Contrails and Aviation-induced Cirrus Clouds
STRATOSPHERIC DEHYDRATION BY THIN CIRRUS CLOUDS
Eric. J. Jensen, Owen. B. Toon, Leonhard Pfister, and Henry B. Selkirk
The extreme dryness of the stratosphere is believed to be caused by freeze-drying of air as it enters the stratosphere through the cold tropical tropopause. Tropical aircraft campaigns as part of the Stratosphere-Troposphere Exchange Project were undertaken to evaluate the roles of mixing and precipitation in the anvils of deep convective systems. However, ice crystals formed in situ near the tropopause may also play an important role in the stratospheric water budget. Recent observations have shown the presence of a persistent thin cirrus cloud layer near the tropical tropopause that is apparently not associated with deep convective systems. Measurements from satellite, lidar, and aircraft show that this cloud layer occurs primarily over the tropical western Pacific. This cirrus layer appears to be present as much as 80% of the time in this region.
Sedimentation of ice crystals in the thin cirrus may provide a significant downward flux of water vapor. It has also recently been suggested that gravity waves generated by convection may drive the formation of ice clouds in the lower stratosphere and that precipitation of crystals in these clouds may serve as a stratospheric dehydration mechanism.
Ames-Moffett contact: Dr. Eric J. Jensen