Relative Humidity Distributions in the Tropical Tropopause Layer Measured During NASA ATTREX

Removal of water vapor (WV) from the gas phase to form cirrus clouds in the tropical tropopause layer (TTL) is the primary control over the concentration and temporal variability of stratospheric WV. A complete understanding of the associated chemical and microphysical processes and quantitative predictive power of the frequency with which these clouds form and the efficiency with which they dehydrate air masses has been elusive, largely due to insufficiencies in measurements both of WV and cirrus microphysical properties. Observations provided by the NASA Airborne Tropical Tropopause Experiment (ATTREX) are helping to significantly close the gap between measurements and theory with respect to TTL cirrus and stratospheric WV. We will show that while the observed humidity inside and outside of TTL cirrus clouds is frequently supersaturated with respect to hexagonal ice, these measurements are in good agreement with conventional theories of ice nucleation and deposition. The ATTREX observations provide valuable statistics of relative humidity (RHi) in the TTL. We use these observations to evaluate the frequency distribution of RHi inside and outside of cirrus clouds, and begin to constrain the error that may be incurred in models that calculate stratospheric WV by assuming RHi does not exceed 100% in the TTL.