar mesospheric clouds contradicts data from satellite measurements. That includes measurements obtained during the Antarctic summer of 1998 and the Arctic summer of 1999. Daniel

Morrison, an atmospheric scientist at Johns Hopkins University Applied Physics Laboratory in Laurel, Md., presented the satellite data last December at the American Geophysical Union’s fall meeting in San Francisco.

Morrison and his colleagues reported that, on average, there was no difference in altitude for summertime polar mesospheric clouds for the two hemispheres. In both regions, the polar mesospheric clouds appeared at heights of about 82.5 km., the researchers found.

More data will be the key to determining whether Gardner’s finding has identified a real difference between the hemispheres in the altitude of such clouds, Morrison says.

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