Research Staff: Eric Jensen and Leonhard Pfister

Stratospheric 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. These studies showed that temperature variations associated with horizontal transport of air in the tropopause layer can drive ice cloud formation and dehydration.

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 measured by the SAGE II satellite. The simulated cloud properties depend strongly on the assumed ice supersaturation threshold for ice nucleation. With effective ice nuclei present (low supersaturation threshold), ice number densities are high (0.1-10 cm^-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 (< 0.01 cm^-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.

The figure shows the simulated distribution of water vapor near the tropopause (about 17 km) in the tropics, along with the frequency of clouds from our model calculations. The driest regions are generally displaced to the west of the cloud formation regions due to the rapid horizontal transport of air parcels near the tropopause. Hence, the water vapor concentration measured in a particular location may be completely unrelated to cloud processes occurring nearby. Multidimensional models, including horizontal transport, vertical transport, and cloud microphysics are required to investigate dehydration of air entering the stratosphere.

Point of Contact: Eric Jensen, 650/604-4392, ejensen@sky.arc.nasa.gov

Figure Caption:

The distribution of water vapor mixing ratio (color shading) near the tropopause (about 17 km) is shown for the tropics (within 20 degrees of the equator) based on our model calculations. The black contours show the frequency (%) of clouds near the tropopause in the model. The clouds form most frequently just east of Indonesia, but the driest regions are displaced to the west due to rapid horizontal transport of air near the tropopause.