Ice nucleation and dehydration in the Tropical Tropopause Layer

Optically thin cirrus near the tropical tropopause regulate the humidity of air entering the stratosphere, which in turn has a strong influence on the Earth’s radiation budget and climate. Recent highaltitude, unmanned aircraft measurements provide evidence for two distinct classes of cirrus formed in the tropical tropopause region: (i) vertically extensive cirrus with low ice number concentrations, low extinctions, and large supersaturations (up to ∼70%) with respect to ice; and (ii) vertically thin cirrus layers with much higher ice concentrations that effectively deplete the vapor in exwith tropical tropopause temperature variability (7). Despite this general understanding, the details of the regulation of stratospheric humidity, including the role of deep convection, transport pathways, and cloud microphysical processes, have been subjects of considerable debate (8–10).