DEHYDRATION OF THE UPPER TROPOSPHERE AND LOWER STRATOSPHERE BY SUBVISIBLE CIRRUS CLOUDS NEAR THE TROPICAL TROPOPAUSE

The extreme dryness of the lower stratosphere is believed to be caused by freeze-drying of air as it enters the stratosphere through the col d tropical tropopause. Previous investigations have been focused on de hydration occurring at the tops of deep convective cloud systems. Howe ver, recent observations of a ubiquitous stratiform cirrus cloud layer near the tropical tropopause suggest the possibility of dehydration a s air is slowly lifted by large-scale motions. In this study, we have evaluated this possibility using a detailed ice cloud model. Simulatio ns of ice cloud formation in the temperature minima of gravity waves ( wave periods of 1 - 2 hours) indicate that large numbers of ice crysta ls will likely form due to the low temperatures and rapid cooling. As a result, the crystals do not grow larger than about 10 mu m, fallspee ds are no greater than a few cm-s(-1), and little or no precipitation or dehydration occurs. However, ice clouds formed by large-scale verti cal motions (with lifetimes of a day or more) should have fewer crysta ls and more time for crystal sedimentation to occur, resulting in wate r vapor depletions as large as 1 ppmv near the tropopause. We suggest that gradual lifting near the tropical tropopause, accompanied by form ation of thin cirrus, may account for the dehydration.