NEWS INDEX Archives 2004 January

Puzzling height of polar clouds linked to solar radiation

James E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@uiuc.edu

1/26/04

CHAMPAIGN, Ill. — Scientists have discovered why polar mesospheric clouds over the South Pole are nearly two miles higher than those over the North Pole. A variation in solar radiation – a result of Earth’s elliptical orbit – is responsible, they say.

In the Jan. 29 online version of the journal Geophysical Research Letters, scientists from the University of Illinois at Urbana-Champaign and the British Antarctic Survey report new laser radar (lidar) measurements from Rothera, Antarctica, that support earlier findings concerning the puzzling heights of polar mesospheric clouds.

“We found that seasonal variations in cloud height are directly related to the vertical upwelling velocities in the mesopause region above the South Pole,” said Chester Gardner, a professor of electrical and computer engineering at Illinois. “The higher altitudes of the polar mesospheric clouds appear to be the direct result of increased solar radiation during the austral summer, when Earth is closest to the sun.”

Polar mesospheric clouds are the highest on Earth, forming at an altitude of about 52 miles. They form over the summertime polar caps when temperatures fall below minus 125 degrees Celsius. The brightness of the clouds and the geographic extent over which they are seen have been increasing over the past four decades.

While these clouds have little effect on Earth’s radiation budget, the fact that they are increasing is probably an indicator of long-term global climate change, Gardner said. “This has been attributed to increasing levels of atmospheric carbon dioxide and methane, which in the upper atmosphere lead to cooler temperatures and more water vapor.”
Greenhouse gases – such as carbon dioxide and methane – warm the lower atmosphere, but radiate this heat into space in the thin upper atmosphere. “As carbon dioxide levels rise, we expect the upper atmosphere to get colder,” Gardner said.

In addition, methane is broken up by the sun’s ultraviolet radiation, freeing hydrogen that can react with oxygen to form water vapor. “As methane levels rise, more water vapor will be created,” Gardner said. “These two scenarios may explain why polar mesospheric clouds are being seen more frequently and over larger geographical areas.”

The formation of polar mesospheric clouds is a complex process that depends on the temperature, water vapor and vertical wind structure of the mesopause region, Gardner said. As the cloud particles grow in size and mass, they slowly fall to lower altitudes where the combined effects of increasing atmospheric density and the upwelling air mass provide sufficient buoyancy to cause them to collect in thin layers.

Gardner’s group at Illinois made the first measurements of polar mesospheric clouds over the North Pole with an airborne lidar system in June and July of 1999. Six months later, the instrument was taken to the Amundsen-Scott South Pole Station, where measurements were made during the 1999-2000 and 2000-2001 summer seasons. The polar mesospheric clouds above the South Pole were consistently one to two miles higher than those over the North Pole.

In a paper published earlier this year in the Journal of Geophysical Research, Gardner, Xinzhao Chu, a research scientist at Illinois, and Ray Roble, a senior scientist at the High Altitude Observatory of the National Center for Atmospheric Research in Boulder, Colo., compared the cloud measurements with predictions from a global circulation model of the upper atmosphere.

The researchers used the NCAR Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model to explore the temperatures and vertical wind distributions at the North and South poles. Developed by Roble and his colleagues, this is the latest in a series of sophisticated three-dimensional, time-dependent models that simulate the circulation, temperature, and compositional structure of the upper atmosphere and ionosphere.

“The model showed that the primary forcing is the change in solar radiation as Earth orbits the sun,” Roble said. “The eccentricity of Earth’s orbit brings it closest to the sun in early January, and gives rise to a 6 percent annual variation in the intensity of sunlight striking the atmosphere. The increased sunlight over the South Pole creates higher vertical wind velocities, which push the clouds to a greater altitude.”

To better characterize the geographic differences in polar mesospheric cloud parameters, the researchers needed to make additional measurements at lower latitudes in Antarctica. Through a collaboration between the U. of I. and the British Antarctic Survey, the Illinois lidar was relocated in December 2002 to Rothera, Antarctica, nearly 1,500 miles from the South Pole.

Measurements taken during the austral summer of 2002-2003 revealed that the Rothera polar mesospheric clouds were much weaker, less frequent and not as high as those observed at the South Pole. The measurements also showed that in late January the temperatures in the mesopause were warmer at Rothera compared with the South Pole.

“The weaker polar mesospheric clouds at Rothera may be related to differences in temperature and water vapor in the mesopause region at Rothera compared with the South Pole,” said Patrick Espy, a senior scientist with the British Antarctic Survey. “Available water vapor is strongly influenced by the local temperature, and the mesopause temperature at Rothera was about 12 degrees Celsius warmer than at the South Pole.”

Although the clouds were lower at Rothera than at the South Pole, they were considerably higher than at similar latitudes in the Northern Hemisphere, Gardner said. “The variation with latitude in the Southern Hemisphere occurs because the vertical wind speed decreases with distance from the pole. But this doesn’t account for the difference in cloud height between the two poles.”

The seasonal variation in vertical wind speed is caused by Earth’s position in space, relative to the sun, Gardner said. Both the eccentricity of Earth’s orbit and the tilt of the planet’s axis create heating and cooling effects that either reinforce or counteract one another, depending on the time of year.

As summer approaches in the Northern Hemisphere, for example, the North Pole tilts toward the sun and receives more direct solar radiation. But, because Earth is moving away from the sun, this radiation also grows weaker, creating a counteracting effect. As summer approaches in the Southern Hemisphere, the South Pole tilts toward the sun, and the planet also moves closer to the sun. In this case, the two effects reinforce one another.

“During summer over the South Pole, the 6 percent higher solar intensity heats the lower atmosphere more, giving rise to greater vertical upwelling velocities,” Gardner said. “These higher wind speeds are just sufficient to raise the clouds one to two miles higher than those over the North Pole.”

In addition to Gardner, Chu and Espy, co-authors of the paper were Graeme J. Nott, Jan C. Diettrich, Mark A. Clilverd and Martin J. Jarvis, all with the British Antarctic Survey. The National Science Foundation and the British Antarctic Survey funded the work.