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 with a detection threshold of 0.4 µm am a Video Ice Particle Sampler with a detection threshold of 5–10 µm. The measurements show a rapid transition from solution droplets to ice crystals characteristic of homogeneous ice nucleation throughout the temperature range from −35° to −56°C. The temperature dependence of the relative humidity and the droplet sizes when ice nucleation occurs is consistent with theoretical and experimental expectations for homogeneous freezing. An expression is given for the peak RH with respect to water in the wave clouds (RHhn), which decreases from 100% above −39°C to 73% at −56°C; RHhn represents the condition required for ice nucleation in the wave clouds and is shown to be more consistent with the homogeneous freezing of sulfuric acid solution droplets than ammonium sulfate solution droplets.
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 approximately equal to (RHhn–10)%. Measurements from a balloonborne Formvar ice crystal replicator are reported that show the vertical structure of cirrus generally consists of three microphysically distinct regions: a highly ice-supersaturated ice production region near cloud top, an ice-supersaturated ice crystal growth region, and a sublimation region near cloud base formed by fallout of ice into ice-subsaturated air.
A negative feedback is observed, and studied numerically, between ice crystal concentration and ice-supersaturation; the RH condition for new ice nucleation in cirrus is most likely to occur near cloud top when existing ice concentrations are low. A second relationship, wherein the ice production rate depends on the RH, leads the authors to propose a conceptual model of the formation and evolution of circus clouds.