Ice Nucleation in Cold Cirrus Near the Tropical Tropopause Layer

Recent in situ measurements of thin cirrus near the tropical tropopause from the CRAVE and TC4 field experiments indicate ice number concentrations less than 100/L that are far lower than homogeneous- freezing nucleation theory predicts. In addition, the measured size distributions are much broader than theoretical models predict. Consistent with the apparent overestimate of ice concentrations by numerical models, we find that simulated extinctions are substantially larger than those indicated by CALIPSO. All of this evidence points to a fundamental problem with our theoretical understanding of ice cloud formation processes at low temperature with important implications for cloud radiative properties and dehydration of air entering the stratosphere. Various hypotheses for why ice concentrations are so low (including heterogeneous nuclei and glassy states of organic-containing aerosols) will be presented.