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Ice Cloud Formation and Dehydration in the Tropical Tropopause LayerStratospheric water vapor is important not only for its greenhouse forcing, but also because it plays a significant role in stratospheric chemistry. several recent studies have focused on the potential for dehydration due to ice cloud formation in air rising slowly through the tropical tropopause layer. Holton and Gettelman showed that temperature variations associated with horizontal transport of air in the tropopause layer can drive ice cloud formation and dehydration, and Gettelman et al. recently examined the cloud formation and dehydration along kinematic trajectories using simple assumptions about the cloud properties. In this study, we use a Lagrangian, one-dimensional cloud model to further investigate cloud formation and dehydration as air is transported horizontally and vertically through the tropical tropopause layer. Time-height curtains of temperature are extracted from meteorological analyses. The model tracks the growth and sedimentation of individual cloud particles. The regional distribution of clouds simulated in the model is comparable to the subvisible cirrus distribution indicated by SAGE II. The simulated cloud properties depend strongly on the assumed ice supersaturation threshold for ice nucleation. with effective nuclei present (low supersaturation threshold), ice number densities are high (0.1--10 cm(circumflex)-3), and ice crystals do not grow large enough to fall very far, resulting in limited dehydration. With higher supersaturation thresholds, ice number densities are much lower (less than 0.01 cm(circumflex)-3), and ice crystals grow large enough to fall substantially; however, supersaturated air often crosses the tropopause without cloud formation. The clouds typically do not dehydrate the air along trajectories down to the temperature minimum saturation mixing ratio. Rather the water vapor mixing ratio crossing the tropopause along trajectories is typically 10-50% larger than the saturation mixing ratio.
Document ID
20020054347
Acquisition Source
Ames Research Center
Document Type
Conference Paper
Authors
Jensen, Eric
(NASA Ames Research Center Moffett Field, CA United States)
Pfister, Leonhard
(NASA Ames Research Center Moffett Field, CA United States)
Gore, Warren J.
Date Acquired
August 20, 2013
Publication Date
January 1, 2002
Subject Category
Meteorology And Climatology
Meeting Information
Meeting: European Geophysical Society XXVII General Assembly
Location: Nice
Country: France
Start Date: April 20, 2002
End Date: April 28, 2002
Sponsors: European Geophysical Society
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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