A closer look at cirrus clouds

Cirrus clouds—thin strands or sheets usually composed of ice crystals—form high in the atmosphere. Satellite observations have shown that these clouds are very prevalent near the tropopause (atmospheric boundary between the troposphere and stratosphere) throughout most of the tropics. One important aspect of cirrus clouds is that they restrict the amount of water vapor that is transported from the upper troposphere into the lower stratosphere.

A new study led by NCAR scientist Steve Massie, published in the Journal of Geophysical Research in January 2010, employs satellite technology to observe cirrus in greater detail than before. Using data from the HIRDLS (HIgh Resolution Dynamics Limb Sounder) and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) instruments currently aboard NASA satellites, the research team determined monthly distributions of cirrus clouds for the time period of September 2006 to August 2007. Experiments with data from both instruments found similar geographic distributions.

The researchers analyzed seasonal variations in the vertical and horizontal scales of cirrus clouds, finding that horizontal lengths are largest in December–February and shortest in summer. Their seasonal maps of laminar cirrus (isolated, shallow cirrus clouds) are the most complete maps of this type of cirrus to date.

Future changes in the distributions of cirrus clouds will likely modulate changes in stratospheric water vapor and chemistry, tying cirrus clouds to an important physical component of climate change in the stratosphere.

“A common theme in the history of science is that improvements in observations frequently reveal physical details not previously known,” Massie says. “The HIRDLS and CALIPSO observations are a good example of this.”


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