The water vapor content of the stratosphere is governed by dehydration via ice formation and sedimentation in the cold tropopause transition layer (TTL). In-situ aircraft measurements have routinely suggested that in atmospheric conditions, ice nucleation and growth at very low temperatures (T &lt; 200 K) is not sufficient to draw background water vapor down to presumed saturation vapor pressure over ice. Hypotheses to explain persistent supersaturations include mechanisms that inhibit ice growth or nucleation, or deposition of alternate ice phases with higher saturation vapor pressure. We report on measurements of ice formation in the ISOCLOUD experiments at the AIDA cloud and aerosol chamber (Karlsruhe, Germany), designed to investigate this anomalous supersaturation in ultracold cirrus. A series of cirrus formation experiments over temperatures from 235-190 K showed no temperature dependence in ice growth (accommodation coefficient), and no deviations from expected saturation vapor pressure.  Although some vapor pressure discrepancies in in-situ measurements may be due to experimental error, TTL cirrus formation remains somewhat problematic. We discuss the implications of the ISOCLOUD experiments in light of observed low ice particle number densities in high-altitude cirrus.

Ice formation in ultracold cirrus

249th National Meeting (2015)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.