Relative humidity and temperature influences on cirrus formation and evolution observations from wave clouds and FIRE II

Measurements in orographic wave clouds, and in cirrus sampled during FIRE II, are used to investigate ice nucleation in the upper troposphere. The dynamically and microphysically simpler quasi-steady-state wave clouds provide relatively ideal conditions for observing characteristics of ice nucleation. Conclusions from the wave cloud study are applied to help understand the formation and evolution of ice in the cirrus clouds observed during FIRE II. The wave cloud study extends analyses reported by Heymsfield and Miloshevich down to -56°C, in part by using an improved droplet size spectrometer. Aircraft measurements made in cirrus during FIRE II show highly ice-supersaturated regions in clear air, placing a lower bound on the RH required for cirrus formation.